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New analytical expressions, symmetry relations and numerical solutions for the rotational overlap integrals
Abstract In this article, extremely simple analytical formulas are obtained for rotational overlap integrals which occur in integrals over two reduced rotation matrix elements. The analytical derivations are based on the properties of the Jacobi polynomials and beta functions. Numerical results and special values for rotational overlap integrals are obtained by using symmetry properties and recurrence relationships for reduced rotation matrix elements. The final results are of surprisingly simple structures and very useful for practical applications. © 2011 Wiley Periodicals, Inc. Int J Quantum Chem, 2012
A DFT study on the structure-property relationship of aminonitropyrazole-2-oxides
Abstract Density functional theory (DFT) calculations at the B3LYP/aug-cc-pVDZ level have been carried out to study the geometry and electronic structures, stability, sensitivity and band gap of the possible isomers of aminonitropyrazole-2-oxides. Kamlet-Jacob equations were used to determine the performance properties of model compounds. The performance properties of model compounds P5, P18, P20, P21, P22, and P23 are higher compared with 2,4,6,8,10,12-hexanitro-2,4,6,8,10,12-hexaazaisowurtzitane (CL-20) and octanitrocubane (ONC). The heat of explosion, density, detonation velocity and detonation pressure are related to the number and positions of NO2 and NH2 groups in pyrazole-2-oxide. © 2011 Wiley Periodicals, Inc. Int J Quantum Chem, 2012
A constrained variational approach to the designing of low transport band gap materials: A multiobjective random mutation hill climbing method
Abstract Neutral polythiophene (PT) and polyselenophene (PSe) are semiconductors with band gaps of about 2 eV. We have proposed and implemented a constrained variational method in which total energy of neutral PT or PSe oligomers is minimized under the constraint that the band gap measured by HOMO–LUMO energy difference is also a minimum in each case. The constrained (bimodal) minimization has been carried out by an adaptive random mutation hill climbing method within the basic framework of Su-Schrieffer-Heeger type of model. We show that the “band-gap constrained minimization” automatically creates electron deficient quinoid regions (QR) in the PT or PSe chains, embedded in aromatic regions (ARs), on both sides. We have investigated how the number and distribution of such QRs can reduce the band gap. Band gap constrained electronic structure calculations thus provide designing clues for low transport band gap materials based on molecular chromophores. © 2011 Wiley Periodicals, Inc. Int J Quantum Chem, 2012
A CASSCF/CASPT2 study on the low-lying electronic states of the CH3SS and its cation
Abstract Complete active space self-consistent field (CASSCF) and multiconfiguration second-order perturbation theory (CASPT2) calculations with contracted ANO-RCC basis set were performed for low-lying electronic states of CH3 SS and its cation in C s symmetry. For the ground state X2 A″ of CH3 SS, the calculated S-S stretching mode is in good agreement with experimental reports. The electron transitions of CH3 SS+ , X1 A′ → 11 A″, X1 A′ → 21 A′, and X1 A′ → 21 A″, are predicted at 1.055, 3.247, and 3.841 eV. Moreover, the calculated adiabatic and vertical ionization potential and adiabatic affinity are in reasonable agreement with the experimental data. The CASSCF/CASPT2 potential energy curves (PECs) were calculated for S2 -loss dissociation from the X2 A″, 12 A′, and 22 A″ states. The electronic states of the CH3 radical and S2 molecule as the dissociation products were carefully determined by checking energies and geometries of the asymptote products. The S2 -loss PEC for CH3 SS indicate that S2 -loss dissociation occurs from the X2 A″ state leading to CH3 (12 A″) + S2 (X3 Σ2 A′ and 22 A″ leading to CH3 (12 A″) + S2 (1 Δg ). © 2011 Wiley Periodicals, Inc. Int J Quantum Chem, 2012.
Gauge function optimization 2: An accurately solvable model
Abstract The gauge function is optimized by using the Kennedy–Kobe (KK) theory [Kennedy and Kobe, Phys Rev A, (1984) 30, 51] for the anisotropic harmonic oscillator model in a magnetic field. We extend the KK model to describe various electron density distributions, which resemble to those for the ground and excited states of a HC 0 continuous function at the nucleus. © 2011 Wiley Periodicals, Inc. Int J Quantum Chem, 2012
On the importance of orbital localization in QC-DMRG calculations
Abstract We investigate the importance of orbital localization in the application of the Density Matrix Renormalization Group (DMRG) technique to ab initio studies of molecular electronic structure. Our previous implementation of DMRG has been generalized to allow for the use of localized nonorthogonal orbitals. Simple cycles of equidistant hydrogen atoms, which are good examples of one dimensional metal-like lattices with fully delocalized electronic structures, have been taken as test models. In this study, the efficiency of the DMRG method and the importance of orbital localization for the generation of the DMRG building blocks are confirmed. However, it is found that the convergence of the procedure based on nonorthogonal orbitals is slower and requires more DMRG components than the standard orthogonal formulation. Symmetrically orthonormalized atomic orbitals are shown to be a good compromise solution: they satisfy the requirement of orbital localization for the generation of the DMRG blocks and improve the convergence, reducing the number of components of the DMRG expansion. © 2011 Wiley Periodicals, Inc. Int J Quantum Chem, 2012
An experimental and theoretical approach to the molecular structure of 3-{[4-(3-Methyl-3-phenyl-cyclobutyl)-thiazol-2-yl]-hydrazono}-1,3-dihydro- indol-2-one
Abstract The title molecule, 3-{[4-(3-methyl-3-phenyl-cyclobutyl)-thiazol-2-yl]-hydrazono}-1,3-dihydro-indol-2-one (C22 H20 N4 O1 S1 ), was prepared and characterized by 1 H NMR, 13 C NMR, IR, UV–visible, and single-crystal X-ray diffraction. The compound crystallizes in the monoclinic space group P21 with a = 8.3401(5), b = 5.6976(3), c = 20.8155(14) Å, and β = 95.144(5)°. Molecular geometry from X-ray experiment and vibrational frequencies of the title compound in the ground state has been calculated using the Hartree–Fock with 6-31G(d, p) and density functional method (B3LYP) with 6-31G(d, p) and 6-311G(d, p) basis sets, and compared with the experimental data. The calculated results show that optimized geometries can well reproduce the crystal structural parameters, and the theoretical vibrational frequencies values show good agreement with experimental data. Density functional theory calculations of the title compound and thermodynamic properties were performed at B3LYP/6-31G(d, p) level of theory. © 2011 Wiley Periodicals, Inc. Int J Quantum Chem, 2012
N-arylamides and N-arylcarbamates NCO internal rotation barrier study by molecular modeling
Abstract Amides and carbamates present an energetic barrier associated to Nt -butyl phenylcarbamates, and their N -methylderivatives was studied by AM1 and B3LYP/6-31G(d,p) calculations. The effect of aryl p- substituents (MeO, Me, Cl, Br, CN, and NO2 ) was also studied. Amide barriers were found by DFT calculation between 12 and 21 kcal/mol. Carbamates, on the other hand, showed barriers between 11 and 15 kcal/mol. AM1 underestimates the energetic barriers and provides values around half those obtained by B3LYP/6-31G(d,p) calculations. Electron withdrawing substituents on aryl group decrease the barrier. © 2011 Wiley Periodicals, Inc. Int J Quantum Chem, 2012
Theoretical study on the ion–molecule reaction of NH+ with CH2O
Abstract An in-depth theoretical study is carried out at the B3LYP/6-311G(d,p), M062X/aug-cc -pVDZ and CCSD(T)/6-311++G(3df,2dp) (single-point) levels as an attempt to explore the mechanism of the little-understood ion–molecule reaction between NH+ and CH2 O. Various possible reaction pathways are taken into account. It is shown that six dissociation products, including P 1 2 N + CH2 OH+ ), P 2 4 N + CH2 OH+ ), P 3 3 NH + CH2 O+ ), P 4 2 + HCO+ ), P 5 P 9 P 4 P 3 P 2 P 1 P 5 P 9 + + CH2 + . Furthermore, our calculated result is compared with the previous experimental research, and, meanwhile, it provides a useful guide for understanding analogous reaction, NH+ with CH2 NH. © 2011 Wiley Periodicals, Inc. Int J Quantum Chem, 2012
A new estimate on the indirect Coulomb energy
Abstract Here we prove a new lower bound on the indirect Coulomb energy in quantum mechanics, in terms of the single particle density of the system. The new universal lower bound is an alternative to the classical Lieb–Oxford bound (with a smaller constant, C = 1.45 < C LO = 1.68) but involving an additive kinetic energy term of the single particle density as well. © 2011 Wiley Periodicals, Inc. Int J Quantum Chem, 2012
Density functional theory studies on structures and absorption spectra of [Au(tpy)Cl]2+ and its derivatives: Role of basis set, functional, solvent effect, and spin orbit effect
Abstract The geometries of [Au(tpy)Cl]2+ (tpy = 2,2′:6′,2″-terpyridine) and its derivatives (1 –4 ) were optimized using relativistic density functional theory (DFT) at both scalar and two-component spin orbit coupling (SOC) level of theory via zero order regular approximation (ZORA). The combination of OPTX exchange, PW91c correlation functional (denoted as OP91), all-electron ZORA TZ2P basis set was found to be the optimal combination for geometry. The results reveal that both SOC and substituents have little effect on the geometry of complexes 1 –4 . Then, their absorption spectra were investigated by scalar relativistic time dependent DFT (TDDFT)/SAOP/TZ2P in vacuum, in CH2 Cl2 , CH3 CN solvents by means of conductor like screening model. The calculations indicate that the nature of the low-lying spin-allowed excited states is gold-perturbed intraligand transition, namely charge reorganization. This fact also demonstrates that the influence of the polarity of solvent on absorption spectra of 1 –4 is negligible. The spin orbit TDDFT was also performed to get further insight into the effect of SOC on the absorption spectra. It is found that the SOC has little influence on the simulation of electronic spectrum of complexes 1 –4 due to no significant involvement of d-orbitals during electronic transition. Our conclusions are reliable and are in good agreement with the previous experimental results and theoretical investigations. © 2011 Wiley Periodicals, Inc. Int J Quantum Chem, 2012
Theoretical thermochemistry: Enthalpies of formation of a set of nitrogen-containing compounds
Abstract Gas-phase standard state formation enthalpies of 63 nitrogen-containing compounds (NCCs) were studied by computational methods. Gaussian-n and complete basis set composite methods were applied. After the calculation of the set of NCCs, the results were analyzed in several ways. All the seven selected methods depicted various precisions in this work. According to the calculations and further data processing, G4 was proven to be appropriate and capable of formation enthalpy calculations on NCCs with a mean absolute deviation of 0.63 kcal/mol. Thus, G4 calculation may help us on choosing experimental values and conducting predictions. Isodesmic reactions using G4 methods are also conducted and further correction was given. The high accuracy of G4 method makes it reliable for calculating gas-phase enthalpies of formations and allows them to serve as a valuable check on the accuracy of reported experimental data. © 2011 Wiley Periodicals, Inc. Int J Quantum Chem, 2012
Double icosahedron-based motif of Nin (n = 20−30)
Abstract A genetic algorithm (GA) coupled with a tight-binding (TB) interatomic potential was used to search for the low-energy structures of the medium-sized Nin n = 20−30) clusters. The low-energy candidate structures from the GA/TB search were further optimized by using the density functional theory calculations with the Perdew, Burke, and Ernzerhof exchange-correlation energy functional. The obtained lowest-energy structures of the medium-sized Nin n = 20−30) clusters are shown to exhibit double icosahedron-based motif. The properties of the nickel clusters including binding energies, second differences in energy, and especially magnetic properties have also been studied. © 2011 Wiley Periodicals, Inc. Int J Quantum Chem, 2012
Use of noninteger n-generalized exponential type orbitals with hyperbolic cosine in atomic calculations
Abstract The efficiency of noninteger n-generalized exponential type orbitals (NGETO) r n *−1e r μ ) as radial basis functions in atomic ground state total energy calculations is studied. By the use of these functions, the combined Hartree-Fock-Roothaan calculations have been performed for some closed and open shell neutral atoms and their anions and cations with Z ≤ 21. The performance of new basis functions within the minimal basis framework has been compared with numerical Hartree-Fock (NHF) results. Our total energy values are significantly close to NHF results. The presented minimal basis total energies obtained from the noninteger NGETO with HC are notably better than minimal basis functions total energies previously reported in the literature. It is found that the accuracy of new noninteger NGETO with HC almost correspond to the accuracy of the conventional double-zeta functions. All the nonlinear parameters are fully optimized. © 2011 Wiley Periodicals, Inc. Int J Quantum Chem, 2012
Wigner crystallization of quadratically dispersing electrons in graphene
Abstract The graphene surface with the unpaired π electrons presents an ideal two-dimensional electron system. Although the effective massless Dirac fermions are important, they are not the only carriers that describe the quantum transport in graphene. Above zero energy, the current carrying carriers in graphene are the usual electrons. This electron density may vary depending on the surface defects and π–σ interaction, and this may lead to a possible Wigner crystallization on the surface of graphene. Calculations for nonmagnetic, ferromagnetic, and antiferromagnetic Wigner crystals are carried out based on the Koster–Kohn variational principle for direct calculation of Wannier functions. The effect of positive background due to the carbon ions is suitably treated. From our results, we find that Wigner crystallization is possible in grapheme, if we consider the electrons on the surface, which obey quadratic dispersion relation. The electron crystal with ferromagnetic phase and face centered square lattice structure has the lowest energy. © 2011 Wiley Periodicals, Inc. Int J Quantum Chem, 2012
Delocalization errors in a hubbard-like model: Consequences for density-functional tight-binding calculations of molecular systems
Abstract It has previously been shown that self-consistent-charge density-functional tight-binding (SCC-DFTB) suffers from a self-interaction error that leads to artificial stabilization of delocalized states. The effects of the error are similar to those appearing for many density functionals. In SCC-DFTB, the delocalization error is inherently related to the use of a Hubbard-like term to describe on-site charge interactions. The mathematical simplicity of this Hubbard-like term makes it easy to estimate if a complex system is subject to artificial stabilization of delocalized states and to quantitatively predict the delocalization error in the system energy at large fragment separation. The error is directly proportional to the on-site charge interaction term but decreases as the fragments become more asymmetric. The difference in orbital energies required to eliminate the delocalization error becomes equal to the Hubbard-like parameter of the fragment with the highest electron affinity. However, in most cases, the localized state will be favored by spin polarization, fragment repulsion, solvent effects, and large reorganization energies, in analogy to density functional theory, from which SCC-DFTB is derived. The presented analysis gives an early indication whether the standard approach is suitable, or if a different method is required to correct the delocalization error. In addition to cationic dimers, we discuss the effects of the delocalization error for asymmetric systems, bond dissociation of neutral molecules, and the description of mixed valence transition metal systems, exemplified by the enzyme cytochrome oxidase. © 2011 Wiley Periodicals, Inc. Int J Quantum Chem, 2012
Off-center shallow donors in a spherical Si quantum dot with dielectric border
Abstract Within the effective mass approximation and using a finite element method, the ground state energy and electron cloud localization of the shallow donors in a Si quantum dot (QD) with dielectric border are calculated. Simultaneous effects of dielectric mismatch (DM) at the core–shell interface, the impurity radial position, and the external electric field on the electronic properties are investigated. We found that (i) for a freestanding QD, the binding energy is strongly enhanced due to the additional interactions of the electron with the polarization charges; (ii) the electron cloud distribution can be easily modulated by varying the impurity position; (iii) the electric field-induced shift in energy levels increases with the DM. Therefore, the electronic energy levels of the nanocrystal could be tuned by properly tailoring the heterostructure parameters (DM with the surrounding matrix, impurity location) as well as by varying the electric field strength. © 2011 Wiley Periodicals, Inc. Int J Quantum Chem, 2012
Exact S-wave solution of the trigonometric pöschl-teller potential
Abstract The trigonometric Pöschl-Teller (PT) potential describes the diatomic molecular vibration. By using the Nikiforov-Uvarov method, we have obtained the exact analytical s-wave solutions of the radial Schrödinger equation (SE) for the trigonometric PT potential. The energy eigenvalues and corresponding eigenfunctions are calculated in closed forms. Some numerical results are presented too. © 2011 Wiley Periodicals, Inc. Int J Quantum Chem, 2012
New graphical representation of a DNA sequence based on the ordered dinucleotides and its application to sequence analysis
Abstract Graphical representation of a DNA sequence is a powerful tool for basic biological research. Based on the ordered dinucleotides, we propose a novel three dimensional (3D) graphical representation without circuit or degeneracy. Simultaneously, we derive the projection curve of the 3D graph. These two curves have good visualization for longer DNA sequences. The utility of the proposed curves is illustrated by mutation analysis, similarity analysis, and evolutionary relationships of different species. The results indicate that our method is efficient and powerful. © 2011 Wiley Periodicals, Inc. Int J Quantum Chem, 2012
Wiener indices of trees and monocyclic graphs with given bipartition
Abstract The Wiener index of a connected graph is defined as the sum of distances between all unordered pairs of its vertices. It has found various applications in chemical research. We determine the minimum and the maximum Wiener indices of trees with given bipartition and the minimum Wiener index of monocyclic graphs with given bipartition, respectively. We also characterize the graphs whose Wiener indices attain these values. © 2011 Wiley Periodicals, Inc. Int J Quantum Chem, 2012
Interfacial dipole and band bending in model pentacene/C60 heterojunctions
Abstract Quantum-chemical calculations are performed at the semiempirical Valence-Bond/Hartree–Fock level on model 1D aggregates to assess the electronic structure at pentacene/C60 heterojunctions. We show that the asymmetry of the electrostatic potential at the interface profoundly impacts the energy landscape explored by the charges, as they move away from the interface. Depending on the orientation of the pentacene molecules with respect to the interface, electrostatic effects may favor either the charge recombination or separation process. © 2012 Wiley Periodicals, Inc.
Hund's rule in the doubly excited states of the helium isoelectronic
Abstract The relative magnitudes of the interelectronic repulsions in doubly excited states of the helium isoelectronic sequence that correspond to a common (zero order) configuration are explored. For the isoelectronic sequences of singly excited states an inversion of the relative magnitude of this quantity has been observed to take place at the low end of the sequence, compared to the prediction of first-order perturbation theory. It was demonstrated that this inversion is associated with expansion of the inner shell concomitant with contraction of the outer shell. Correlation does not upset this inversion. A heuristic analysis of the behavior of the singly excited states near the critical charge at which the outer electron becomes unbound suggests that the inversion in the relative magnitudes of the singlet and triplet interelectronic repulsions is indeed inevitable. A similar argument shows that for the harmonic quantum dot such an inversion is highly unlikely (and is actually ruled out by numerical calculations). The behavior observed in intrashell doubly excited states of two-electron atoms exhibits markedly different features. Within the Hartree–Fock level no inversion of the relative magnitudes of the interelectronic repulsions takes place, in agreement with the absence of an expanding inner shell. However, correlation appears to play a more decisive role in this case, yielding a reversal of the relative magnitudes of the interelectronic repulsion that the independent particle model does not anticipate. Inserting a closed inner shell, that is, examining the singlet and triplet terms of the four electron configuration 1s2 2p2 and the six electron configuration 1s2 2s2 2p2 , fully supports the role of the inner shell in giving rise to an inversion of the relative magnitudes of the interelectronic repulsions even at the Hartree–Fock level. © 2012 Wiley Periodicals, Inc.
Possible DNA damage by oxidation products of guanine: A density functional and electron propagator theoretical study
Abstract The stability and reactivity of seven guanine oxidation products (GOP), which contain 8-oxo-7,8-dihydroguanine (8-oxoG) have been studied and compared with the four nucleobases, such as adenine, cytosine, guanine, and thymine. It has been possible with the use of density functional theory and electron propagator theory (EPT), to evaluate their relation with certain ionization induced process, which produce damage to DNA. Using the application of Koopmans's theorem, EPT provides alternatively a reliable way to calculate the vertical ionization potential (VIP) and vertical electron affinity (VEA). This process has been used to obtain other reactivity indexes, such as: electronegativity and hardness. In the nucleobases and the GOP studies, we observed the following: guanine and 8-oxo-7,8-dihydroguanine were the lowest VIP, and 8-oxoG was the lowest hardness. This let us confirm that these species are the most susceptible to change their electron densities and transform in other GOP. Particularly, the GOP, Sp(R), Sp(S), and Z were the highest VIP. It allows us to say that they are the most stable. Z and Iz were the highest VEA; this suggests that they have a big capacity to accept electrons and form anionic centers in DNA. The GOP, which was considered in this study, showed hardness values between 9.1 and 10.4 because of π-conjugation; therefore, these GOP could be good candidates to participate in DNA transversions. © 2012 Wiley Periodicals, Inc.
Protonation of 5-methylhydantoin and its thio derivatives in the gas phase: A theoretical study
Abstract The gas phase proton affinities of 5-methylhydantoin and its thio derivatives were theoretically studied through the use of high-level density functional theory calculations. The structure of all possible tautomers and their conformers were optimized at the B3LYP/6-311+(d,p) level of theory. Final energies were obtained at the B3LYP/6-311+(2df,2p) level. The imidazolidone derivatives 5-methyl-2,4-dioxo imidazolidine, 5-methyl-2-oxo-4-thio imidazolidine, 5-methyl-2-thio-4-oxo imidazolidine, and 5-methyl-2,4-dithio imidazolidine possess moderately strong proton affinities. Protonation at sulfur would be larger than protonation at oxygen. The most stable protonated forms of 2O4O and 2S4O have the proton attached to the heteroatom in position 2, whereas protonation of 2O4S and 2S4S preferentially takes place at position 4. The barriers for proton migration between the different tautomers are rather large. The energy decomposition analysis analysis of the O+ and S+ interactions suggests that the bonding interactions come mainly from the covalent bond formation. The contribution of the Coulomb attraction is rather small. © 2012 Wiley Periodicals, Inc.
The systematic examination of the basicity of heterocyclic compounds helps understanding of the reaction mechanisms occurring in several biological systems, such as the tautomerisation process of 5-methylhydantoin and its thio derivatives. For that system, the energy decomposition analysis of the OH+ interactions suggests that the bonding interactions come mainly from the covalent bond formation, while the contribution of the Coulomb attraction is rather small.
The origins and later developments of molecular orbital theory
Abstract Hund in 1926 speculated on the assignment of quantum numbers for the electronic states of diatomic molecules, basing his assignments on the correlation between the diatomic states and those of the united atom limit. Mulliken, who had earlier considered the problem of this assignment using old quantum theory, followed Hund in 1927, in the new quantum theory, correlating the states of a diatomic molecule from the united to the separated atoms. In 1932, he introduced the term orbital for the one-electron states of an atom or molecule. He was the first to seriously explore the orbitals of polyatomic molecules (1932). Burrau in 1927 made the first successful attempt to solve the Schrodinger's equation for H2 . Lennard-Jones (LJ) (1929) introduced the linear combination of atomic orbital model, and following the aufbau principle showed that this explained the paramagnetism of O2 . Slater (1929) wrote many-electron wave functions as determinants of spin-orbitals, and LJ (1949) showed that with this formulation molecular orbitals (MOs) could be transformed into bond-localized functions; this provided the link to the valence bond approach and the traditional view of the chemical bond. Huckel (1930) was the first to develop a semiempirical MO model for π-electron hydrocarbons, and this was later extended by others for all-electron wave functions. Boys (1950) saw the implication of Gaussian functions for calculating the electron repulsion integrals needed for ab initio calculations, and a later approach by Kohn and coworkers (1964 and 1965) produced a density functional MO theory in which electron repulsion is calculated from the whole electron density. © 2011 Wiley Periodicals, Inc. Int J Quantum Chem, 2011
Coupled-cluster theory for the polarizable continuum model. III. A response theory for molecules in solution
Abstract A coupled-cluster (CC) response functions theory for molecular solutes described with the framework of the polarizable continuum model (PCM) is presented. The theory is an extension to the dynamical molecular properties of the PCM-CC analytic derivatives recently proposed for the calculation of static molecular properties (Cammi, Jr Chem Phys 2009, 131, 164104). The theory is presented for linear and quadratic response functions, and the operative expressions of these response functions can accurately account for the nonequilibrium solvation effects. The excitation energies and transition moments of the solvated chromophores have been determined from the linear response functions. Accurate expressions for gradients of excitation energies for the evaluation of the excited state properties have been also discussed. © 2012 Wiley Periodicals, Inc. Int J Quantum Chem, 2012
Experimental and quantum chemical calculational studies on N-[4-(3-Methyl-3-phenyl-cyclobutyl)-thiazol-2-yl]-N′-pyridin-3ylmethylene-hydrazine
Abstract The title molecule, N-[4-(3-Methyl-3-phenyl-cyclobutyl)-thiazol-2-yl]-N′-pyridin-3ylmethylene- hydrazine (C20 H20 N4 S1 ), was characterized by 1 H-NMR, 13 C-NMR, IR, UV-visible, and X-ray determination. In addition to the molecular geometry from X-ray experiment, the molecular geometry, vibrational frequencies and gauge including atomic orbital 1 H- and 13 C-NMR chemical shift values of the title compound in the ground state have been calculated using the Hartree-Fock and density functional method (B3LYP) with 6-31G(d, p) basis set. The calculated results show that optimized geometries can well reproduce the crystal structural parameters. By using time-dependent density functional theory method, electronic absorption spectrum of the title compound has been predicted. © 2011 Wiley Periodicals, Inc. Int J Quantum Chem, 2011
Theoretical studies of ground and excited electronic states in a series of heteroleptic iridium complexes using density functional theory
Abstract A series of new heteroleptic iridium(III) complexes [Ir(C⁁ N)2 (N⁁ N)]PF6 (1 -6 ) (each with two cyclometalating C⁁ N ligands and one neutral N⁁ N ancillary ligand, where C⁁ N = 2-phenylpyridine (ppy), 5-methyl-2-(4-fluoro)phenylpyridine (F-mppy), and N⁁ N = 2,2′-dipyridyl (bpy), 1,10-phenanthroline (phen), 4,4′-diphenyl-2,2′-dipyridy (dphphen) were found to have rich photophysical properties. Theoretical calculations are employed for studying the photophysical and electrochemical properties. All complexes are investigated using density functional theory. Excited singlet and triplet states are examined using time-dependent density functional theory. The low-lying excited-state geometries are optimized at the ab initio configuration interaction singles level. Then, the excited-state properties are investigated in detail, including absorption and emission properties, photoactivation processes. The excited state of complexes is complicated and contains triplet metal-to-ligand charge transfer, triplet ligand-to-ligand charge transfer simultaneously. Importantly, the absorption spectra and emission maxima can be tuned significantly by changing the N⁁ N ligands and C⁁ N ligands. © 2011 Wiley Periodicals, Inc. Int J Quantum Chem, 2011
Rayleigh light scattering from hydrogen-bonded dimers of small astrophysical molecules
Abstract High level ab initio calculations of the Rayleigh scattering activities of the hydrogen-bonded dimers of formic acid (HCOOH), nitrosyl hydride (HNO), and hydrogen cyanide (HCN) molecules have been performed. All these molecules have already been detected in interstellar space and are of great importance from the astrochemical point of view. The geometries of the homo- and hetero-dimers have been optimized using Hartree–Fock and second-order Møller-Plesset perturbation theory. Dipole moment, mean dipole polarizability, and polarizability anisotropy have been calculated utilizing Pople-type 6-311++G(d,p) and Dunning's aug-cc-pVDZ basis sets for all the complexes. The polarizabilities are then used to calculate and analyze the Rayleigh scattering parameters. The results for the dimers, HCN···HCN, HCOOH···HCOOH, HNO···HNO, HCN···HCOOH, HCN···HNO, and HNO···HCOOH are compared with those of the isolated molecules, HCN, HCOOH, and HNO to see the effect of hydrogen bond formation on the molecular interaction with radiation. © 2011 Wiley Periodicals, Inc.
Hydrogen-bonded complexes of nicotine with simple alcohols
Abstract Ab initio and density functional theory studies have been performed on the hydrogen-bonded complexes of neutral and protonated nicotine with ethanol, methanol, and trifluromethanol to explore their relative stability in a systematic way. Among all the hydrogen-bonded nicotine complexes considered here, protonated forms in nicotine–ethanol and nicotine–methanol, and neutral form in nicotine–trifluromethanol complexes have been found to be the most stable. In the former two complexes, the proton attached to the pyrrolidine nitrogen acts as a strong hydrogen bond donor, whereas the pyrrolidine nitrogen atom acts as a hydrogen bond acceptor in the latter case. Neutral complex of nicotine with trifluromethanol has been found to possess a very short hydrogen bond (1.57 Å) and basis set superposition error corrected hydrogen bond energy value of 19 kcal/mol. The nature of the various hydrogen bonds formed has been investigated through topological aspects using Bader's atoms in molecules theory. From the calculated topological results, excellent linear correlation is shown to exist among the hydrogen bond length, electron density, and its Laplacian at the bond critical points for all the complexes considered. The natural bond orbital analysis has been carried out to investigate the charge transfer in the nicotine alcohol complexes. In contrast to the blue shifting behavior that is generally exhibited by other C3 carbon atom, the C
Theoretical investigation on the structures, densities, and detonation properties of polynitrotetraazaoctahydroanthracenes
Abstract The polynitrotetraazaoctahydroanthracenes were optimized to obtain their molecular geometries and electronic structures at density functional theory–B3LYP/6-31+G(d) level. Detonation velocities (D ) and detonation pressures (P ) were estimated for this nitramine compounds using Kamlet-Jacobs equations, based on the theoretical densities (ρ) and heats of formation. It is found that there are good linear relationships between volume, density, detonation velocity, detonation pressure and the number of nitro group. Thermal stability of the compounds was investigated by calculating the bond dissociation energies and energy gap (ΔE LUMO–HOMO ). The simulation results reveal that molecule H performs similarly to famous explosive RDX. These results provide basic information for molecular design of novel high energetic density compounds. © 2011 Wiley Periodicals, Inc.
Benzene–kaolinite interaction properties
Abstract In this work, we present a theoretical study of interaction of benzene with kaolinite and an analysis of the vibrational spectra, electrostatic potential maps, and self consistent field (SCF) orbitals. B3LYP and MP2 benzene–kaolinite results indicate a preferential interaction of benzene on kaolinite octahedral surface. Population analysis indicates that the atoms modified their charges when the monoaromatic molecule and clay mineral are close to tetrahedral and octahedral surface of kaolinite, which suggests that the adsorbed molecule becomes slightly positive. Infrared vibrational data show the reduction in the band frequencies related to the kaolinite external hydroxyls, indicating a weak interaction of benzene with clay mineral. It also was verified, from the potential electrostatic maps, a change in electron density in both benzene and kaolinite. Electron localization function analysis was also carried out for this interaction. © 2011 Wiley Periodicals, Inc.
Vibrational mode analysis for the multichannel reaction of CH3Cl + OH
Abstract The recently presented ab initio calculations for the reaction system of CH3 Cl + OH (Dehestani and Shojaie, Int J Quantum Chem, in press) are applied to the vibrational mode analysis. Extending previous work, we use the vibrational mode analysis to elucidate the relationships of the reactants, the transition state, the intermediates (IM), and the products. The extensive investigation shows that the reaction mechanism is reliable. © 2011 Wiley Periodicals, Inc. Int J Quantum Chem, 2011
π-Electron currents in larger fully aromatic benzenoids
Abstract Recently, we have reported on calculation of π-electron ring currents in several smaller fully benzenoid hydrocarbons having up to eight fused benzene rings and five Clar π-aromatic sextets. In contrast to early HMO ring current calculations and more recent ab initio calculations of π-electron density, our current calculations are based on a graph theoretical model in which contributions to ring currents comes from currents associated with individual conjugated circuits. In this contribution, we consider several larger fully benzenoid hydrocarbons having from 9 to 13 fused rings and from six or seven π-aromatic sextets. © 2011 Wiley Periodicals, Inc. Int J Quantum Chem, 2011
Local angular momentum as ring strain descriptor
Abstract A new method is demonstrated to quantify local ring strain, which is based on the expectation value of orbital angular momentum along the internuclear axis. In contrast to energy based methods which provide overall ring strain, this method is able to identify the local strain in every part of the ring. The formalism is benchmarked on several cycloalkanes in which the presence of ring strain is well understood. The ring strain plays a decisive role in carbon nanotubes (CNTs) properties; for instance, the hydrogen storage capability of CNTs is related to their diameter, which in turn has a close relation to the ring strain in their C
Theoretical investigation on identical anionic halide-exchange SN2 reaction processes on N-haloammonium cation NH3X+ (X = F, Cl, Br, and I)
Abstract CCSD(T) calculations have been used for identically nucleophilic substitution reactions on N -haloammonium cation, X− + NH3 X+ (X = F, Cl, Br, and I), with comparison of classic anionic SN 2 reactions, X− + CH3 X. The described SN 2 reactions are characterized to a double curve potential, and separated charged reactants proceed to form transition state through a stronger complexation and a charge neutralization process. For title reactions X− + NH3 X+ , charge distributions, geometries, energy barriers, and their correlations have been investigated. Central barriers ΔE − + NH3 X+ are found to be lower and lie within a relatively narrow range, decreasing in the following order: Cl (21.1 kJ/mol) > F (19.7 kJ/mol) > Br (10.9 kJ/mol) > I (9.1 kJ/mol). The overall barriers ΔE E comp decrease in the order F (645.6 kJ/mol) > Cl (515.2 kJ/mol) > Br (495.8 kJ/mol) > I (467.6 kJ/mol), and are found to correlate well with halogen Mulliken electronegativities (R 2 = 0.972) and proton affinity of halogen anions X− (R 2 = 0.996). Based on polarizable continuum model, solvent effects have investigated, which indicates solvents, especially polar and protic solvents lower the complexation energy dramatically, due to dually solvated reactant ions, and even character of double well potential in reactions X− + CH3 X has disappeared. © 2011 Wiley Periodicals, Inc. Int J Quantum Chem, 2011
Water oxidation on N-Doped TiO2 nanotube arrays
Abstract Photocatalytic splitting water into hydrogen and oxygen by utilizing solar energy is regarded as an effective strategy to solve oil crisis. By utilizing density functional calculations, we herein present the systemic studies with respect to water splitting mechanism on N-doped TiO2 nanotube arrays (NTAs), and focus on activation energy, thermodynamic properties, and effects of N-doping on reaction process. Our results reveal that the impurity 2p states of doped nitrogen effectively change electronic structure of TiO2 NTAs, which act as an electron acceptor and facilitate weakly bound electrons of valence band to be easily excited to acceptor level, as well as enhance the first H2 O adsorption and dissociation on the inside wall of N-doped TiO2 NTAs. Therefore, it is found that the rate-determining step of water splitting is the formation reaction of HOO* on N-doped TiO2 NTAs rather than the formation of HO* from the first H2 O. © 2011 Wiley Periodicals, Inc. Int J Quantum Chem, 2011
The influence of hydrophobic amino acid side groups on the acidity of the aromatic imidazole ring of histidine: A theoretical study
Abstract In this study we have calculated the acidity constant (pK a) of imidazole ring in Histidine-Hydrophobic amino acid dipeptides using the quantum chemistry and continuum solvation methods. Density functional theory calculations with the large basis sets are used to determine the Gibbs free energy of deprotonate in the gas and liquid phases. Based on our results ΔG S values are located between −69.38 and −18.82 kcal mol−1 which are related to His+ –Gly and His forms, respectively. pK a of the dipeptides in the aqueous phase was obtained from the calculated gas-phase and solvation free energies through a thermodynamic cycle and the solvation model chemistry of Martin Karplus et al. Solvation effects are treated using a self-consistent reaction field formalism involving polarized continuum models. According to our calculations pK a values are between 5.50 and 8.19 that are belong to His+ –ILe and His+ –Ala forms, respectively. Natural bond orbital analysis of dipeptides reveals that the electron delocalization in imidazole ring is the most effective factor in determination of acidity order for these compounds. Structural analysis confirmed that the orientation of carbonyl group with respect to imidazole ring is an effective factor in imidazole ring stability. © 2011 Wiley Periodicals, Inc. Int J Quantum Chem, 2011
The adiabatic-to-diabatic transformation angle and the berry phase for coupled jahn–teller/renner–teller systems: The F + H2 as a case study
Abstract The approach to calculate improved, two-state, adiabatic-to-diabatic transformation angles (also known as mixing angles), presented before (see Das et al., J Chem Phys 2010, 133, 084107), was used here while studying the F + H2 system. However, this study is characterized by two new features: (a) it is the first of its kind in which is studied the interplay between Renner–Teller (RT) and Jahn–Teller (JT) nonadiabatic coupling terms (NACT); (b) it is the first of its kind in which is reported the effect of an upper singular RT-NACT on a lower two-state (JT) mixing angle. The fact that the upper NACT is singular (it is shown to be a quasi-Dirac δ-function) enables a semi-analytical solution for this perturbed mixing angle. The present treatment, performed for the F + H2 system, revealed the existence of a novel parameter, η, the Jahn–Renner coupling parameter (JRCP), which yields, in an unambiguous way, the right intensity of the RT coupling (as resembled, in this case, by the quasi-Dirac δ-function) responsible for the fact that the final end-of-the contour angle (identified with the Berry phase) is properly quantized. This study implies that the numerical value of this parameter is a pure number (independent of the molecular system): η =
Spectroscopic constants and molecular properties of the X2Π and a4Σ− electronic states of the SiF radical
Abstract The potential energy curves (PECs) of the X2 Π and a 4 Σ− electronic states of the SiF radical have been studied by an ab initio quantum chemical method. The calculations have been made using the complete active space self-consistent field (CASSCF) method, which is followed by the valence internally contracted multireference configuration interaction (MRCI) approach in combination with several correlation-consistent basis sets. The effects on the PECs by the core-valence correlation and relativistic corrections are included. The way to consider the relativistic correction is to use the third-order Douglas–Kroll Hamiltonian approximation. The relativistic corrections are made at the level of cc-pV5Z basis set. The core-valence correlation corrections are performed using the cc-pCV5Z basis set. To obtain more reliable results, the PECs determined by the MRCI calculations are also corrected for size-extensivity errors by means of the Davidson modification (MRCI+Q). These PECs are extrapolated to the complete basis set limit by the total-energy extrapolation scheme. Using these PECs, the spectroscopic parameters are determined and compared with those reported in the literature. With these PECs obtained by the MRCI+Q/CV+DK+56 calculations, the vibrational levels, inertial rotation, and centrifugal distortion constants of the first 20 vibrational state of each electronic state are calculated when the rotational quantum number J equals zero. Comparison with the Rydberg-Klein-Rees (RKR) data shows that the present results are reliable and accurate. The molecular constants of the X2 Π and a 4 Σ− electronic states determined by the MRCI+Q/CV+DK+56 calculations should be good prediction for future laboratory experiment. © 2011 Wiley Periodicals, Inc. Int J Quantum Chem, 2011
One-, two-, and three-electron bonding: An “in vitro” theoretical study using noninteger nuclear charges evidences the crucial role of electronegativity in the strength of symmetrical bonds
Abstract Fictitious hydrogen atoms H*A of variable nuclear charge 0.5 ≤ ZA ≤ 2 (and thus of variable electronegativity) are used to study the intrinsic dependency of chemical bonding on electronegativity. Dissociation energy and equilibrium distance are reported for symmetrical 1-, 2- and 3-electron H*A H*A systems and 2-electron dissymmetrical H*A -H ones. Dealing with symmetrical systems, the strongest two-electron bonds are found for ZA ≈ 1.2. Oneelectron and three-electron strongest bonds occur respectively with low (ca. 0.7) and high (ca. 1.7) ZA values and can become stronger than the corresponding 2-electron system. Comparison with data on real systems leads to conclude that electronegativity is a prevailing atomic property in the control of the dissociation energy of symmetrical 1-, 2- and 3-electron bonds. A simplified mathematical model at Hartree-Fock or Heitler-London level with a minimal basis set reproduces these trends semi-quantitatively and provides the overall shape of the dissociation curves. Finally some points are qualitatively discussed from MO analysis, which emphasize the dependence of the bonding/antibonding properties on the nucleus charge ZA and their occupancy number. © 2011 Wiley Periodicals, Inc. Int J Quantum Chem, 2011
The investigation of nonadiabatic effects for the N + ND → N2 + D reaction
Abstract Nonadiabatic quantum dynamical calculations have been carried out on the two coupled potential energy surfaces (12 A′ and 22 A′) (Mota et al., J Theor Comput Chem 2009, 8, 849) for the title reaction. Initial state-resolved reaction probabilities and cross sections for ground and excited states for collision energies of 0.005–1.0 eV are determined, respectively. Nonadiabatic transition is enhanced about four times by isotopic substitution of N + NH by N + ND reaction. It turns out that the nonadiabatic effects exert no significant contribution in the N + ND → N2 + D reaction. © 2011 Wiley Periodicals, Inc. Int J Quantum Chem, 2011
Theoretical study of the electronic structure of LiX and NaX (X = Rb, Cs) molecules
Abstract Adiabatic potential energy, spectroscopic constants, dipole moments, and vibrational levels have been computed for the lowest electronic states of alkali dimers LiX and NaX (X = Rb, Cs). Calculations have been carried with the use of an ab initio approach with core-potential potentials and full-valence configuration. Thus, these systems are treated as two-electron systems. A good agreement is obtained for some lowest states of the molecules studied with available theoretical works. The existence of numerous avoided crossings between electronic states for 1 Σ symmetries is related to the charge-transfer process in each molecule between its two ionic systems (Li+ X− , Li− X+ ) and (Na+ X− , Na− X+ ). © 2011 Wiley Periodicals, Inc. Int J Quantum Chem, 2011
Comprehensive study of the interaction between hydrogen halides and methanol derivatives
Abstract The R2 2 , OMe, CHO, CN, C2 H5 , CH3 , H; X = F, Cl, Br) complexes are considered here as the interest sample for the consideration of different measures of H-bond strength. The intermolecular interaction energies are predicted by using MP2/6-31++G(d,p) and B3LYP/6-31++G(d,p) methods with basis set superposition error and zero-point energy corrections. The results showed that intermolecular hydrogen bonds for complexes with HF are stronger than such interactions in complexes with HCl and HBr. Quantum theory of “Atoms in Molecules” and natural bond orbitals method were applied to analyzed H-bond interactions. The gas phase thermodynamic properties of complexes were predicted using quantum mechanical computations. The obtained results showed a strong influence of the R and X substituents on the thermodynamic properties of complexes. Numerous correlations between topological, geometrical, thermodynamic properties and energetic parameters were also found. © 2011 Wiley Periodicals, Inc. Int J Quantum Chem, 2011
Diffusion monte carlo study of the hydrogen molecules adsorbed on C4H3Li
Abstract Diffusion Monte Carlo (DMC) simulations were used to calculate the binding energies for hydrogen molecules adsorbed on the lithium metal–organic complex C4 H3 Li. The calculations use all-electron DMC techniques where every electron is explicitly included in the simulation. Also we have systematically studied it using density functional theory (DFT) methods, revealing that each C4 H3 Li can hold up to four H2 molecules and the adsorption distance is about 2.2 Å. The DMC binding energies are in the range of 0.055–0.143 eV and are compared with those obtained with DFT using various exchange-correlation functionals, with values ranging from 0.029 to 0.504 eV. These results indicate that caution is required applying DFT methods to weakly bound systems such as hydrogen storage materials based on lithium-doped metal–organic frameworks. © 2011 Wiley Periodicals, Inc. Int J Quantum Chem, 2011
Computational study of the patterns of weaker intramolecular hydrogen bonds stabilizing acylphloroglucinols
Abstract Acylphloroglucinols (ACPLs) are polyphenolic compounds derivative from phloroglucinol, characterized by the presence of at least one COR group and exhibiting a variety of biological activities, which makes them interesting for drug development possibilities. This study investigates patterns in the ways in which weaker intramolecular hydrogen bonds contribute to their conformational stabilization, considering the Cin vacuo and in three different solvents (chloroform, acetonitrile, and water) show that, whereas Cin vacuo are performed at various levels to enable performance comparisons; calculations in solution use the polarizable continuum model. © 2011 Wiley Periodicals, Inc. Int J Quantum Chem, 2011
Quartic lattice interactions, soliton-like excitations, and electron pairing in one-dimensional anharmonic crystals
Abstract In this study, it is shown that two added, excess electrons with opposite spins in one-dimensional crystal lattices with quartic anharmonicity may form a bisolectron, which is a localized bound state of the paired electrons to a soliton-like lattice deformation. It is also shown that when the Coulomb repulsion is included, the wave function of the bisolectron has two maxima, and such a state is stable in lattices with strong enough electron (phonon/soliton)–lattice coupling. Furthermore, the energy of the bisolectron is shown to be lower than the energy of the state with two separate, independent electrons, as even with account of the Coulomb repulsion the bisolectron binding energy is positive. © 2011 Wiley Periodicals, Inc. Int J Quantum Chem, 2011
Quantum molecular similarity analysis and quantitative definition of catecholamines with respect to biogenic monoamines associated: Scale alpha and beta of quantitative convergence
Abstract In this article, we propose to make a quantitative definition of catecholamines with respect monoamine biogenic associated, this quantification allows define the alpha and beta scales, which expresses mathematically the correlation group of catecholamines with respect to their common fragment (phenylethylamine) and the tyrosine precursor, in addition performed quantum molecular similarity analysis using four descriptors, quantified by: molecular quantum similarity indices of overlap, Coulomb, overlap-interaction, Coulomb-interaction, overlap-distances Euclidian, and Coulomb-distances Euclidian, using the topo-geometrical superposition approach as method of alignment, also proposes a methodology for defining individual groups quantitatively, based on quantum similarity descriptors and supporting the interpretations with experimental results of structural path of reaction. Take in account the importance of creating tools that allow the quantification of biological relationships, and allow the creation of scales of group relations, this scales may be useful in organic synthesis of catecholamines for determine similarities, considering the search for experimental conditions as an area of growing scientific interest. © 2011 Wiley Periodicals, Inc. Int J Quantum Chem, 2011
The rotation-vibration spectrum of diatomic molecules with the tietz-hua rotating oscillator
Abstract The Tietz-Hua (TH) potential is one of the very best analytical model potentials for the vibrational energy of diatomic molecules. By using the Nikiforov-Uvarov method, we have obtained the exact analytical s-wave solutions of the radial Schrödinger equation for the TH potential. The energy eigenvalues and the corresponding eigenfunctions are calculated in closed forms. Some numerical results for diatomic molecules are also presented. © 2011 Wiley Periodicals, Inc. Int J Quantum Chem, 2011
The IR spectrum of supercritical water: Combined molecular dynamics/quantum mechanics strategy and force field for cluster sampling
Abstract Supercritical water was analyzed recently as a gas of small clusters of waters linked to each other by intermolecular hydrogen-bonds, but unexpected “linear” conformations of clusters are required to reproduce the infra-red (IR) spectra of the supercritical state. Aiming at a better understanding of clusters in supercritical water, this work presents a strategy combining classical molecular dynamics to explore the potential energy landscape of water clusters with quantum mechanical calculation of their IR spectra. For this purpose, we have developed an accurate and flexible force field of water based on the TIP5P 5-site model. Water dimers and trimers obtained with this improved force field compare well with the quantum mechanically optimized clusters. Exploration by simulated annealing of the potential energy surface of the classical force field reveals a new trimer conformation whose IR response determined from quantum calculations could play a role in the IR spectra of supercritical water. © 2011 Wiley Periodicals, Inc. Int J Quantum Chem, 2011
Theoretical study of the chemical reactivity and molecular quantum similarity in a series of derivatives of 2-adamantyl-thiazolidine-4-one using density functional theory and the topo-geometrical superposition approach
Abstract A theoretical study for the determination of chemical reactivity of a series of 2-adamantyl-thiazolidine-4-one derivatives was performed. These molecules are inhibitors of inverse transcriptase activity of human immunodeficiency virus type 1 (HIV-1) that prevents the replication of HIV-1 in cells without causing an appreciable effect on other retroviral particles or cellular polymerases. To determine the nature of the bond, enzyme substrate was used for quantum similarity molecular calculations, using the alignment of topo-geometrical superposition approach, solving the problem of finding the best alignment. The reactivity descriptors provide an appropriate methodology to evaluate molecular quantum similarity of these molecular structures, in the context of the density functional theory, taking into account the electronic effects quantified by the quantum similarity descriptor of Coulomb, which turned out to be the best molecular descriptor to describe the electron correlation structures; the determination of the reactive sites is of great importance for the determination of chemical reactivity, providing a bridge between the fields of chemical reactivity, and molecular quantum similarity allows us to find new characteristics and properties of reactivity on these kinds of molecules. © 2011 Wiley Periodicals, Inc. Int J Quantum Chem, 2011
Microsolvation of DMSO: Density functional study on the structure and polaraizabilities
Abstract The structure and stability for the association of water with dimethyl sulfoxide (DMSO) are investigated using the density functional M06-2X level theory. Stable complexes are formed by the formation of hydrogen bonding between water and oxygen atom of DMSO molecule, while the electrostatic force between water and DMSO plays a vital role in deciding the structure. The water-DMSO interactions are stronger than the interwater hydrogen bonds, which can be inferred from the shorter DMSO-water bond distance compared with the water–water bond distance. The calculated solvent association energy does not saturate, and it remains favorable to attach additional water molecules to the existing water network. The calculated IR spectra shifts supports the formation stronger hydrogen bonding, while the electrostatic potential (ESP) plot supports the existence of weaker electrostatic interaction in the studied clusters. The polarizabilities for the ground state clusters were found to increase monotonically with the cluster size. The presence of additional electrostatic bonding between water and DMSO, devastates the linear hydrogen-bonding network. © 2011 Wiley Periodicals, Inc. Int J Quantum Chem, 2011
Dispersion coefficients for Li+–H and Li+–He systems with coulomb and screened coulomb potentials
Abstract Exploiting powerful computational aspects and highly correlated exponential wave functions for two-electron atoms, we have investigated the effects of screened Coulomb interaction on the hexadecapole polarizability of Li+ (11 S), and the dispersion coefficients C 6 , C 8 , C 10 , and C 12 for interaction of Li+ with H and He atoms in their ground states. The dispersion coefficients and hexadecapole polarizability for different screening parameters ranging from 0 to 1.0 a + , and the dispersion C 12 coefficients for Li+ –H and Li+ –He system are reported for the first time in the literature. The C 6 , C 8 , and C 10 coefficients for the unscreened cases are comparable with the reported results. © 2011 Wiley Periodicals, Inc. Int J Quantum Chem, 2011
Wavelet transform of quantum chemical states
Abstract By using the technique of integral within an ordered product of operators, we recast classical wavelet transform to a matrix element of the squeezing-displacing operator U (μ,s ) between the mother wavelet vector 〈ψ| and the state vector |f 〉 to be transformed, i.e., we propose that 〈ψ|U (μ,s )|f 〉 can be considered as a new kind of spectrum for analyzing the quantum state |f 〉. In this way, we propose the wavelet- transform spectrum for quantum chemical states. As an example, we carry out the numerical calculation of wavelet-transform spectrum for the binomial state. It seems to us that this kind of spectrum can be used to recognize a variety of quantum chemical states. © 2011 Wiley Periodicals, Inc.
Impurity hamiltonian for transition metal complexes based on the exact exchange for correlated electrons hybrid functional
Abstract We present a theoretical method that combines density functional theory (DFT) and full configuration interaction (CI). Such a combination is based on the impurity model to (i) incorporate the advantages of DFT in description of the dynamic correlation and (ii) reduce the number of the two-electron integrals. The impurity model has been introduced with the help of a special type of the hybrid functional, namely, the exact exchange for correlated electrons (EECE), and the orthogonal fragment orbitals formalism. The EECE functional, originally based on the formulation of the local density approximation + U (LDA + U) method without the empirical parameter U, has been rewritten in this work in a form that incorporates the projected electron–electron repulsion energy operator. This representation has been compared with other types of nonglobal hybrids: position dependent and range separated hybrids. Further, the obtained Hamiltonian of the generalized Anderson model has been transformed to the form that affords the implementation of the linear scaling density-matrix renormalization group algorithm. © 2011 Wiley Periodicals, Inc.
Molecular level understanding of the role of aldehyde in vegetable-aldehyde–collagen cross-linking reaction
Abstract The role of formaldehyde (HCHO) in vegetable-aldehyde–collagen cross-linking reaction was investigated at the B3LYP/6-31+G(d) level, where lysine (LYS) was used as model of collagen and catechin (EC) as model of condensed vegetable tannin. Atomic charge and Frontier molecular orbital analysis show that intermediates formed by HCHO reacting with LYS or EC, that is, MLYS , MEC-6 , and MEC-8 , still have both nucleophilic and electrophilic sites, which are elements to form ternary cross-linking in vegetable-aldehyde–collagen system. The analysis of energy gap between HOMO (highest occupied molecular orbit) and LUMO (lowest unoccupied molecular orbit) indicate that the intermediate of HCHO–LYS residues (MLYS ) can further react with free HCHO to form product P-N(CH2 OH)2 (P-N-represents amino acid residue; N represents nitrogen atom on side chain), but the reaction of intermediate MLYS with free EC is difficult to take place. So, the probability of forming ternary cross-linking structure of amino acid residue–HCHO–EC is small, if HCHO is added before vegetable tannin in vegetable-aldehyde–collagen system. However, the reactions of EC–HCHO intermediates (MEC-6 and MEC-8 ) with free amino acids, HCHO–amino acid residue intermediate (MLYS ), as well as with other EC–HCHO intermediates (MEC-6 and MEC-8 ), are very easy to take place. The reaction enthalpy also shows that the cross-linking tendency is favorable in thermodynamics. So, it can be deduced that covalent cross-linking among amino side chain of collagen and vegetable tannin may take place when aldehyde is added after vegetable tannin. In this way, a multiple point cross-linking reaction occurs to create a high stabilization of collagen. © 2011 Wiley Periodicals, Inc. Int J Quantum Chem, 2011
Theoretical insights into the visible near-infrared absorption spectra of Bis(hexafluoroacetylacetonate) copper(II) in pyridine
Abstract The density functional theory calculations were performed to investigate the specific solvent effects on the optical absorption spectrum of copper(II) hexafluoroacetylacetonate complex in pyridine. The effects of single and double coordination of pyridine molecules at axial position of bis(hexafluoroacetylacetonate) copper(II) indicate that both positions and intensities of 3d–3d electronic transitions are strongly dependent on the coordination environment around the copper(II) complex. The results indicate that the nature of the electron-acceptor atoms in the equatorial ligands plays an important role in the number of solvent molecules in the first solvation shell of copper(II) acac systems. © 2011 Wiley Periodicals, Inc. Int J Quantum Chem, 2011
Exact solution of the modified Pöschl–Teller potential in the tridiagonal representation
Abstract The Schrödinger equation with the modified Pöschl-Teller (MPT) potential is studied by working in a complete square integrable basis that supports a tridiagonal matrix representation of the wave operator. The resulting three-term recursion relation for the expansion coefficients of the wavefunction is presented, and the wavefunctions are expressed in terms of the Jocobi polynomial. The discrete spectrum of the bound states is obtained by diagonalization of the recursion relation. © 2011 Wiley Periodicals, Inc. Int J Quantum Chem, 2011
DFT studies of cobalt hydride intermediate on cobaloxime-catalyzed H2 evolution pathways
Abstract Chemical, electrochemical, and photochemical methods all had been utilized to explore proton reduction catalysis by cobaloxime complexes. It was postulated in these studies that the initial step toward making H2 was protonation of CoI to form a CoIII hydride intermediate. However, in the following steps, different results from electrochemical studies had led to both monometallic and bimetallic pathways. In this article, theoretical computation method (BP86/6-31G*) was firstly performed on possible cobalt hydride intermediates involved in the reactive pathway of cobaloxime-catalyzed H2 evolution. The monometallic pathway B was excluded, both monometallic pathway A and bimetallic pathway were the possible process. However, the Gibbs free energy change for generation of H2 following monometallic pathway A was much more negative than that following bimetallic pathway. The calculation on monometallic pathway A indicated that the main driving force of the reaction (i) came from the step of the reduction of 11 . The proton transfer steps were also studied in detail. The protonation of cobalt hydride intermediates could directly happen on the dimethylglyoximate part. All the results refer to gas-phase calculations, not considering the solution. © 2011 Wiley Periodicals, Inc. Int J Quantum Chem, 2011
Calculation of a divergence-free lamb shift
Abstract A divergence-free theory of the Lamb shift is presented, and calculations are carried out for hydrogen-like ions. The theory uses two Lorentz-invariant equations of motion, one for the electron in the presence of the photon, which is Dirac's four-space time-dependent equation, and the other for the photon in the presence of the electron. The electron–photon interaction is calculated from the Dirac current. The distribution of photon frequencies is bounded by the distribution of electron frequencies intrinsic to the Dirac current, thereby guaranteeing a divergence-free result for the Lamb shift. © 2011 Wiley Periodicals, Inc. Int J Quantum Chem, 2011
Complexes of C60 with cyclic oligoisothianaphthenes. A theoretical study
Abstract Complexes of C60 with cyclic and linear oligoisothianaphthenes containing 8 and 12 repeating units have been modeled at M05-2X/6-311G**//M05-2X/6-31G* level of theory. Basis set superposition error (BSSE) corrected binding energies of neutral donor-acceptor complexes vary from 6 to 14 kcal/mol depending on the complex arquitecture and donor type. The inclusion complexes formed by C60 and cyclooligoisothianaphthenes containing 8 repeating units were found to be the most stable ones due close matching between host and guest molecules. Only weak charge transfer from oligoisothianaphthene to C60 fragment (< 0.04 electron) is detected in the ground state while almost complete electron transfer from oligoisothianaphthenes fragments to C60 has been found in the excited state. One electron oxidation or reduction in most of the cases slightly increases binding energies of the complexes with positive charge being totally concentrated at oligoisothianaphthenes fragment while in anion radicals the negative charge is located entirely at C60 moiety. © 2011 Wiley Periodicals, Inc. Int J Quantum Chem, 2011
Spin–orbit absorption spectroscopy of transition metal hydrides: A TD-DFT and MS-CASPT2 study of HM(CO)5 (M = Mn, Re)
Abstract The spin–orbit (SO) effects on the absorption spectra of HMn(CO)5 and HRe(CO)5 have been studied by means of ab initio multistate complete active space perturbation second order (MS-CASPT2) and time-dependent density functional theory (DFT) calculations. For both molecules and in contrast to the experimental spectra, the spin-free theoretical absorption spectra differ significantly, especially in the region of the intense band observed at 47,000 cm−1 . The main features of the lowest and highest parts of the absorption spectra are well reproduced making a reasonable assignment of the bands observed at 34,500 and 51,300 cm−1 for HMn(CO)5 and 37,000 and 50,630 cm−1 for HRe(CO)5 . Whereas, the lowest band of HMn(CO)5 has a significant metal-centered (MC) character, the band centered around 37,000 cm−1 in HRe(CO)5 corresponds to nearly pure metal-to-ligand-charge-transfer (MLCT) states. The intense bands observed above 48,000 cm−1 in both complexes have mixed character with MLCT, sigma bond to ligand charge transfer, sigma bond to sigma bond charge transfer, and metal to sigma bond charge transfer contributions, which may vary as a function of the level of calculation (DFT, MS-CASPT2, SO). This is attributed to the ability of the method at describing electronic correlation effects in various types of excited states in a balanced way. The SO absorption spectra are more realistic for both molecules with a red shift of the start of the absorption and a broadening of the bands due to a large mixing between the singlet–triplet states and an increase of the density of electronic states contributing to the absorption. This red shift is more important in HMn(CO)5 than in HRe(CO)5 due to the MC contribution to the lowest states in first-row transition metal complexes. Our calculations point to the presence of an absorption of intermediate intensity, not observed experimentally, around 40,000–42,000 cm−1 , mostly of MLCT character. © 2011 Wiley Periodicals, Inc. Int J Quantum Chem, 2011
Ab initio electron energy-loss spectra and depolarization effects: Application to carbon nanotubes
Abstract We perform ab initio calculations of the optical absorption and electron energy-loss spectra of (m , 0) single-walled carbon nanotubes (with m ≠ 3n for m = 7-25) in the framework of a “sum over states” (SOS) treatment of the Kohn-Sham (KS) single-particle orbitals and energies (CRYSTAL program). This approach tested on hexagonal boron nitride enables to fully assign the interband transitions in the imaginary part of the dielectric constant, in terms of atomic orbitals. As these calculations could not take into account the local field effects (depolarization effects), which take place for perpendicular polarizations in 2D and 1D periodic systems, we apply a simple method based on the Clausius-Mossotti formula, relating the SOS and coupled-perturbed KS polarizability values. This approach reproduces the main features of the spectra of boron nitride (001) surface and carbon nanotubes. © 2011 Wiley Periodicals, Inc. Int J Quantum Chem, 2011
Density functional study of the phenylethyl + O2 reaction: Kinetic analysis for the low-temperature autoignition of ethylbenzenes
Abstract Quantum chemical calculations at the CBS-QB3 level of theory have been carried out to investigate the potential energy surfaces for the reactions of α- and β-phenylethyl radicals with molecular oxygen. For the α-phenylethyl + O2 reaction, all of the transition states for the isomerization reactions of α-phenylethylperoxy radicals were positioned above the total energy of the reactants of α-phenylethyl + O2 . For the β-phenylethyl + O2 reaction, on the other hand, most of the transition states were positioned below the total energy of the reactants of β-phenylethyl + O2 . The RRKM rate constant analysis revealed that the backward reaction forming α-phenylethyl + O2 was dominant in the α-phenylethyl radicals + O2 reaction system at the temperature range between 300 and 1500 K, whereas the reaction pathway forming cyclic O2 structures (5b ) was dominant in the β-phenylethyl radicals + O2 reaction system at the same temperature range. In the reactions of both α- and β-phenylethyl radicals with molecular oxygen, the HO2 elimination reaction channels became more and more important when the temperature increased up to around 1000 K. Further decomposition channels of the cyclic O2 structures (5b ) were investigated using the density functional B3LYP theories and found that all of these decomposition reactions could proceed without any large activation barriers. The transition state structures forming such cyclic O2 structures in the phenylpropyl + O2 have also been calculated, and it was found that these cyclic O2 structures were one of the major products on the high-temperature reactions of β- and γ-phenylpropyl radicals with molecular oxygen. © 2011 Wiley Periodicals, Inc. Int J Quantum Chem, 2011
Theoretical study of the fragmentation and isomerization of ethyltoluene radical cations
Abstract This theoretical study was held with purpose to explore fragmentation and isomerization pathways of ethyltoluene radical cation. B3LYP level of theory with 6-31G(d) and 6-311++G(df,p) basis sets was used to calculate potential energy surface of o -, m -, and p -isomers. Single-point energies calculations were carried out using CCSD//B3LYP/6-31G(d). It was found that methyl loss from these seven-membered ring ions requires less energy than either ring methyl loss or direct CH3 detachment from ethyl moiety of ethyltoluene radical cation, which is consistent with previous experimental studies. © 2011 Wiley Periodicals, Inc. Int J Quantum Chem, 2011
Critical charges of simple coulomb molecular systems: One-two electron case
Abstract We consider some Coulomb systems with several infinitely massive centers of charge Z and one or two electrons: (Z ,e ), (2Z ,e ), (3Z ,e ), (4Z ,e ), (2Z ,e ,e ), and (3Z ,e ,e ). It is shown that the physical, integer charges Z = 1,2,… do not play a distinguished role for the total energy and for the equilibrium configuration of a system, giving no indication of a charge quantization. By definition, a critical charge Z cr for a given Coulomb system (nZ ,e ) or (nZ ,e ,e ) is a charge which separates the domain of the existence of bound states from the domain of unbound states (the domain of stability), the continuum (the domain of instability). For all the above-mentioned systems critical charges Z cr as well as equilibrium geometrical configurations are found. Furthermore, an indication to a branch point singularity at Z = Z cr with exponent 3/2 was obtained. It is demonstrated that in the domain of the existence the optimal geometrical configuration for both (nZ ,e ) at n = 2,3,4 and (nZ ,e ,e ) at n = 2,3 corresponds to the Platonic body. © 2011 Wiley Periodicals, Inc. Int J Quantum Chem, 2011
Structures, magnetic properties, and electronic counting rule of metals-encapsulated cage-like M2 Si18 (M = Ti-Zn) clusters
Abstract The geometries, magnetic properties and stabilities of the transition metal (TM) atoms encapsulated M2 Si18 (M = Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn) clusters have been systematically calculated by using the density function theory with generalized gradient approximation. Only when the doping metal atom has no more than half-full d electronic shell, a double hexagonal prism cage-like M2 Si18 structure could form. The total moments of M2 Si18 are either 0 or 2μB . Co2 Si18 is the most stable cluster among all 3d doped M2 Si18 clusters. The model of shell closure at the TM atom may be helpful to understand the stability of M2 Si18 clusters. © 2011 Wiley Periodicals, Inc. Int J Quantum Chem, 2011
The adsorption of CO2, H2CO3, HCO3− and CO32− on Cu2O (111) surface: First-principles study
Abstract The adsorption of CO2 , and its derivatives, H2 CO3 , HCO2 O (111) surface has been investigated by first-principles calculations based on the density functional theory at B3LYP hybrid functional level. The Cu2 O (111) surface has been modeled using an embedded cluster method,in which the quantum clusters plus some ab initio ion model potentials were inserted in an array of point charges. On the surface, H2 CO3 was dissociated into an H+ and an HCO2 species, HCO2 on Cu2 O (111) surface can start from the form of HCO
Competitive interaction between halogen and hydrogen bonds in NH2Br-HOX (X = F, Cl, and Br) complex
Abstract The NH2 Br-HOX (X = F, Cl, and Br) complexes have been investigated with quantum chemical calculations at the MP2/aug-cc-pVTZ level. Five isomers are observed for the Cl and Br complexes, whereas only two isomers are found for the F complex. The geometrical, energetic, and spectroscopic parameters have been analyzed for these complexes. The hydrogen-bonded complexes are more stable than the halogen-bonded ones. In most complexes, the associated O
On the time-dependent solutions of the Schrödinger's equation. II. The one-mode field perturbed harmonic oscillator
Abstract The solution of the time-dependent Schrödinger's equation for a perturbed harmonic oscillator is obtained using a solvable Lie algebra. We choose a harmonic oscillator interacting with a one-mode field, where the perturbation happens to be periodic in time. This leads to one of the simplest Floquet problems. Using the Wei–Norman theorem, the Floquet wave function is obtained as well as the semiclassical Floquet shift in the energy. © 2011 Wiley Periodicals, Inc. Int J Quantum Chem, 2011
Theoretical study of methoxy group influence in the gas-phase elimination kinetics of methoxyalkyl chlorides
Abstract The unimolecular gas-phase elimination kinetics of 2-methoxy-1-chloroethane, 3-methoxy-1-chloropropane, and 4-methoxyl-1-chloroburane has been studied by using density functional theory (DFT) methods to propose the most reasonable mechanisms of decomposition of the aforementioned compounds. Calculation results of 2-methoxy-1-chloroethane and 3-methoxy-1-chloropropane suggest dehydrochlorination through a concerted nonsynchronous four-centered cyclic transition state (TS) to give the corresponding olefin. In the case of 4-methoxyl-1-chloroburane, in addition to the 1,2-elimination mechanism, the anchimeric assistance by the methoxy group, through a polar five-centered cyclic TS, provides additional pathways to give 4-methoxy-butene, tetrahydrofuran and chloromethane. The bond polarization of the Cδ + ··· Clδ − , is the limiting step of these elimination reactions. The significant increase in rate together with the formation of a cyclic product tetrahydrofuran in the gas-phase elimination of 4-methoxyl-1-chloroburane is attributed to neighboring group participation of the oxygen of the methoxy group in the TS. The theoretical calculations show a good agreement with the reported experimental results. © 2011 Wiley Periodicals, Inc. Int J Quantum Chem, 2011
Substituent effect on the reaction mechanism of proton transfer in formamide
Abstract MP2 and B3LYP methods at 6-311++G** basis set have been used to explore proton transfer in keto-enol forms of formamide and to investigate the effect of substituent, i.e., H, F, Cl, OH, SH, and NH2 on their transition states. Additionally, the vibrational frequencies of aforementioned compounds are calculated at the same levels of theory. It is proposed that the barrier heights values in kJ/mol for F, Cl, OH, and SH substituents are significantly greater than that of the bare tautomerization reaction, implying the importance of the substituents effect on the intramolecular proton transfer. © 2011 Wiley Periodicals, Inc. Int J Quantum Chem, 2011
Dynamic changing features of the molecular face for interaction of a rare gas atom with a hydrogen molecule
Abstract We perform a systematical investigation on the dynamic changing feature of the molecular shape and size and electron density distribution in the process of a rare-gas atom (He, Ne, Ar, and Kr) approaching a hydrogen molecule by an ab initio method. In terms of the molecular face (MF) theory, the polarization effect in the above processes is vividly demonstrated. There is a good linear correlation between our average variation rate (S aver ) of molecular intrinsic characteristic radius at the contacting point and the experimental polarizability of the rare-gas atoms. This indicates that the MF theory can well represent the intermolecular polarization effect. Interestingly, the pictures of shape changing, charge-flow, and exchange repulsion processes especially on the reacting active areas have been clearly observed. © 2011 Wiley Periodicals, Inc. Int J Quantum Chem, 2011
Multistep conformational interconversion mechanism of cyclododecane. A simple and fast analysis using potential energy curves
Abstract An ab initio and Density Functional Theory (DFT) study of the conformational properties of cyclododecane was carried out. The energetically preferred equilibrium structures, their relative stability, and some of the transition state (TS) structures involved in the conformational interconversion pathways were analyzed from RHF/6-31G(d), B3LYP/6-31G(d,p) and B3LYP/6311++G(d,p) calculations. Aug-cc-pVDZ//B3LYP/6311++G(d,p) single point calculations predict that the multistep conformational interconversion mechanism requires 11.07 kcal/mol, which is in agreement with the available experimental data. These results allow us to form a concise idea about the internal intricacies of the preferred forms of cyclododecane, describing the conformations as well as the conformational interconversion processes in the conformational potential energy hypersurface. Our results indicated that performing an exhaustive analysis of the potential energy curves connecting the most representative conformations is a valid alternate tool to determine the principal conformational interconversion paths for cyclododecane. This methodology represents a satisfactory first approximation for the conformational analysis of medium- and large-size flexible cyclic compounds. © 2011 Wiley Periodicals, Inc. Int J Quantum Chem, 2011
First-principles simulations of two photon absorption spectra of dynamic structural chromophores in green fluorescent protein
Abstract The bridge photoisomerization of the chromophores of fluorescent proteins has been suggested as the possible mechanism of radiationless decay in fluorescent proteins. It indicates that this internal structure changing of chromophores great influence the optical properties of fluorescent proteins. The two-photon absorption (TPA) properties of fluorescent proteins might also be influenced by the bridge photoisomerization of the chromophores. In this work, we simulate the dynamic conformations through rotating the bridge bond of chromophore of green fluorescent protein, and employ the time dependent density functional theory combining with the sum-over-states method to study their TPA characters. With our study, we find that the TPA characters of chromophore will be improved through controlling rotation of the bridge bond of chromophore. © 2011 Wiley Periodicals, Inc. Int J Quantum Chem, 2011
Theoretical prediction on HAlS+ and HSAl+ cations using multiconfiguration second-order perturbation theory
Abstract Some low-lying states of the HAlS+ and HSAl+ cations have been studied for the first time by large-scale theoretical calculations using three methods: complete active space self-consistent field (CASSCF), complete active second-order perturbation theory (CASPT2), and density functional theory Becke's three-parameter hybrid function with the nonlocal correlation of Lee–Yang–Parr (B3LYP) with the contracted atomic natural orbital (ANO-L) and cc-pVTZ basis sets. The geometries of all stationary points along the potential energy surfaces (PESs) were optimized at the CASSCF/ANO-L and B3LYP/cc-pVTZ levels. The ground and the first excited states of linear HAlS+ are predicted to be X 2 Π and A 2 Σ+ states, respectively. For the linear HSAl+ structure, the first excited state is A 2 Σ+ . The X 2 Π state of linear HSAl+ is a second-order saddle point, because it has two imaginary frequencies. Two bent global minima M1 and M2 were found along the 12 A ′ and 12 A ″ PESs, respectively. The CASPT2/ANO-L potential energy curves of isomerization reactions were calculated as a function of HAlS bond angle. According to our calculations, the ground-state HAlS+ is linear, whereas the ground-state HSAl+ is bent. © 2011 Wiley Periodicals, Inc. Int J Quantum Chem, 2011
Density functional theory study of the regio- and stereoselectivity of diels–alder reactions of 5-Aryl-2-pyrones
Abstract A theoretical study of the mechanism and regio- and stereoselectivity of Diels–Alder reactions of 5-aryl-2-pyrones (Ar = Ph, 4-(MeS)-Ph) with substituted alkenes (CHZ = CH2 , Z = COMe, OAc) is performed at the B3LYP/6-31G(d) level. The analysis of the relevant stationary points of the potential energy surface and intrinsic reaction coordinate calculations show that these cycloadditions are undergoing through asynchronous concerted mechanisms yielding to the formation of the 5-endo isomers as the major cycloadducts. The calculation of activation and reaction energies indicates that the 5-endo cycloadducts are favored both kinetically and thermodynamically. The obtained results are in good agreement with experimental outcomes. © 2011 Wiley Periodicals, Inc. Int J Quantum Chem, 2011
Theoretical study of low-lying electronic states of the LiRb+ molecular ion: Structure, spectroscopy and transition dipole moments
Abstract The electronic structure and the spectroscopic properties for low-lying electronic states of the LiRb+ molecular ion, dissociating into Li (2s , 2p , 3s , 3p , 3d , 4s , and 4p ) + Rb+ and Li+ + Rb (5s , 5p , 4d , 6s , 6p , 5d , and 7s ), have been investigated using an ab initio approach based on non-empirical pseudo potentials for the Li and Rb cores and parametrized l -dependent polarization potential. We have determined the adiabatic potential energy curves and their spectroscopic constants for many electronic states of 2 Σ+ , 2 Π, and 2 Δ symmetries. A satisfying agreement, for the spectroscopic constants, has been obtained for the ground and the first excited states with the available theoretical works. Potential energy curves were presented, for the first time, for the higher excited states. In addition, we have localised and analysed the avoided crossings between electronic states of 2 Σ+ and 2 Π symmetries. Their existences can be related to the interaction between the potential energy curves and to the charge transfer process between the two ionic systems Li+ Rb and LiRb+ . Moreover, we have determined the transition dipole moments from X2 Σ+ and 22 Σ+ states to higher excited states of 2 Σ+ and 2 Π symmetries. For our best knowledge, no experimental data on the LiRb+ molecular ion is available. These theoretical data can help experimentalists to optimize photoassociative formation of ultracold LiRb+ molecular ion and their longevity in a trap or in an optical lattice. © 2011 Wiley Periodicals, Inc. Int J Quantum Chem, 2011
Basis set dependence of phosphate frequencies in density functional theory calculations
Abstract The addition of extravalence, polarization and diffuse functions, were studied in order to conclude how they affect the P
Receptor binding affinity based comparative QSAR study of testosterone derivatives
Abstract The quantitative structure activity relationship models of 22 testosterone derivatives have been made with the help of topological and quantum chemical parameters. The molecular modeling and geometry optimization have been carried out with CAChe Pro software. The calculations of topological and quantum chemical parameters have been done by MOPAC 2007. The statistical parameters are calculated by STATISTICA and SSP software. The study indicates that the topological parameters better predict the receptor binding affinity of testosterone derivatives, whereas quantum chemical parameters better predict androgenic potency of testosterone derivatives as indicated by correlation coefficient, standard error, standard error of estimation, p value, t value, and degree of freedom of the quantitative structure activity relationship (QSAR) models. The predicted activity values obtained by these QSAR models are close to observed activity. © 2011 Wiley Periodicals, Inc. Int J Quantum Chem, 2011
Novel physically adapted STO##-3G basis sets. Efficiency for prediction of second-order electric and magnetic properties of aromatic hydrocarbons
Abstract Efficient scheme for construction of physically justified STO## -3Gel and STO## -3Gmag basis sets has been proposed. It is based upon the analysis of analytical form of the first-order correction functions to unperturbed STO basis sets under the perturbation by electric or magnetic fields. The test calculations of polarizability, magnetic susceptibility and chemical shifts performed for a series of aromatic compounds within the developed basis set in the framework of Hartree-Fock and Density Functional Theory (DFT) approaches show good agreement of the predicted properties with experiments. © 2011 Wiley Periodicals, Inc. Int J Quantum Chem, 2011
The Rayleigh-Schrödinger perturbation series of quasi-degenerate systems
Abstract We describe the first representation of the general term of the Rayleigh-Schrödinger series for quasidegenerate systems. Each term of the series is represented by a tree and there is a straightforward relation between the tree and the analytical expression of the corresponding term. The combinatorial and graphical techniques used in the proof of the series expansion allow us to derive various resummation formulas of the series. A relation with several combinatorial objects used for special cases (degenerate or non-degenerate systems) is established. © 2011 Wiley Periodicals, Inc. Int J Quantum Chem, 2011
The effect of the functional, basis set, and solvent in the simulation of the geometry and spectroscopic properties of VIVO2+ complexes. chemical and biological applications
Abstract The geometry of 32 VIV O2+ complexes with different donor set, electric charge, geometry, arrangement of the ligands with respect to the Vd (Δd = d calcd − d exptl ) and absolute deviation |Δd | (|Δd | = |d calcd − d exptl |) from the experimental values and of the corresponding standard deviations (SD(Δd ) and SD(|Δd |)), is: B3P86 ∼ PBE0 ∼ MPW1PW91 > B3PW91 ≫ TPSS > B3LYP ≫ HCTH. In the gas phase the prediction of V51 V and 14 N hyperfine coupling constants and 14 N nuclear quadrupolar coupling constant). In most of the cases, the structures optimized in solution closely approach the experimental ones and this can be of great help in the simulations of naturally occurring vanadium compounds and metal site of V-proteins, like amavadin and the reduced form of vanadium bromoperoxidase (VBrPO). © 2011 Wiley Periodicals, Inc. Int J Quantum Chem, 2011
Direct (through-space) and indirect (through-bridge) components of the chemical bond multiplicities
Abstract It is argued that the chemical bond concept embodies all types of dependencies between general basis functions, called “atomic orbitals” (AO), which are used to construct the bonding subspace of the occupied molecular orbitals (MOs), which in the single-determinant LCAO MO type approach fully describes the bonding pattern of a molecule. The chemical interaction between the specified AO can be both of the direct and indirect origin: the former results from the explicit dependency between the interacting AO, due to their constructive interference in a molecule, and the latter has its roots in the implicit dependencies between basis functions in the molecular bonding subspace. The indirect (through-bridge) contributions complement the familiar direct (through-space) bond-orders in the resultant pattern of bond multiplicities. The implicit components, realized via AO intermediates, are examined using the “quadratic” Wiberg approach to the localized bond-orders and their information sources are investigated in the orbital communication theory of the chemical bond, in terms of the cascade communications between AO. The explicit and implicit dependencies between basis functions in the bonding subspace are linked to the relevant elements of the molecular charge-and-bond-order matrix. The conditional probabilities of the direct and bridge (cascade) probability propagations (“communications”) between AO, which determine the associated molecular information channels, are derived and used to determine the information-theoretic (IT) bond descriptors. These entropy/information indices of the bond covalency and ionicity accordingly reflect the average “noise” and the amount of the information flow in the molecular communications between AO. The multistage propagations involving full cascades of parallel AO bridges are shown to conserve the direct, stationary scattering probability between the given pair of terminal AO. This demonstrates the internal consistency of the bridge perspective on propagation of the electronic information in molecules. The associated Wiberg-type (quadratic) bond-order measures and IT bond multiplicities for these two types of interactions between AO are illustrated in the Hückel theory for selected π-electron systems (benzene, butadiene, and linear polyenes). In all these representative molecular systems, the resultant bond indices, combining the direct and indirect multiplicity contributions, are shown to generate a more balanced bonding perspective compared with the corresponding patterns resulting from the direct (Wiberg) bond-order approach alone. As illustrated for linear polyenes, the direct bonding can be realized at relatively short distances, while the indirect mechanism effectively extends the range of chemical interactions in molecular systems. The AO-resolved communications have been examined in a general basis set case, and the “transition” communication system for the “promolecule” → molecule transformation has been proposed. © 2011 Wiley Periodicals, Inc. Int J Quantum Chem, 2011
One-electron addition on short-loop selenylsulfide and diselenide-linked biomolecules: From diethyldichalcogens to Grx3-like selenopeptides
Abstract One-electron uptake by thiaselenide or diselenide linkages of a series of short-loop selenopeptides, is examined by means of hybrid QM:QM′ and QM:MM calculations. A special emphasis is given to the identification of key factors tuning adiabatic electron affinity, in comparison with established trends for disulfides. The conclusions are threefold: first, electron capture is intrinsically favored by 0.20 and 0.36 eV upon sulfur-to-selenium mono- or bi-substitution. This order of reactivity, established on diethyldichalcogens, is globally transferable to linear dipeptides, with a dramatic increase of ∼1eV of adiabatic electron affinities. Cyclization impact is then quantified along a representative set of short-loop peptides: conformational strain tends to reinforce dichalcogen propensity to act as an “electron buffer,” although this contribution is attenuated for seleno-containing linkages because of their longer equilibrium distance. Collective electrostatic effects, such as the dipole created by a N-terminal α-helix, result in the same shifting, e.g., ∼1eV as a dodecalanine is grafted onto a given tetrapeptide. Structural features for neutral and radical anion species are also analyzed, in line with their spectroscopic aftermaths. This analysis delineates some useful trends for assisting design of seleno agents and artificial enzymes. © 2011 Wiley Periodicals, Inc. Int J Quantum Chem, 2011
Thermodynamic and kinetic analysis on the reaction of dimethyl sulfide radical with oxygen
Abstract Thermochemical properties of methylthiol-methylhydroperoxide, CH3 SCH2 OOH and its two terminal radicals resulting from the loss of hydrogen atoms, viz. CH3 SCH2 OO· and C·H2 SCH2 OOH, are important to understand the stability, reaction paths, and kinetics of the reactions of dimethyl sulfide and related sulfur-containing hydrocarbons in the atmosphere and the combustion processes. Thermochemical properties ΔH o f , S (T ) and C p (T ) and internal rotation potentials for reactants, intermediates, and transition states in the methylthiomethyl (CH3 SC·H2 ) radical + O2 reaction system are analyzed to evaluate reaction paths and kinetics versus temperature and pressure. Isodesmic reaction analysis is used to determine the enthalpies of formation (ΔH o f298 ) using density functional B3LYP/6-311G(d,p) and complete basis set extrapolation CBS-QB3 computational methods. Entropy and heat capacities C p (T ) are determined using geometric parameters and vibration frequencies obtained at B3LYP/6-311G(d,p) level of calculation. ΔH f o 298 values estimated at the CBS-QB3 level (units: kcal mol−1 ) are CH3 SC·H2 (32.7), CH3 SCH2 OOH (–29.1), CH3 SCH2 OO· (5.2), C·H2 SCH2 OOH (11.5). Quantum Rice–Ramsperger–Kassel analysis is used to calculate energy-dependent rate constants, k (E ), and master equation is used to account for collisional stabilization of peroxy adduct and the hydroperoxide isomer. The methylthiomethyl radical associates with O2 to form a methylthiomethyl peroxy radical with a well depth of 27.49 kcal mol−1 . The peroxy radical can dissociate back to reactants, isomerize via hydrogen shift to form a hydroperoxide methyl radical C·H2 SCH2 OOH, decompose via hydrogen transfer, and undergo subsequent low-energy beta scission to form CH3 SC(3 S(2 O product set. The C·H2 SCH2 OOH isomer can decompose via an elimination transition state to CH2
Toward anharmonic computations of vibrational spectra for large molecular systems
Abstract The subtle interplay of several different effects makes the interpretation and analysis of experimental spectra in terms of structural and dynamic characteristics a very challenging task. In this context, theoretical studies can be very helpful, and this is the reason behind the rapid evolution of computational spectroscopy from a highly specialized research field toward a versatile and widespread tool. However, in the case of vibrational spectra of large molecular systems, the most popular approach still relies on a harmonic treatment, because of the difficulty to explore the multidimensional anharmonic potential energy surface. These can be overcome considering that, in many cases, the vibrational transitions are well localized and only some of them are observed experimentally. To this aim, the procedure for the simulation of vibrational spectra of large molecular systems beyond the harmonic approximation is discussed. The quality of system-specific reduced dimensional anharmonic approaches is first validated by comparison with computations taking into account all modes simultaneously for anisole and glycine. Next, the approach is applied to two larger systems, namely glycine adsorbed on a silicon surface and chlorophyll-a in solution, and the results are compared with experimental data showing significant improvement over the standard harmonic approximation. Our results show that properly tailored reduced dimension anharmonic approaches stand as feasible routes for state-of-the-art computational spectroscopy studies and allow to take into account both anharmonic and environmental effects on the spectra even for relatively large molecular systems. © 2011 Wiley Periodicals, Inc. Int J Quantum Chem, 2011
Coupled cluster and DFT calculations of 14N nuclear quadrupole coupling constants
Abstract The dependence of 14 N quadrupole coupling constants calculated using coupled cluster theory on the level of approximation is examined for a series of small molecules. For HCN, HNC, CH3 CN, and CH3 NC, we use the coupled cluster singles-and-doubles with a noniterative perturbative triples correction—CCSD(T)—approach, and we analyze the basis set dependence of the results. For aziridine, diazirine, and cyclopropyl cyanide, we use the CCSD(T) approach, but smaller basis sets, and for the largest studied molecules—quinuclidine and hexamine—we present CCSD results. The differences between computed and experimental values for the best basis sets used are ≈ 5% at the CCSD level and decrease noticeably at the CCSD(T) level. The - N≡C bonds are an exception—in this case the quadrupole coupling constants are very small, hence the differences between theory and experiment become larger (up to 9%). We also consider the performance of density functional theory, comparing the results for different density functionals with the coupled cluster values of the same constants. Most of the functionals provide results systematically improved with respect to the Hartree–Fock values, with 14 N coupling constants in - N≡C bonds being again an exception. © 2011 Wiley Periodicals, Inc. Int J Quantum Chem, 2011
Theoretical study of disubstituted pyrrolopyrimidines as focal adhesion kinase inhibitors
Abstract An in silico molecular modeling study of selected 7H-pyrrolo[2,3-d]pyrimidines with FAK inhibitory activities was performed. Rigid docking of each inhibitor at the FAK catalytic site was employed to obtain the most appropriate starting structures, followed by molecular mechanics-based energy minimizations associated with molecular dynamics at the FAK binding site using the AMBER force field. Theoretical values of interaction energies obtained from the geometry optimization calculations for the protein-inhibitor complexes were compared with published IC50 values for FAK and showed a reasonable correlation. Based on these results and in view of the geometry of the most potent inhibitors, two new molecular structures were designed as possible FAK inhibitors and submitted to the same theoretical procedures. © 2011 Wiley Periodicals, Inc. Int J Quantum Chem, 2011
Theoretical calculation of the pKa values of some drugs in aqueous solution
Abstract In this work, calculations of pK a values have been performed on benzoic acid and its para-substituted derivatives and some drugs by using Gaussian 98 software package. Gas-phase energies were calculated with HF/6-31 G** and B3LYP/6-31 G** levels of theory. Free energies of solvation have been computed using the polarizable continuum model (PCM), conductor-like PCM (CPCM), and the integral equation formalism-PCM at the same levels which have been used for geometry determination in the gas-phase. The results that show the calculated pK a values using the B3LYP are better than those using the corresponding HF. In comparison to the other models, the results obtained indicate that the PCM model is a suitable solvation model for calculating pK a values. For the investigated compounds, a good agreement between the experimental and the calculated pK a values was also observed. © 2011 Wiley Periodicals, Inc. Int J Quantum Chem, 2011
Computational study of hexanal peroxy radical–water complexes
Abstract The results of an ab initio study on a family of hydroxy peroxy radical–water complexes formed from the oxidation of E-2-hexenal, which constitutes an important component of biogenic atmospheric emissions, are reported. Binding energies for the β-hydroxy-γ-peroxy hexanal (β- and γ-positions are relative to the carbonyl) radical–water complex and the γ-hydroxy-β-peroxy hexanal radical–water complex are predicted to be to 3.8 and 3.6 kcal/mol, respectively, computed at the MP2/6-311++G(2d,2p)//B3LYP/6-311++G(2d,2p) computational level. Natural bond orbital reveals that conventional hydrogen bonding between the water and the hydroxy and aldehyde functional groups of the radical are primarily responsible for the stability of the complex. It can be shown that the peroxy moiety contributes very little to the stability of the radical–water complexes. Thermochemistry calculations reveal estimated equilibrium constants that are comparable to those recently reported for several hydroxy isoprene radical–water complexes. The results of this report suggest that the hexanal peroxy radical–water complexes are expected to play a significant role in the complex chemistry of the atmosphere. © 2011 Wiley Periodicals, Inc. Int J Quantum Chem, 2011
Real wave packet and flux analysis studies of the H + F2 → HF + F reaction
Abstract The H + F2 → HF + F reaction on ground state potential energy surface is investigated using the quantum mechanical real wave packet and Flux analysis method based on centrifugal sudden approximation. The initial state selected reaction probabilities for total angular momentum J = 0 have been calculated by both methods while the probabilities for J > 0 have been calculated by Flux analysis method. The initial state selected reaction probabilities, integral cross sections and rate coefficients have been calculated for a broad range of collision energy. The results show a large rotational enhancement of the reaction probability. Some resonances were seen in the state-to-state reaction probabilities while state-to-all reaction probabilities and the reaction cross section do not manifest any oscillations and the initial state selected reaction rate constants are sensitive to the temperature. © 2011 Wiley Periodicals, Inc. Int J Quantum Chem, 2011
Density functional computational studies on 2-[(2,4-Dimethylphenyl)iminomethyl]-3,5-dimethoxyphenol
Abstract Density functional calculations of the structure, molecular electrostatic potential, and thermodynamic functions have been performed at B3LYP/6-31G(d) level of theory for the title compound of 2-[(2,4-dimethylphenyl)iminomethyl]-3,5-dimethoxyphenol (I ). To investigate the tautomeric stability, optimization calculations at B3LYP/6-31G(d) level were performed for the enol and keto forms of I . Calculated results reveal that the enol form of I is more stable than its keto form. The predicted nonlinear optical properties of I are much greater than ones of urea. The changes of thermodynamic properties for the formation of the title compound with the temperature ranging from 200 to 500 K have been obtained using the statistical thermodynamic method. At 298.15 K, the change of Gibbs free energy for the formation reaction of I is 32.973 kJ/mol. The title compound can not be spontaneously produced from the isolated monomers at room temperature. The tautomeric equilibrium constant is computed as 0.868 at 298.15 K for enol-imine↔keto-amine tautomerization of I . In addition, natural bond orbital analysis of I was performed using the B3LYP/6-31G(d) method. © 2011 Wiley Periodicals, Inc. Int J Quantum Chem, 2011
Electronic structure, dielectric properties and infrared vibrational spectrum of fayalite: An ab initio simulation with an all-electron Gaussian basis set and the B3LYP functional
Abstract The electronic structure, the static and high frequency dielectric tensors, and the infrared (IR) spectrum of fayalite Fe2 SiO4 , the Fe-rich end-member of olivine solid solutions, are explored at an ab initio quantum mechanical level, by using an all-electron Gaussian type basis set, the B3LYP hybrid DFT functional, and the CRYSTAL09 code. Mulliken population analysis and spin density maps illustrate the electronic structure, characterized by a nearly pure d6 , high-spin configuration of the transition metal atom. The full set of IR wavenumbers and intensities is computed. The availability of highly accurate synchrotron radiation data (Suto et al., Astron Astrophys 2002, 389, 568) permits a very accurate comparison between simulated and measured quantities, in primis wavenumbers (ν) and oscillator strengths (f ). The mean absolute difference
−1 , and the maximum absolute difference |Δνmax | never exceeds 12 cm−1 , whereas the summed absolute difference ΔF between f exp and f calc is around 10%. Modes not detected in the experiment turn out to be (i) characterized by low computed intensity, or (ii) very close to a large intense peak. Computed and experimental IR reflectance curves are in striking agreement also. The nature of the vibrational modes is investigated by means of isotopic substitutions, which clarify the participation of the various atomic species to each mode. © 2011 Wiley Periodicals, Inc. Int J Quantum Chem, 2011
Structural and binding properties of metal ion chelators relevant to Alzheimer's disease. A theoretical investigation
Abstract The Copper (II) and Zinc (II) affinities for clioquinol and N,N'-bis[(5β-D-glucopyranosyloxy-2-hydroxy)benzyl]-N,N'-dimethyl-ethane-1,2-diamine ligands, recently proposed as good chelators in the therapy of Alzheimer's disease, have been studied theoretically by employing density functional theory. Results show that both ligands have a higher affinity for the copper dication and indicate that the copper complex assumes a near planar coordination geometry while the zinc prefers tetrahedral topologies. Furthermore, from the computed O
A visual representation for RNA secondary structure and its application
Abstract The graphical representation of biological sequences is an important subject in the area of genome studies. We propose a novel visual representation for RNA secondary structures. Some symmetric properties and information on the base distribution and compositions can be intuitively reflected by the projection graphs of the points corresponding to the RNA secondary structures. Then our method is applied to compute the similarity of 12 classical samples and 11 real RNA secondary structures. The results indicate that our method can not only effectively analyze the similarity between RNA secondary structures but also show a high consistency with other literatures. Moreover, our method only needs the geometrical center of the characteristic curve of the RNA secondary structure to compute the similarity matrix, which means a low computational complexity. © 2011 Wiley Periodicals, Inc. Int J Quantum Chem, 2011
NH and NCl homolytic bond dissociation energies and radical stabilization energies: An assessment of theoretical procedures through comparison with benchmark-quality W2w data
Abstract The performance of a large variety of contemporary density functional theory (DFT), double-hybrid DFT, and high-level Gaussian-n (Gn ) procedures has been evaluated for the calculation of bond dissociation energies (BDEs) and radical stabilization energies (RSEs) associated with Nn schemes, the modified G4 procedures (G4-5H and G4(MP2)-6X) perform somewhat better than the corresponding standard G4 procedures for the N–1 . However, for the N–1 . The BDEs of both N−1 for both bond types. Nearly all of the considered DFT procedures perform significantly better for the computation of RSEs, due to a significantly larger degree of error cancelation compared with the BDEs. For the RSEs, BH&HLYP, M05-2X, M06, M06-2X, BMK, PBE0, B2-PLYP, B2GP-PLYP, B2T-PLYP, and ROB2-PLYP are the best performers, with MADs ≤ 4.2 kJ mol−1 . Reliable values of N2 N2 N
Three-unit semicircles curve: A compact 3D graphical representation of DNA sequences based on classifications of nucleotides
Abstract A new three-dimensional graphical representation of DNA sequences, three-unit semicircles (TUS)-curve, which maps a given sequence into a dot sequences embedded in three-unit semicircles, is proposed based on three biclassifications of nucleotides. TUS-curve has the merit of compactness and could avoid the degeneracy and loss of information. The geometrical center of the curve, which indicates the distribution of base frequencies of the corresponding DNA sequence, is extracted and applied to analyze the similarity of various species. Phylogenetic tree of 11 species based on their first exons of β-globin genes showed that the TUS-curve is a powerful tool to get valuable biological information. © 2011 Wiley Periodicals, Inc. Int J Quantum Chem, 2011
Modeling of properties of amino acids with random, semirandom, and molecular connectivity descriptors
Abstract Sets of random indices, sets of molecular connectivity indices (MCIs), and mixed sets of random and MCIs have been used to model nine properties of amino acids. The number of amino acids throughout the concerned properties is not constant, it ranges from 21 for the isoelectric point to eight for the unfrozen water content. The quality of the descriptors has been submitted to the leave-one-out (loo) and to the training/evaluation statistical tests. Molecular connectivity descriptors made of MCIs only show exceptionally good model characteristics for the volume side-chain throughout both tests, while the model of the solubility holds only at the loo level. Random descriptors made of random indices, and semirandom descriptors, made of random and MCIs show good descriptive characteristics of all other properties at the loo level, while they fail at the training/evaluation level. Molecular connectivity descriptors achieve a good model at the training/evaluation level for the hydration potential and for the specific rotation. Practically, at the loo level all properties, excluding the unfrozen water content, can satisfactorily be modeled, while at the training/evaluation level, only side-chain volume, hydration potential, and specific rotation can be modeled in a satisfactory way. To notice is the importance, at both statistical levels, of the hydrogen content of the amino acids, which is encoded with a perturbation parameter at the level of the valence delta number of the valence MCIs. © 2011 Wiley Periodicals, Inc. Int J Quantum Chem, 2011
Theoretical study on HO2-initiated atmospheric oxidation of halogenated carbonyls
Abstract The initial reactions of HO2 with oxalyl dichloride, oxalyl dibromide, acetyl chloride, and acetyl bromide are studied for the first time using the density functional theory with CAM-B3LYP and wB97XD, CBS-QB3, and G3 theoretical methods and the transition state theory. The reactions occur via the similar mechanism that the prereactive complex is formed before the transition state and product. The calculated results demonstrate that the HO2 radical with ClC(O)C(O)Cl and BrC(O)C(O)Br reactions is feasible and could play a significant role in the atmosphere because the barriers are 0.13 kcal/mol, −0.05 kcal/mol with respect to the free reactants, respectively at the CBS-QB3 level of theory. In addition, the rate constants of the HO2 + ClC(O)C(O)Cl and HO2 + BrC(O)C(O)Br reactions are computed to be 1.37 × 10−15 cm3 mol−1 s−1 , 1.70 × 10−15 cm3 mol−1 s−1 at 298 K. However, the HO2 reactions with acetyl chloride and acetyl bromide are of no importance because of higher activated barrier and slower rate constant. © 2011 Wiley Periodicals, Inc. Int J Quantum Chem, 2011
Ab initio effective rotational Hamiltonians: A comparative study
Abstract Two independent methods to obtain ab initio effective rotational Hamiltonians have been implemented recently. The first one is based on a generalization of perturbation theory to noncommutative rings, the other one on contact transformation (CT) techniques. In principle, both methods are able to give rotational Hamiltonians including centrifugal distortion effects of arbitrary high orders. These methods are compared, for the first time, in this article with regard to calculations of the rotational levels of methane vibrational ground state. © 2011 Wiley Periodicals, Inc. Int J Quantum Chem, 2011
Microscopic theory for insulator to metal transition in cuprates
Abstract “Hubbard U” is equal to the energy of the insulator to metal transition in Mott insulators. Hubbard U is, therefore, also the excitation energy for metal atom to other metal atom transitions and the key concept in photoinduced conductivity spectra of pure and doped cuprates. In this article, the electronic structure of Cu(II) and Cu(III) sites is first discussed. It is shown that Hubbard U depends on the location of the excitation in the CuO2 plane relative to the Cu(III) sites. Far from the Cu(III) sites, the ground-state wave function continues to be of spin-coupled type. Near to a Cu(III) site, it changes character and is mixed with charge components. The Hubbard gap thus depends on locality in the CuO2 plane. Close to a Cu(III) site, it tends to zero and induces local conductivity. Far from a Cu(III) site, it is large but converges to zero as the doping levels are raised. In fact, the Hubbard gap has many features in common with the pseudogap. © 2011 Wiley Periodicals, Inc. Int J Quantum Chem, 2011
Structure and reactivity of baicalein radical cation
Abstract Neutral baicalein, corresponding radical cation, and three possible radicals obtained by proton removal from the radical cation were investigated using density functional theory. The structure and UV spectrum of baicalein were very well reproduced by the B3LYP/6-311+G(2df,p) level of theory. The results showed that the loss of an electron from baicalein molecule induced the transfer of H4 to O5. The reasons for this rearrangement were pointed out. It was found that delocalization of spin density is most pronounced in the thermodynamically most stable C6
Quantifying the performance of conventional DFT methods on a class of difficult problems: The interaction (hyper)polarizability of two water molecules as a test case
Abstract We have investigated the predictive capability of a widely used group of density functional theory (DFT) methods on the interaction (hyper)polarizability of the two water molecules in the water dimer. We find that compared with conventional ab initio methods, DFT gives a different picture of the dipole moment and (hyper) polarizability for the water monomer and dimer. In addition, the DFT methods tested in this work predict for the water dimer a systematically higher value of the differential interaction-induced second hyperpolarizability per water molecule, DHPM = [
2 O)2 ]/2 −
2 O) than the ab initio ones. The interaction-induced mean polarizability and second hyperpolarizability is predicted to be small and positive for all DFT methods used here. The respective quantities predicted by conventional post-Hartree–Fock ab initio methods are also quite small but negative. © 2011 Wiley Periodicals, Inc. Int J Quantum Chem, 2011
The Importance of the DFT method on the computation of the second hyperpolarizability of semiconductor clusters of increasing size: A critical analysis on prolate aluminum phosphide clusters
Abstract The importance of the density functional theory (DFT) methods on the computation of cluster hyperpolarizabilities is discussed. The performance of the conventional BLYP, BP86, BPW91, B3LYP, B3PW91, and B3P86 functionals in the computation of the second hyperpolarizability of aluminum phosphide prolate clusters up to 60 atoms is compared with the “half and half functionals” BHandH and BHandHLYP and to the long-range corrected functionals LC-(BLYP, BP86, BPW91), CAM-B3LYP, and wB97XD. The presented results demonstrate that when long-range corrections are incorporated on pure and hybrid functionals their performance is vastly affected. What is more, the obtained DFT results are compared with second-order Møller–Plesset perturbation theory (MP2) all electron calculations. It is shown that all the long-range outcomes are bracketed by the MP2 and Hatree–Fock (HF) values. The relative ordering of the obtained longitudinal hyperpolarizabilities follows strictly the trend MP2 > CAM-B3LYP > wB97XD > LC-(BLYP, BP86, BPW91) > HF. © 2011 Wiley Periodicals, Inc. Int J Quantum Chem, 2011
Vibrational contribution to static and dynamic (Hyper)polarizabilities of zigzag BN nanotubes calculated by the finite field nuclear relaxation method
Abstract The vibrational contribution to static and dynamic (hyper)polarizabilities for the zigzag (n ,0) family of BN nanotubes, with n ranging from (6,0) to (36,0), has been obtained. Calculations were done by the finite field nuclear relaxation (FF-NR) method for periodic systems, newly implemented in the CRYSTAL code, using the Coupled Perturbed Kohn-Sham (CPKS) scheme at the B3LYP/6-31G* level for the required electronic properties. Both transverse and transverse-longitudinal tensor components are determined by applying finite, i.e. static, fields in the transverse direction. The magnitude of the vibrational term increases with the radius of the nanotube as determined by the increase in the field-induced geometric deformation. The resulting vibrational (hyper)polarizability varies from being dominant to negligible, when compared with the corresponding static electronic contribution. This depends upon the radius, as well as the property and the component, in a systematic manner. The extension to longitudinal components, not yet available, will be implemented next. © 2011 Wiley Periodicals, Inc. Int J Quantum Chem, 2011
Pyrrolidine-based dye-sensitized solar cells: A time-dependent density functional theory investigation of the excited state electronic properties
Abstract This work reports a theoretical study of the excited state properties of a series of original conjugated metal-free organic dyes containing the pyrrolidine (PYR) moiety. These compounds have recently been developed for dye-sensitized solar cells (DSSCs). Our polarizable continuum model time-dependent density functional theory-based procedure made it possible to efficiently and accurately evaluate (i) the vertical excitation energies, (ii) the related redox potentials, and (iii) the free enthalpies of injection. It turns out that the Becke-Half-and-Half-Lee-Yang-Parr functional, combined to the 6-311+G(2d,2p) basis set, gives reliable auxochromic shifts when the bulk solvation effects are included in the model. Indeed, the theoretical procedure provides excitation energies with a mean absolute deviation limited to ∼0.10 eV only. In addition, we give insights into the geometrical and electronic structures of the dyes, and we unravel the structural modifications that optimize the properties of PYR-based DSSCs. This investigation aims at improving the electron injection process, as well as the light harvesting efficiency (LHE) of the dyes. For this purpose, we considered a set of 17 new dyes, and starting from the basic five-block [PYR]-[phenyl]-[ethylene]-[thiophene]-[cyano acrylic acid] system (PYR-m′ structure), several modifications leading to better electron injection and comparable LHE properties have been proposed. © 2011 Wiley Periodicals, Inc. Int J Quantum Chem, 2011
Theoretical study of the polymerization of p-tert-butyl-anisol
Abstract The stereoselective formation of dimers and trimers of p -tert -butyl-anisol by electrochemical oxidation has been theoretically studied. For the purpose, density functional theory calculations with the PBE exchange-correlation functional have been performed with a triple zeta plus polarization basis set. A mechanism has been proposed by validating one suggestion of the experimentalists, whereas other reaction pathways suggested by experimentalists have been ruled out. © 2011 Wiley Periodicals, Inc. Int J Quantum Chem 000: 000–000, 2011
Computational study of the catalytic effect of platinum on the decomposition of DNT
Abstract The catalytic decomposition of dinitrotoluene (DNT; 3-4-DNT), a by-product of the explosive trinitrotoluene (trotyl), on a platinum surface is investigated computationally. Reaction paths have been computed for a DNT molecule interacting with a Pt-cluster under varying temperatures using quantum-chemical density functional theory. Two possible initiation steps where DNT split either into nitro-tolyl and NO2 , or in nitro-tolyl-oxidanyl and NO, are considered. The energy barrier for the catalytic process is found to decrease significantly for the Pt catalyzed reaction compared with the uncatalyzed reaction. © 2011 Wiley Periodicals, Inc. Int J Quantum Chem 000: 000–000, 2011
Analytical potential energy surface for the reaction with intermediate complexes NH3 + Cl → NH2 + HCl: Application to the kinetics study
Abstract We present for the first time an analytical potential energy surface (PES) for the reaction of hydrogen abstraction from ammonia by a chlorine atom. It has a very complicated shape with various maxima and minima. The functional form used in the development of the PES considered the stretching and bending nuclear motions, and the parameters in the calibration process were fitted to reproduce exclusively high-level ab initio electronic structure calculations obtained at the CCSD(T) = FULL/aug-cc-pVTZ//CCSD(T) = FC/cc-pVTZ single point level. Thus, the surface is completely symmetric with respect to the permutation of the three ammonia hydrogen atoms, and no experimental information is used in the process. The ab initio information used in the fit includes a wide spectrum of properties (equilibrium geometries, relative energies, and vibrational frequencies) of the reactants, products, saddle point, intermediate complexes in the entry and exit channels, points on the reaction path, and points on the reaction swath. By comparison with the reference results, we show that the resulting PES reproduces not only the ab initio data used in the fitting procedure but also other thermochemical and kinetics results computed at the same ab initio level, which were not used in the fit—equilibrium constants, rate constants, and kinetic isotope effects. This represents a severe test for the new surface. As a first application, we perform an extensive kinetics study using variational transition-state theory with semiclassical transmission coefficients over a wide temperature range, 200–2000 K, on this analytical PES. The forward rate constants reproduce the sparse experimental measurements, while the reverse ones reproduce the change of activation energy with temperature reported in another theoretical study, although unfortunately there are no experimental data for comparison. Finally, we analyze the influence of the intermediate complexes and the spin–orbit correction on the kinetics results. In summary, these results indicate that the PES adequately describes this reaction, and the reasonable agreement with experiment lends further confidence to this new surface. © 2011 Wiley Periodicals, Inc. Int J Quantum Chem, 2011
Infrared spectra of charge-solvated versus salt-bridge conformations of glycine-, serine-, and cysteine-Ca2+ complexes
Abstract The structure, relative stability, and anharmonic vibrational frequencies of the most stable complexes between glycine, serine, and cysteine with Ca2+ have been calculated by means of DFT approaches. The global minimum of the potential energy surface for glycine-Ca2+ complex corresponds to the salt-bridge (SB) form, whereas for serine- and cysteine-Ca2+ complexes is a charge-solvated (CS) structure in which the metal dication is bound to the carbonyl group of the acidic function, the amino group and the OH (SH) group of the alcohol (enethiol) function. The energy gap between the CS global minimum and the SB form decreases significantly on going from serine to cysteine. Hence, for the latter both CS and SB forms could coexist in the gas phase mainly at high temperatures. Anharmonicity effects are lower than 10%, and do not affect significantly the assignment of the fundamental vibrational modes. The calculated infrared spectra of the SB and CS forms of glycine-, serine-, and cysteine-Ca2+ complexes show very distinctive characteristics, which should permit to unambiguously characterize them by IR multiphoton dissociation (IRMPD) techniques. © 2011 Wiley Periodicals, Inc. Int J Quantum Chem, 2011
Quantum chemical modeling studies of ethylbenzene dehydrogenase activity
Abstract Quantum chemical modeling is used to predict the reactivity of ethylbenzene dehydrogenase (EBDH), a molybdenum enzyme that catalyzes the stereoselective oxidation of alkylaromatic and alkylheterocyclic hydrocarbons to the (S )-enantiomers of secondary alcohols. The reaction mechanism is studied for four different substrates: ethylbenzene, 4-ethylphenol, allylbenzene, and 4-ethylpyridine with a cluster model of the active site. The modeling predicts radical C
Product channels in the reaction of the CH3SO radical with NO2: DFT and ab initio studies
Abstract The stationary points involved in the CH3 SO + NO2 radical-radical reaction were examined at the B3LYP/6-311++G(2df,2pd) and CCSD/cc-pVDZ levels of theory. Singlet potential energy surface was calculated using the CCSD(T)/aug-cc-pVTZ//CCSD/CC-pVDZ single-point calculations and the CBS-QB3 composite method. The association between radicals is confirmed to be a barrierless process and resulted in the two low-energy intermediates, CH3 S(O)NO2 and CH3 S(O)ONO. The CH3 S(O)ONO intermediate decomposes directly to the CH3 SO2 + NO products with the N−1 . Although the dissociation of the C3 SO2 requires an energy of 16.3 kcal mol−1 , the formation of final CH3 + SO2 + NO products is very likely, because their energy level is by 3.4 kcal mol−1 still lower relative to that of the reactants. The formation of the CH2 SO + HONO products from CH3 S(O)ONO can proceed through the three-center transition state, but it is not important due to significant barrier well above the energy level of reactants. Eventually, CH2 SO + HONO may be generated in the direct H-abstraction from CH3 SO radical by NO2 . © 2011 Wiley Periodicals, Inc. Int J Quantum Chem, 2011
Quantum theoretical study of mechanism of the reaction between guanine radical cation and carbonate radical anion: Formation of 8-oxoguanine
Abstract The reaction between guanine radical cation (G+· ) and carbonate radical anion (CO3 −·
Promising anchoring groups for ZnO-based hybrid materials: A periodic density functional theory investigation
Abstract We, herein, explore the potential use of new anchoring groups for the development of hybrid organic–inorganic materials and more specifically for the chemisorption of dyes in dye-sensitized solar cells. The structural and the electronic properties of four different compounds (the 1,2-benzenediole-, catechol-, the pentane-2,4-dione (acetylacetone), and the corresponding thio derivatives) adsorbed on a {100} clean ZnO surface were investigated by the means of density functional theory in a periodic framework. Subsequent—harmonic—infrared (IR) spectral calculations of the adsorbed and isolated systems pointed out that the adsorption process may be followed by IR techniques. From our analysis, all anchoring groups seem to be suitable as anchoring groups in hybrid devices both from a structural and electronic point of view, although additional requirements may be important for specific applications. © 2011 Wiley Periodicals, Inc. Int J Quantum Chem, 2011
Isomerization of dehydrofulvene radicals to the phenyl radical, and application to the growth of polycyclic aromatic hydrocarbons
Abstract In this work, the isomerization mechanisms of dehydrofulvene radicals to the phenyl radical are described. This study shows, for 6-dehydrofulvene, a low-energy isomerization path going through a resonantly stabilized bicyclic system. In a second part, the different established mechanisms are applied to the closing of a second and third aromatic ring. This application highlights the possibility of forming new products, such as 2-naphthyl radical, and the effect of aromatic cycles on the new mechanisms. © 2011 Wiley Periodicals, Inc. Int J Quantum Chem, 2011
A theoretical study of the electronic structure of GaN nanorods
Abstract We present a theoretical study of the evolution of the electronic structure of wurtzite GaN nanorods for different lengths (2.4–15.4 nm) in the [0001] direction and different diameters (0.97–2.25 nm). This study includes both a hybrid density functional theory study and a comparison with the k.p empirical band structure method. From the quantum chemical calculations, surface effects are found to be important. When these have been compensated for the electronic structure properties as a function of rod length or diameter approximately follow the trend expected from the quantum confinement effect. The k.p method predicts a similar behavior although deviations are apparent for smaller sizes. © 2011 Wiley Periodicals, Inc. Int J Quantum Chem, 2011
Molecular dynamics study of aggregation of amyloidogenic peptides in explicit water
Abstract The presence of amyloid fibrils is associated with a number of degenerative and serious human diseases. Nonetheless, the process of fibrillogenesis is still poorly understood. In this study, we present the results of a molecular dynamics investigation into the self-aggregation of the amyloidogenic peptide FLVHSS, a six-residue peptide belonging to the sequence of human islet amyloid polypeptide (hIAPP), capable of self-association to form amyloid-like fibrils. The simulations were performed at T = 300 K, using multiple replicas of FLVHSS monomer with random initial spatial distribution and orientation, in explicit water, set in a confined space volume (simulation box). Different simulations were carried out by varying initial atomic coordinates or initial atomic velocities and simulation box size. The formation of a unique aggregate (cluster) was observed within few nanoseconds from the beginning of all simulations. Structural analysis of the clusters clearly showed an elongated scalene ellipsoidal shape and a preferential parallel alignment of the peptides, roughly perpendicular to the main axis of the cluster and in extended conformation. The spatial arrangement of peptides in the simulated clusters appears to be similar to the spatial disposition of molecules in amyloid fibrils. An elongated scalene ellipsoidal shape was observed also in the final aggregate of the cluster growth simulation we also reported herein, indicating a tendency of cluster to grow along its major geometrical axis. © 2011 Wiley Periodicals, Inc. Int J Quantum Chem, 2011
Bond order uniform grids for quantum reactive scattering
Abstract The adoption of evenly spaced grids in the so-called bond order coordinates for the calculation of diatomic eigenfunctions in reactive scattering problems is investigated. These grids (used here for the first time in reactive dynamics) sample the accessible space better than traditional bond length coordinates. A comparison between the bond order and the bond length method on the test H + H2 system using an asymptotically Morse-like potential singles out the advantages of adopting the former when a limited number of grid points is chosen. © 2011 Wiley Periodicals, Inc. Int J Quantum Chem, 2011
CCSD(T)-F12 investigations on HBNH and its isotopologues
Abstract CCSD(T)-F12 ab initio computations are performed to investigate the isomerization path between HBNH and BNH2 , and the six-dimensional potential energy surface of the electronic ground state X̃ 1 Σ+ of the linear isomer. The equilibrium geometries and harmonic wavenumbers obtained at the CCSD(T)-F12a/aug-cc-pVTZ level of theory are nearly those computed with the CCSD(T)/aug-cc-pV5Z level, but with a smaller amount of CPU time for the evaluation of the total energy. The analytical representation of the potential energy surface is used in variational calculations of the rovibrational energy levels of H11 BNH and isotopologues. The computed rovibrational energy levels are compared with the available experimental data, and a good agreement is generally noticed. We also propose an assignment of the rovibrational energy levels of H11 BNH up to 3000 cm−1 above the zero point energy. © 2011 Wiley Periodicals, Inc. Int J Quantum Chem, 2011
A molecular dynamics study of sodium dodecyl sulfate-methane system in water using the improved lennard jones formulation
Abstract The behavior of the sodium dodecyl sulfate (SDS)-methane system in water has been investigated using molecular dynamics calculations performed on a local cluster as well as on the EGI-grid distributed platform. To this end, the model adapted molecular polarizability centers for force-fields has been adopted to provide in a reliable and suitable analytical form the various intermolecular potentials involved. Structural properties of the system have been then investigated by varying the number of water molecules solvating the SDS and the CH4 molecules. © 2011 Wiley Periodicals, Inc. Int J Quantum Chem, 2011
Dynamic monte carlo simulations of NO decomposition on Pt(100): Temperature-programmed desorption spectra
Abstract The decomposition of absorbed NO on a Pt(100) surface is studied by using a dynamic Monte Carlo method on a square lattice at low pressure conditions. The N2 temperature-programmed desorption spectra were simulated considering the presence or absence of lateral interactions. Moreover, the effect on NO dissociation rate, the limiting step in the whole reaction, is inhibited by coadsorbed NO, N, and O molecules. The dissociation rate for NO and N2 desorption are enhanced by the presence of adsorbed atoms as nearest neighbors. In these simulations, values of experimental parameters, such as adsorption, desorption, and diffusion of the reactants, are included. The phenomenon is studied varying the temperature in the range of 300–550 K. Our simulations are positively compared with experimental spectra and calculated mean field models. © 2011 Wiley Periodicals, Inc. Int J Quantum Chem, 2011
Conformational dependence of the electronic coupling in guanine–tryptophan complexes: A DFT study
Abstract Radical cation states (electron holes) generated in DNA lead to mutagenic effects. In DNA–protein complexes, the holes can migrate from DNA to aromatic amino acid residues preventing the formation of the oxidative lesions. The efficiency of hole transfer (HT) may significantly depend on the arrangement of redox sites. This dependence is mainly determined by sensitivity of the electronic coupling of donor and acceptor to structural changes. Based on DFT calculations of a number of guanine–tryptophan (G–Trp) complexes, we explore the conformational dependence of HT electronic coupling in this dyad. Stacked and T-shaped structures are considered. The electronic coupling in the system is shown to be responsive to the mutual arrangement of G and Trp. Although in most cases the probability of HT in T-shaped conformations is predicted to be lower than in π stacks, several T-type structures are found where HT should be very efficient. © 2011 Wiley Periodicals, Inc. Int J Quantum Chem, 2011
Evaluation of DFT methods to study reactions of benzene with OH radical
Abstract Several density functional theory” (DFT) methods are applied to two different reaction channels involving OH• + C6 H6 , and the results compared with high-level ab initio calculations. The OH• adds directly to one C atom in the first channel, first forming an encounter complex with the OH• poised above the aromatic plane. B3LYP, BH&HLYP, and MPW1K compute an accurate estimate of the overall exothermicity, whereas M05-2X, PBE0, and PBEPBE overestimate this quantity to some degree. With the exceptions of PBEPBE and PBE0, the other methods produce an acceptable barrier to addition. All approaches except BH&HLYP correctly predict an exothermic H• abstraction, although PBEPBE is too exothermic. The BH&HLYP barrier to H• abstraction is too high while the MPW1K, PBE0, and B3LYP values are better, and M05-2X the best. © 2011 Wiley Periodicals, Inc. Int J Quantum Chem, 2011
DFT and Ab Initio calculations of spectroscopic properties of tetramethyltin and of its cation
Abstract DFT and ab initio calculations are performed on tetramethyltin (SnMe4 , TMSn) and its cation. A set of spectroscopic constants for both species are derived. They include equilibrium geometries, rotational constants and vibrational wavenumbers. All quantities are in close agreement with the available experimental data. For the cation, our calculations confirm the C3v charge transfer structure proposed earlier through the analysis of electron paramagnetic resonance (EPR) experimental data. Using multi reference configuration interaction and time dependant density functional theory (TD-DFT) methodologies, the vertical electronic excitation energies of TMSn and TMSn+ are determined. For the singlet-singlet neutral molecule, our calculated transition energies are distinctly lower than those previously computed. For the TMSn (
1 A1 → 11 T2 ) absorption transition, our computed excitation energy coincides, however, with the experimental value. Predictive data are also given for the TMSn triplets. At the best level of theory, the vertical and the adiabatic ionization energies of TMSn are computed 9.86 eV and 8.74 eV, respectively. © 2011 Wiley Periodicals, Inc. Int J Quantum Chem 000:000–000, 2011
Spectroscopic properties of mono- and bis-azopyrroles
Abstract We use density functional theory and time-dependent density functional theory to evaluate the structure and electronic properties of a series of recently-synthesized mono- and bi-azopyrrole dyes. The selected ab initio approach relies on hybrid functionals, large basis sets and systematic modeling of solvent effects. For simulating the absorption spectra, three functionals are benchmarked (PBE0, BMK, and CAM-B3LYP), and it turns out that these three hybrids are able to provide accurate theoretical estimates (average deviations ∼ 0.1 eV) of the longest wavelength of maximal absorption. New structures are proposed to minimize the transition energy and the impact of the relative position of donor and acceptor groups is discussed. The linear and nonlinear optical properties of the proposed dyes are investigated with a long-range corrected approach. © 2011 Wiley Periodicals, Inc. Int J Quantum Chem, 2011
Transferability of conformational dependent charges from protein simulations
Abstract We have studied the transferability of atomic charges for proteins, fitted to the quantum mechanical electrostatic potential and extensively averaged over a set of structures sampled by molecular dynamics (MD) and over all residues of the same kind in the protein sequence (xAvESP). Previously, such charges were obtained for one single protein (avidin). In this study, we use five additional proteins. The aim of this study is fourfold. First, we provide xAvESP charges for all amino acids, including amino- and carboxy-terminal variants of all, as well as alternative protonation states of His, Asp, Glu, Lys, Arg, Cys, and Tyr. Second, we show that the xAvESP charges averaged over the five new proteins are similar to charges obtained in the same way for avidin, with a correlation coefficient of 0.997. This shows that the charges are transferable and system-independent. Electrostatic protein–ligand interaction energies calculated with charges obtained from different proteins differ by only 1–3 kJ/mol on average. The xAvESP charges correlate rather well with Amber charges (except for the N atom of amino-terminal residues that are erroneous in Amber), although they are obtained in a more general way. Third, the conformational dependence of the charges is significant and gives rise to quite large differences in energies. However, these differences are to a large extent screened by solvation effects. For example, the solvent-screened electrostatic interaction energy between the protein galectin-3 and five different ligands varies with the charge sets by less than 3 kJ/mol on average. Finally, we show that the xAvESP charges give a comparable root-mean-squared deviation as the Amber charges for the MD simulations of 18 protein–ligand complexes, they give comparable or slightly worse backbone N
The role of hydrogen bonds in protein–ligand interactions. DFT calculations in 1,3-dihydrobenzimidazole-2 thione derivatives with glycinamide as model HIV RT inhibitors
Abstract The structures, redshifts, binding energies, Bader analysis, and shared-electron numbers (SENs) of 1,3-dihydrobenzimidazole-2-thione (DBS) derivatives hydrogen bonded to glycinamide were calculated by the means of DFT methods. The DBS–glycinamide complex serves as a model for human immunodeficiency virus reverse transcriptase inhibitors of the N -dimethylallyl-6-methyl-4,5,6,7-tetrahydroimidazo-[4,5,1-jk][1,4]-benzodiazepin-2(1H)-thione family. A correlation between experimental Gibbs free energies, associated biological activities and the energy of the hydrogen bond obtained with the SEN method showed a linear relationship for different substitution patterns. Our results suggest that efficient inhibitors are those substituted in the 8-position with electron-withdrawing small substituents. © 2011 Wiley Periodicals, Inc. Int J Quantum Chem, 2011
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C(O) bond rotation, which determines two different equilibrium geometries. In this work, the conformational equilibrium of formanilide, acetanilide, methyl and t-butyl phenylcarbamates, and their N-methylderivatives was studied by AM1 and B3LYP/6-31G(d,p) calculations. The effect of aryl p-substituents (MeO, Me, Cl, Br, CN, and NO2) was also studied. Amide barriers were found by DFT calculation between 12 and 21 kcal/mol. Carbamates, on the other hand, showed barriers between 11 and 15 kcal/mol. AM1 underestimates the energetic barriers and provides values around half those obtained by B3LYP/6-31G(d,p) calculations. Electron withdrawing substituents on aryl group decrease the barrier. © 2011 Wiley Periodicals, Inc. Int J Quantum Chem, 2012
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O bond and type of ligand was calculated by density functional theory methods. 32 V
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C
