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Semiempirical electronic structure calculation on Ca and Pb apatites
A systematic study is made on the electronic structure of stoichiometric calcium and lead apatites, using the tight binding extended Hückel method (eHT). The aim is to investigate the applicability of the semiempirical theory to study this family of compounds. A10(BO4)6X2 (A = Ca, Pb) apatites, differing by substitutions in the BO4 tetrahedral unit (B = P, As, and V) and X-channel ion (X = OH, Cl), are considered. The calculations show that eHT is suitable to describe basic properties especially concerning trends with atomic substitution and geometry changes. Band structure, Mulliken charge distribution, and bond orders are in good agreement with results of ab initio density functional theory (DFT) found in the literature. Large variations in the optical gap due to vanadium and lead substitutions are newly found. Changes in the anion X-channel affect the optical gap, which is in close agreement with DFT results. Analysis involving subnets are performed to determine the role of halogenic orbitals in the electronic structure of chloroapatites, showing evidence of covalent Cl bonding. It was also found that Pb[bond]OH bonding in hydroxy-vanadinite Pb10(VO4)6(OH)2, recently synthesized, is weaker than that of Ca[bond]OH in vanadate Ca10(VO4)6(OH)2. Arsenium is found to be more weakely bound to the O-tetrahedron than phosphorous, although Ca[bond]O bond is increased with the substitution. We investigate, in addition, the electronic structure of a model system Ca10(AsO4)6(OH)2, obtained from direct As substitution in the vanadate Ca10(VO4)6(OH)2. © 2008 Wiley Periodicals, Inc. Int J Quantum Chem, 2009
First principles study on the structure and electronic properties of 2-nitrimino-1-nitroimidazolidine
The properties of 2-Nitrimino-1-nitroimidazolidine are calculated by using SIESTA code, which adopts the standard Kohn-Sham self-consistent density functional method in the local density approximation. The structures and electronic properties are analyzed, and the factors that affect the impact sensitivity are discussed based on the crystal structure, band energy, and projected density of state. The reason for the smaller impact sensitivity compared to RDX (hexahydro-1,3,5-trinitro-1,3,5-triazine) is also explored from several respects such as the weakest bond dissociation energy in single molecule, and hydrogen bond, band gap in the crystal. © 2008 Wiley Periodicals, Inc. Int J Quantum Chem, 2009
Theoretical investigation of carotenoid ultraviolet spectra
We have determined the [pi] band-gaps of the main carotenoids present in poly(methyl)methacrylate/buriti blends, namely, trans-[beta]-carotene, 13-cis-[beta]-carotene, 9-cis-[beta]-carotene, phytofluene, and zeaxanthin. Semiempirical, model Hamiltonian, and density functional calculations were carried out to study these structures. The geometries were fully optimized using AM1, PM3, and B3LYP/6-31G(d,p) methods. The TD-DFT and ZINDO/S methods were applied for the calculation of the electronic absorption spectra of the optimized B3LYP geometries. The calculated spectra using the polarizable continuum model for the solvent effects were compared with the available experimental. © 2008 Wiley Periodicals, Inc. Int J Quantum Chem, 2009
Development of eclipsed and staggered forms in some hydrogen bonded complexes
Intermolecular hydrogen bonding in X3CH···NH3 (X = H, F, Cl, and Br) complexes has been studied by B3LYP, B3PW91, MP2, MP3, MP4, and CCSD methods using 6-311++G(d,p) and AUG-cc-PVTZ basis sets. These complexes could exist in both eclipsed (EC) and staggered (ST) forms. The differences between binding energies of EC and ST forms are negligible and all EC and ST shapes correspond to minimum stationary states. The order of stabilities of them is in an agreement with the results of atoms in molecules (AIM) and natural bond orbital (NBO) analyses. On the basis of low differences between binding energies, ST forms are more stable than EC forms in all complexes with the exception of Br3CH···NH3, which behaves just opposite. Although the differences between binding energies are negligible, they are consistent with the results of AIM analysis. © 2008 Wiley Periodicals, Inc. Int J Quantum Chem, 2009
A reasonable criterion of reactivities at the defective region of single-walled carbon nanotubes
Defect directional curvature KD-def was proposed as a reasonable criterion for the reactivities and adduct structures at the defective region of single-walled carbon nanotubes (SWCNTs). B3LYP/6-31G* calculations for the [2 + 1] and [1 + 1] additions of a series of 11-layer (n, n) (n = 4-8) and six-layer (10,0) SWCNTs with Stone-Wales defects showed that the KD-def or its mean KM-def was a good index to judge the adduct structures and the reactivities. Adducts of the [2 + 1] additions were divided into two types: one was the adduct with the C-X-C configuration and corresponding to the large KD-def and the large binding energy, and another was the adduct with the closed -3MR structure and corresponding to the small KD-def and the small binding energy. It must be pointed out that, besides mainly relying on the KD-def, the adduct structures and the reactivities of the [2 + 1] additions had been weakly affected by topologic structures. The calculated results for the [1 + 1] additions of the 11-layer (5,5) SWCNT with defect A revealed that the binding energies monotonously increased with the KM-def. © 2008 Wiley Periodicals, Inc. Int J Quantum Chem, 2009
Common generating function for three-dimensional hydrogen atom complete wave functions
The Schrödinger equation for the three-dimensional hydrogen atom is separable and integrable in spherical, spheroconal, parabolic, and prolate spheroidal coordinates. The formal correspondence of the eigenfunctions in parabolic coordinates with two harmonic-oscillator eigenfunctions in circular coordinates leads to the identification of a generating function for the hydrogen atom eigenfunctions. Its expansions yielding the complete wave functions in the respective coordinates are explicitly constructed. © 2008 Wiley Periodicals, Inc. Int J Quantum Chem, 2009
Rayleigh-Ritz variation method and connected-moments polynomial approach
We show that the connected-moments polynomial approach proposed recently is equivalent to the well known Rayleigh-Ritz variation method in the Krylov space. We compare the latter with one of the original connected-moments methods by means of a numerical test on an anharmonic oscillator. © 2008 Wiley Periodicals, Inc. Int J Quantum Chem, 2009
Electron density and its derivatives at the nucleus for spherically confined hydrogen atom
It is shown that the energy of a hydrogen-like atom confined inside a spherical cavity of radius, R, and potential barrier, V0, is quantitatively defined by the ratio . Here, the conventional spherical density (r) is scaled as [eta]l(r) = and the ratio of the second derivative [eta]l[Prime](r) to [eta]l(r) is evaluated at the nucleus. Numerical results of the ratios are presented for 1s, 2s, 2p, and 3d states at several values of V0. For such states, the characteristic radii of confinement leading to the well-defined values of energy are identified. © 2008 Wiley Periodicals, Inc. Int J Quantum Chem, 2009
Structure and stability of high-spin Aun(n = 2-8) clusters
The structures and relative stability of the maximum-spin n+1Aun and nAu+n (n = 2-8) clusters have been determined by density-functional theory. The structure optimizations and vibrational frequency analysis are performed with the gradient-corrections of Perdew along with his 1981 local correlation functional, combined with SBKJC effective core potential, augmented in the valence basis set by a set of f functions. We predicted the existence of a number of previously unknown isomers. The energetic and electronic properties of the small high-spin gold clusters are strongly dependent on sizes. The high-spin clusters tend to holding three-dimensional geometry rather than planar form preferred in low-spin situations. In whole high-spin Aun (n = 2-8) neutral and cationic species, 5Au4, 2Au+2, and 4Au+4 are predicted to be of high stability, which can be explained by valence bond theory. © 2008 Wiley Periodicals, Inc. Int J Quantum Chem, 2009
Quantum chemical studies of peptide nucleic acid monomers and role of cyclohexyl modification on backbone flexibility
Peptide nucleic acids (PNA) bind sequence specifically to DNA/RNA and are of major interest for all fields of molecular biology and could form the basis for gene-targeted drugs. Modifications are introduced in PNA to overcome problems associated with orientational selectivity in binding, to restrict conformational flexibility of backbone, and to discriminate binding for either DNA or RNA. The addition of geometrical isomers (1R,2S and 1S,2R) of cyclohexyl ring in the backbone of PNA could bring rigidification to PNA backbone and may impart specificity toward RNA. Therefore, quantum chemical studies are aimed to explore the conformational space, to find out preferred stable conformations of PNA and modified (1R,2S and 1S,2R) cyclohexyl PNA monomer. © 2008 Wiley Periodicals, Inc. Int J Quantum Chem, 2009
Convergence and quantum number assignment studies of rovibrational eigenstates in a model of predissociating NeICl van der Waals complex
This report details extensions and further analysis of the results presented in an earlier study (Elander et al., Phys Rev A 2001, 64, 012505). Rovibrational resonances in a model of the triatomic van der Waals complex NeICl were studied with a full quantum mechanical, smooth exterior dilation technique realized in a three-dimensional finite element code. Both exact and approximate results, where Coriolis effects are neglected, are presented, and their structures are discussed. Although the helicity is a good quantum number in a one-equation approximation, it is here demonstrated that this is not in the case when including Coriolis coupling, that is, couplings to other helicity components. The behavior of the resonance widths and the positions together with the structure of the resonance wave functions and their dependence on the total angular momentum and its projection have been studied. The dependence of the width of a given resonance on the total angular momentum was found to be weak while the dependence on the helicity is stronger. © 2008 Wiley Periodicals, Inc. Int J Quantum Chem, 2009
Chemical bond descriptors from molecular information channels in orbital resolution
The inter and intraorbital flows of the Fisher and Shannon informations in the molecular communication networks defined in the condensed atomic orbital (AO) resolution are investigated, and the effect of the basis set overlap is examined. The Schrödinger equation is interpreted as solution of the variation principle for the extreme Fisher information, subject to the wave-function-normalization (geometric) and potential-energy (physical) constraints. Its orbital contributions determine the information scattering of the underlying Fisher channel in the adopted AO basis set. This communication network is compared with the associated information system of the Shannon theory of communication. The new set of conditional probabilities between orbitals is compared to that used in the previous, sequential-cascade approach to the effective information promotion of AO in the molecular environment. Both geometrical and physical probabilities are reexamined using the quantum-mechanical superposition principle. The former characterizes the whole orbital space, whereas the latter describes only its occupied part, which determines the system chemical bonds. It is argued that the squares of corresponding elements of the charge and bond-order (CBO) matrix of the LCAO MO theory replace in the physical set of conditional probabilities the corresponding squares of the overlap integrals, which determine the geometrical set. The chain-rule interpretation of conditional probabilities is given. The modified 2-AO channel reproduces fully the earlier predictions from the molecular information channel in atomic resolution. The familiar bond criteria for an effective mixing of AO into molecular orbitals (MO) are shown to remain valid in the communication theory of the chemical bond (CTCB). © 2008 Wiley Periodicals, Inc. Int J Quantum Chem, 2009
Anharmonic effects on theoretical IR line shapes of medium strong H(D) bonds
We extend a quantum nonadiabatic treatment of damped H-bonds involving combined effects of anharmonicities of both the fast and the slow modes, Fermi resonances and relaxation [Rekik et al., J Mol Liq (in press)] in order to account for stronger H-bonds. For this purpose, we introduce, in the model, the quadratic modulation of both the angular frequency and the equilibrium position of the X - [hellip]Y stretching mode on the intermonomer motions to refine the structure of the spectrum, whereas we have considered in our previous work only the linear modulation of the angular frequency of the fast mode. In this approach, the strong anharmonic coupling theory is used through second-order expansion in the slow-mode coordinate Q of the angular frequency and the equilibrium position of the fast mode. The relaxations of the fast mode (direct damping) and of the H-bond bridge (indirect damping) are incorporated by aid of previous results [Rekik et al., J Mol Struct 2004, 687, 125]. The spectral density is obtained by Fourier transform of the autocorrelation function of the dipole moment of the fast stretching mode. The numerical calculation shows that the modulation of the angular frequency of the fast mode and its equilibrium position by the slow mode coordinate generate an improvement of the fine structure of the spectrum and also provide a direct evidence of the increase of the level density and the spectral broadening. © 2008 Wiley Periodicals, Inc. Int J Quantum Chem, 2009
DFT study on the hydrogen bonds of phenol-cyclohexanone and phenol-H2O2 in the Baeyer-Villiger oxidation
Hydrogen bonds of phenol-cyclohexanone and phenol-H2O2 in the studied Baeyer-Villiger (B-V) oxidation have been investigated by HF, B3LYP, and MP2 methods with various basis sets. The accurate single-point energies were performed using CCSD(T)/6-31+G(d,p) and CCSD(T)/aug-cc-pVDZ on the optimized geometries of MP2/6-31+G(d,p). It has been confirmed that B3LYP/6-31+G(d,p) could be used to study such hydrogen bonds. Energetic analysis of complexes was carried out using the Xantheas method with BSSE corrected by CP method. Orbital energy order ([epsiv]) illuminated that phenol with good hydrogen donor-acceptor property can interact with cyclohexanone or H2O2 to form hydrogen bound complexes, and the binding energies (BE[prime]2) range from -4.38 to -14.06 kcal mol-1. NBO analysis indicated that the redistribution of atomic charges in the complexes facilitated nucleophilic attack of H2O2 on cyclohexanone. The calculated results match remarkably well with the experimental phenomena. © 2008 Wiley Periodicals, Inc. Int J Quantum Chem, 2009
Connection between a few Jeziorski-Monkhorst ansatz-based methods
Different Jeziorski-Monkhorst ansatz-based methods are unified according to how to group terms to eliminate the redundancy problem. It is found that some seemingly different methods used to do MRCC are equivalent. It is argued that the various defining equations are not entirely proper, in the sense that the proper residual condition is not satisfied. This may partially rationalize the unsatisfactory performance of the various methods for single reference systems. In contrast, the MRexpT method satisfies the proper residual condition and it is expected that it will outperform other JM ansatz-based methods in single-reference cases. © 2008 Wiley Periodicals, Inc. Int J Quantum Chem, 2009
Complexity analysis of ionization processes and isoelectronic series
Information-theoretic magnitudes measuring randomness (Shannon entropy, exponential entropy, power entropy), spread (variance), localization (disequilibrium or self-similarity) and intrinsic accuracy (Fisher information) are used to compute several measures of complexity consisting, each one, of two localization-delocalization factors. These proposals have been tested on known, simple, but strongly organized and hierarchical systems (atoms) and processes (ionization). A complete numerical analysis at the Hartree-Fock level is done in position, momentum, and product spaces, where similar trends are followed by all studied complexities. It is also found that the complexity planes clearly reveal shell-filling patterns across the periodic table. Characteristic features accompanying the ionization process are identified, and the physical reasons for the observed patterns are described. We conclude that (i) the studied complexity measures detect not only randomness or localization, but also pattern and organization, and (ii) their study is not only sufficient in the usual position space, but also in the complementary momentum space, to have a complete description of the information-theoretic behavior of these systems. © 2008 Wiley Periodicals, Inc. Int J Quantum Chem, 2009
Theoretical study of electronic spectra of [Pt3([mu]-CO)3(CO)3]n-2 (n = 3-5) complexes
The platinum-platinum attraction and the spectroscopic properties of [Pt3([mu]-CO)3(CO)3]n-2 (n = 3-5) were studied at the PBE level. Theoretical calculations are in agreement with experimental geometries. The absorption spectra of these platinum complexes were calculated by the single excitation time-dependent (TD) density functional method. All complexes showed MLCT transitions interrelated with the intertriangular complexes. The values obtained at the PBE level are in agreement with the experimental color range. © 2008 Wiley Periodicals, Inc. Int J Quantum Chem, 2009
Chiral discrimination in hydrogen-bonded complexes of 2-methylol oxirane with hydrogen peroxide
A systematic quantum chemical study reveals the effects of chirality on the intermolecular interactions between two chiral molecules bound by hydrogen bonds. The methods used are second-order Møller-Plesset perturbation theory (MP2) with the 6-311++g(d,p) basis set. Complexes via the O[bond]H···O hydrogen bond formed between the chiral 2-methylol oxirane (S) and chiral HOOH (P and M) molecules have been investigated, which lead to four diastereomeric complexes. The nomenclature of the complexes used in this article is enantiomeric configuration sign corresponding to English letters. Such as: sm, sp. The relative positions of the methylol group and the hydrogen peroxide are designated as syn (same side) and anti (opposite side). The largest chirodiastaltic energy was [Delta]Echir = -1.329 kcal mol-1 [9% of the counterpoise correct average binding energy De(corr)] between the sm-syn and sp-anti in favor of sm-syn. The largest diastereofacial energy was -1.428 kcal mol-1 between sm-syn and sm-anti in favor of sm-syn. To take into account solvents effect, the polarizable continuum model (PCM) method has been used to evaluate the chirodiastaltic energies, and diastereofacial energies of the 2-methylol oxirane···HOOH complexes. The chiral 2,3-dimethylol oxirane (S, S) is C2 symmetry which offers two identical faces. Hence, the chirodiastaltic energy is identical to the diastereomeric energy, and is [Delta]Echir = 0.563 kcal mol-1 or 5.3% of the De(corr) in favor of s,s-p. The optimized structures, interaction energies, and chirodiastaltic energies for various isomers were estimated. The harmonic frequencies, IR intensities, rotational constants, and dipole moments were also reported. © 2008 Wiley Periodicals, Inc. Int J Quantum Chem, 2009
Molecular parameters of tetraatomic carbonyls X2CO and XYCO (X, Y = H, F, Cl) in the ground and lowest excited electronic states, part 1: A test of ab initio methods
Geometrical parameters of tetraatomic carbonyl molecules X2CO and XYCO (X, Y = H, F, Cl) in the ground (S0) and lowest excited singlet (S1) and triplet (T1) electronic states as well as values of barriers to inversion in S1 and T1 states and S1 [larr] S0 and T1 [larr] S0 adiabatic transition energies were systematically investigated by means of various quantum-chemical techniques. The following methods were tested: HF, MP2, CIS, CISD, CCSD, EOM-CCSD, CCSD(T), CR-EOM-CCSD(T), CASSCF, MR-MP2, CASPT2, CASPT3, NEVPT2, MR-CISD, and MR-AQCC within cc-pVTZ and cc-pVQZ basis sets. The accuracy of quantum-chemical methods was estimated in comparison with experimental data and rather accurate structures of excited electronic states were obtained. MP2 and CASPT2 methods appeared to be the most efficient and CCSD(T), CR-EOM-CCSD(T), and MR-AQCC the most accurate. It was found that at equilibrium all the molecules under study are nonplanar in S1 and T1 electronic states with CO out-of-plane angle ranging from 34° (H2CO, S1) to 52° (F2CO, T1), and height of barrier to inversion varying from 300 (H2CO, S1) to 11,000 (F2CO, T1) cm-1. © 2008 Wiley Periodicals, Inc. Int J Quantum Chem, 2009
Ground and low-lying excited C2v states of FeO2 - A challenge to computational methods
DFT (BPW91 and B3PW91), CCSD, CCSD(T), and MP2 geometry optimizations were performed on the lowest A1, A2, B1, and B2 singlet to septet states (a total of 16 states) of FeO2, using in most cases the 6-311+G(3df) basis set. With few exceptions, the different methods led to similar geometries. The lowest state is 3B1 or 5B2, depending on the method used. The quality of the various computational methods was tested by a comparison of vertical excitation energies with corresponding high-level MRCI values. The best agreement with MRCI was found for BPW91 and B3PW91, with average deviations of about 0.2 eV. MRCI calculations were carried to extremely low configuration selection thresholds (0.025 [mu]h) in order to find the lowest state of FeO2. The full CI estimate (including Davidson correction and energy extrapolation to 0 [mu]h) gave 3A1 as the lowest state, followed by 5B2 (0.03 eV) and 3B1 (0.06 eV). Both 3B1 and 3A1 have geometries and vibrational frequencies consistent with the observed IR spectrum. The calculated adiabatic electron affinity of FeO2 (from 3B1 of FeO2 to 4B2 of FeO2-) is 2.46 eV with CCSD(T), and 2.22 eV with BPW91, compared with the experimental value of 2.36 eV. © 2008 Wiley Periodicals, Inc. Int J Quantum Chem, 2009
Substituent effect on N[bond]H bond dissociation enthalpies of amines and amides: A theoretical study
N[bond]H bond dissociation enthalpies for the substituted ammonia, amine, amides, and their thio- and seleno-analogs have been studied employing ab initio and density functional methods. The orbital interactions involving lone pair of electrons on nitrogen and substituent, electrostatic interactions, spin delocalization, and hydrogen bonding are the important factors affecting the stability of the molecule and the radical. The molecule stabilization effect and radical stabilization effect have been calculated using isodesmic reactions in order to analyze the effect of substituent on the stabilization of the molecule and the radical. © 2008 Wiley Periodicals, Inc. Int J Quantum Chem, 2009
Stability, spectroscopic constants, and dissociation of CO2+: A theoretical study
Stability, spectroscopic constants, and dissociation of CO2+ have been studied in detail using ab initio MP2, CCSD and CCSD(T) methods, and density functional B3LYP method. The stability and the ambiguity between the ground and metastable state of the molecular dication have been discussed. The spectroscopic constants of the molecular dication have been compared with the experimental and theoretical values wherever available. Various charge symmetric and charge asymmetric dissociation pathways of CO2+ have been investigated. After dissociation, the fragmented atoms and ions are considered to be either in their ground or in their metastable state. Interesting results have been obtained for the charge symmetric and charge asymmetric dissociation of the diatomic dication. © 2008 Wiley Periodicals, Inc. Int J Quantum Chem, 2009
The charge capacitance of the chemical bond: Application to bonds containing metals
The charge capacitance of metal containing complexes are studied. For molecules with multiple bonding between the metal atoms it is found that the charge capacitance is correlated to the maximum bond order, natural bond order, and to some extent the effective bond order. Furthermore the charge capacitance of some methylidene metal dihydride complexes are studied. These molecules have agostic interactions of varying strength, and it is concluded that this strength is very well reflected in the charge capacitances of the systems. In accordance with the definition of agostic interactions it is therefor concluded that the charge capacitance holds information about the strength of covalent interactions. The effect therefore on the agostic interactions upon substitution of one of the hydrogen atoms with fluorine in the methylidene metal complexes is studied, and found to reduce the agostic interactions. It is also demonstrated that there is an agostic interaction in an ArCrCrAr complex. The distance dependence of the charge capacitance is also discussed. © 2008 Wiley Periodicals, Inc. Int J Quantum Chem, 2009
A theoretical and experimental study on manipulating the structure and properties of carbon nanotubes using substitutional dopants
We examine the possibility of controlling nanotube growth and simultaneously manipulating the nanotube properties by adding elements in minute amounts (such as nitrogen, phosphorous, and sulfur) that are different from carbon and the metal catalyst during the growth process. This procedure is shown to be capable of producing bamboo-type morphologies, heterodoped carbon nanotubes, and Y-junctions. This also represents a critical step toward tailoring properties and controlling nanotube architectures, thus promoting the development of novel materials with unusual electronic applications. The underlying formation mechanisms that lead to the observed structures and morphologies are elucidated using wide-ranging electronic structure calculations that reveal the fundamentally different nature of nitrogen, phosphorous, and sulfur during carbon nanotube growth. © 2008 Wiley Periodicals, Inc. Int J Quantum Chem, 2009
A simple model for the band structure and D.C. conductivity of an infinite C[double bond]O···H[bond]N chain perpendicular to the protein backbone
The1 Hartree-Fock crystal orbital (CO) method in its linear combination of atomic orbitals form was applied to determine the band structure of histone proteins taking 0.041e charge transfer per nucleotide base from the PO4- groups of poly(guanilic acid) to the arginine, and lysine side chains in histones (see text). Assuming that there are infinite COs, perpendicular to the main chain, formed by the amide groups of one segment of the protein chain bound together by H-bonds with the C[double bond]O groups of another segment of the chain, we have calculated the band structure. From this, we have determined the mobility using the deformation potential approximation. Multiplying this with the mobile electron concentration due to the charge transfer between the PO4- groups of DNA and the positive side chains in histones, we have obtained for the direct current (D.C.) electron conductivity [sigma]fib = 1.07 × 10-9 [Omega]-1 cm for a single fiber and after division by the cross-section of 9.10 × 10-16 cm2, [sigma]spec = 1.18 × 106 [Omega]-1 cm-1 for the specific conductivity. © 2008 Wiley Periodicals, Inc. Int J Quantum Chem, 2009
Theoretical study of interactions between cysteine and perfluoropropanoic acid in gas and aqueous phase
The interaction of perfluoropropanoic acid (PFPA) with the amino acid cysteine was investigated using density functional theory. Previous studies suggest that the peroxisome proliferator chemical, perfluorooctanoic acid, is circulated throughout the body by way of sulfur-containing amino acids. We present conformational analysis of the interactions of PFPA, a small model of perfluorooctanoic acid, with the sulfur-containing amino acid which occur by the process of hydrogen bonding, in which the hydrogen of the sulfhydryl group interacts with the carboxyl oxygen, and the amino nitrogen forms a hydrogen bond with the hydrogen of the [bond]OH group of the fluorinated alkyl. We also show in our structures a recently characterized weak nonbonded interaction between divalent sulfur and a main chain carboxyl oxygen in proteins. B3LYP calculated free energies and interaction energies predict low-energy, high-interaction conformations for complex systems of perfluorinated fatty acid interactions with cysteine. © 2008 Wiley Periodicals, Inc. Int J Quantum Chem, 2009
Efficient generation of Heisenberg Hamiltonian matrices for VB calculations of potential energy surfaces
The spin-Hamiltonian valence bond theory relies upon covalent configurations formed by singly occupied orbitals differing by their spin counterparts. This theory has been proven to be successful in studying potential energy surfaces of the ground and lowest excited states in organic molecules when used as a part of the hybrid molecular mechanics - valence bond method. The method allows one to consider systems with large active spaces formed by n electrons in n orbitals and relies upon a specially proposed graphical unitary group approach. At the same time, the restriction of the equality of the numbers of electrons and orbitals in the active space is too severe: it excludes from the consideration a lot of interesting applications. We can mention here carbocations and systems with heteroatoms. Moreover, the structure of the method makes it difficult to study charge-transfer excited states because they are formed by ionic configurations. In the present work we tackle these problems by significant extension of the spin-Hamiltonian approach. We consider (i) more general active space formed by n ± m electrons in n orbitals and (ii) states with the charge transfer. The main problem addressed is the generation of Hamiltonian matrices for these general cases. We propose a scheme combining operators of electron exchange and hopping, generating all nonzero matrix elements step-by-step. This scheme provides a very efficient way to generate the Hamiltonians, thus extending the applicability of spin-Hamiltonian valence bond theory. © 2008 Wiley Periodicals, Inc. Int J Quantum Chem, 2009
Density functional study of aurophilic interaction in Cl(AuPH3)2+ and in its dimerization
With the help of quantum mechanical calculations, the geometric structures and electronic structures of the closed-shell systems Cl(AuPH3)2+ and [Cl(AuPH3)2+]2 have been determined by DFT and MP2 methods. Experimental structure parameters of the title compounds were reproduced at X[alpha] level. The Mulliken population and HOMO-LUMO gaps were examined. The intermolecular aurophilic interactions in [Cl(AuPH3)2+]2 were analyzed and decomposed. A positive BE value (no bonding) was calculated. When 2PC was added, each pair Au[hellip] Au interaction energy was about 83 kJ/mol. © 2008 Wiley Periodicals, Inc. Int J Quantum Chem, 2009
Nonadditivity of methyl group in single-electron hydrogen bond of methyl radical-water complex
The nonadditivity of methyl group in the single-electron hydrogen bond of the methyl radical-water complex has been studied with quantum chemical calculations at the UMP2/6-311++G(2df,2p) level. The bond lengths and interaction energies have been calculated in the four complexes: CH3[bond]H2O, CH3CH2[bond]H2O, (CH3)2CH[bond]H2O, and (CH3)3C[bond]H2O. With regard to the radicals, tert-butyl radical forms the strongest hydrogen bond, followed by iso-propyl radical and then ethyl radical; methyl radical forms the weakest hydrogen bond. These properties exhibit an indication of nonadditivity of the methyl group in the single-electron hydrogen bond. The degree of nonadditivity of the methyl group is generally proportional to the number of methyl group in the radical. The shortening of the C···H distance and increase of the binding energy in the (CH3)2CH[bond]H2O and (CH3)3C[bond]H2O complexes are less two and three times as much as those in the CH3CH2[bond]H2O complex, respectively. The result suggests that the nonadditivity among methyl groups is negative. Natural bond orbital (NBO) and atom in molecules (AIM) analyses also support such conclusions. © 2008 Wiley Periodicals, Inc. Int J Quantum Chem, 2009
Comparison of polarization consistent and correlation consistent basis sets for noncovalent interactions
The convergence of the polarization consistent family of basis sets to the MP2 complete basis set (CBS) limit has been examined for the dissociation energies of five weakly bound clusters: (HF)2, (HF)3, (H2O)2, (H2O)3, and (C2H2)2. MP2 results obtained with two series of polarization consistent basis sets and the analogous series of Dunning's correlation consistent basis sets are compared to the CBS limits obtained with the explicitly correlated MP2-R12 method. Although the polarization consistent basis sets do converge systematically to the CBS limit, the correlation consistent basis sets with diffuse functions on nonhydrogen atoms (i.e., cc-pVXZ for H and aug-cc-pVXZ otherwise) consistently produced the smallest errors. Counterpoise corrections for basis set superposition error did not improve the performance of either the polarization or correlation consistent basis sets. © 2008 Wiley Periodicals, Inc. Int J Quantum Chem, 2009
Theoretical estimates of the IR spectrum of water intercalated into kaolinite
Calculations at AM1, PM3, and HF/6-31G levels of part of the IR spectrum of the water-kaolinite intercalated system based on a 96-atom cluster of kaolinite with one water molecule are reported. Only the water molecule conformation is optimized. Frequencies and intensities for just the water vibrations and stretchings of four cluster hydroxyls were calculated through partial Hessian matrices and polar tensors obtained by numerical differentiation of energy gradients and dipole moment. The water molecule was found to attach to the cluster mainly through a double hydrogen bond to the siloxane inner surface, partially entering the siloxane ring hexagonal hole. Though the theoretical results predict that the water OH stretching frequencies decrease from the gas-phase state to the intercalated state, they are still higher than expected with respect to the observed spectrum. © 2008 Wiley Periodicals, Inc. Int J Quantum Chem, 2009
Visualizing molecular wavefunctions using Monte Carlo methods
Using explicitly correlated wavefunctions and variational Monte Carlo we calculate the electron density, the electron density difference, the intracule density, the extracule density, two forms of the kinetic energy density, the Laplacian of the electron density, the Laplacian of the intracule density, and the Laplacian of the extracule density on a dense grid of points for the ground state of the hydrogen molecule at three internuclear distances (0.6, 1.4, 8.0). With these values we construct a contour plot of each function and describe how it can be used to visualize the distribution of electrons in this molecule. We also examine the influence of electron correlation on each expectation value by calculating each function with a Hartree-Fock wavefunction and then comparing these values with our explicitly correlated values. © 2008 Wiley Periodicals, Inc. Int J Quantum Chem, 2009
Computational determination of effects of electric fields upon "trigger linkages" of prototypical energetic molecules
For five prototypical energetic molecules (nitrobenzene, methyl azide, methyl nitrate, nitromethane, and dimethylnitramine), we examine computationally the effects of external electric fields upon their "trigger linkage" bonds, the breaking of which is believed to play a key role in detonation initiation. The bonds are, respectively, C[bond]NO2, N[bond]N2, O[bond]NO2, C[bond]NO2, and N[bond]NO2. The calculations are at the B3PW91/6-31G** level. We find that fields along these bonds that reinforce the molecules' intrinsic polarities also lower their energies and increase the bonds' stretching vibration frequencies. This suggests a strengthening of the bonds. Fields in the opposite direction do the reverse. © 2008 Wiley Periodicals, Inc. Int J Quantum Chem, 2009
Size dependent structural and electronic properties of MgO nanotube clusters
MgO nanotube clusters which cross sections are composed of two-, three-, four-, and five-membered rings are constructed and studied by the density functional theory at B3LYP/6-31G(d) level. The variations of bond length present anisotropic effect. Three-membered ring nanotube cluster is the most stable tube among these MgO isomers. Mixed covalent and ionic bonding always exists in MgO nanotube clusters. With increasing length of MgO nanotube clusters, the averaged atomic charge increases, and converge to 1.227; the s-p separation of O bands decreases; whereas energy gap nearby frontier orbitals present dramatic difference corresponding to various structure family. It is possible that MgO nanotube clusters show electronic properties of semiconductor. An interpretation for MgO nanotube clusters fabricated by simply thermal methods is proposed. The structural and electronic properties of MgO nanotube clusters are discussed systematically in details. © 2008 Wiley Periodicals, Inc. Int J Quantum Chem, 2009
Methane elimination from n-pentane
The quantum chemical calculations at the different levels of theory were performed with the target being to determine the vibration frequencies and to estimate the barriers to internal rotations of n-pentane molecules. In connection with the observed losses of CH3 and CH4 from the n-pentane in gas phase, the calculations at the B3LYP level of theory with the 6-31G(d) basis set were used to study the ground-state potential energy surface of the n-pentane. © 2008 Wiley Periodicals, Inc. Int J Quantum Chem, 2009
Theoretical calculations of transition probabilities and oscillator strengths for Ti III and Ti IV
Due to the complicated electronic configuration of atoms and ions of the transition metal elements, the studies for properties such as transition probabilities and oscillator strengths for these atoms and ions are not systematic. Because of the existence in a variety of stellar objects and wide use in the field of astrophysics, titanium has long been of interest for many researchers. In this article within the Weakest Bound Electron Potential Model (WBEPM) theory, comprehensive set of calculations for transition probabilities and oscillator strengths for Ti III and Ti IV are performed. Many of our results had no previous experimental or theoretical values, so these predictive results could be of some value to the workers in this field. © 2008 Wiley Periodicals, Inc. Int J Quantum Chem, 2009
Quantum-chemical study of the singlet oxygen emission
Intensity distribution in rotational lines of the 0-0 band of the a1[Delta]g [rarr] X3[Sigma]g- transition in the oxygen molecule at [lambda] = 1270 nm is studied by quadratic response (QR) method in a framework of multiconfiguration self-consisted field (MCSCF) theory. The distance dependence of the transition magnetic moment and the (0-0)/(0-1) bands intensity ratio are calculated. A short review of previous theory of the red and infrared atmospheric oxygen bands and of their enhancement by collisions is presented to analyze and compare the new results. Enhancement of these bands in O2 collisions with Li2 and N2 molecules is calculated by QR method. Diamagnetic species simulate solvent molecules of different optical polarizability. Specific influence of collisions on vibronic bands is stressed. © 2008 Wiley Periodicals, Inc. Int J Quantum Chem, 2009
CCSD(T) and MRCI studies on the ground and excited states of BrOClO and ClOBrO
In this work, three forms (cis, trans and nonplanar) of ClOBrO and BrOClO were optimized at CCSD(T)/cc-pVTZ level of theory. At the most stable forms (nonplanar form) of ClOBrO and BrOClO, the vertical excitation energies for the lowest six singlet states and two triplet states were calculated at the multireference internally contracted configuration interaction (MRCI) level of theory using cc-pVDZ, Aug-cc-pVDZ, cc-pVTZ, and Aug-cc-pVTZ basis sets. The scalar relativistic effect on the excited states of BrOClO and ClOBrO were estimated. In addition, the potential energy curves of the lowest six singlet states and two triplet states of BrOClO and ClOBrO, as well as BrOOCl were calculated at both MCSCF (complete active space self-consistent field) and MRCI levels of theory using Aug-cc-pVDZ basis set on the active space (18e,12o) along the distances of BrO[bond]ClO, ClO[bond]BrO, and BrO[bond]OCl. The results were compared among BrOOCl, ClOBrO, and BrOClO. The first singlet excited state of BrOOCl is 1.12 eV higher than that of BrOClO and 1.36 eV higher than that of ClOBrO at MRCI/cc-pVTZ level of theory. The first triplet excited state of BrOOCl is 0.77 eV higher than that of BrOClO and 0.86 eV higher than that of ClOBrO at MRCI/cc-pVTZ level of theory. Most of the excited states of BrOClO studied in this work are unbound states; but most of the ClOBrO and BrOOCl excited states studied in this work are weakly bound states at MRCI level of theory. © 2008 Wiley Periodicals, Inc. Int J Quantum Chem, 2009
Prediction of excited state energies for molecular nitrogen using quantum Monte Carlo methods
Using the quantum Monte Carlo (QMC) method, we estimated electronic excitation energies for four low-lying Frank-Condon states of dinitrogen. QMC trial function forms were examined, with single and multideterminant wave functions derived from configuration interaction (CI) and complete active space self-consistent field theory (CASSCF) calculations. Variational Monte Carlo (VMC) and fixed-node diffusion Monte Carlo (FN-DMC) results compare favorably with TDHF, MRDCI, TDDFT, MR-CCSD, EOM-CCSD, and CASSCF which demonstrate the accuracy of QMC for these excited electronic states. The CASSCF constructed trial functions for QMC (QMC-CAS) and MR-CCSD deviate the least from the experimental values presented. Mean absolute deviation (MAD), or the difference between the calculated result and the experimental value divided by the total number of calculated results, is provided for each method. The least favorable MAD is obtained by the multideterminant and single determinant CISD trial function used for VMC calculations at 1.67 and 1.32, respectively. The lowest three MAD's are provided by the multideterminant CASSCF-VMC (0.23), the multideterminant CASSCF-DMC (0.14), and the MR-CCSD (0.13) calculation. © 2008 Wiley Periodicals, Inc. Int J Quantum Chem, 2008
Comparative QSTR study of a series of alcohol derivatives against Tetrahymena pyriformis
The quantitative structure toxicity relationship (QSTR) models of 89 alcohol derivatives have been made with the help of quantum chemical and topological descriptors. The molecular modeling and geometry optimization have been carried out with CAChe pro software. The calculations of quantum chemical descriptors have been done by MOPAC 2002 of topological descriptors by Dragon software. The study indicates that quantum chemical descriptors better predict the toxicity of amino, halogenated and unsaturated alcohol derivatives, while toxicity of acetylenic, diols, and saturated alcohol derivatives are better predicted by topological descriptors as indicated by correlation coefficient and cross validation coefficient (rCV[circ]2) values of the QSTR models. The predicted toxicity (PT) values obtained by these QSTR models are close to observed toxicity. © 2008 Wiley Periodicals, Inc. Int J Quantum Chem, 2009
Use of noninteger n-Slater type orbitals in combined Hartree-Fock-Roothaan theory for calculation of isoelectronic series of atoms Be to Ne
The ground state calculations in the combined Hartree-Fock-Roothaan approach are performed for the neutral and the first 20 cationic members of the isoelectronic series of atoms from Be to Ne using noninteger n-Slater type orbitals. For the total energies obtained, only a small deviation has been found. At the same time, the size of the present noninteger n-Slater type orbitals is smaller than that of the usual extended integer n-Slater functions in literature. All of the nonlinear parameters are fully optimized. The relationship between optimized parameters and atomic number Z is also investigated. For each atom, the total energies are given in tables. © 2008 Wiley Periodicals, Inc. Int J Quantum Chem, 2009
Theoretical study on the reactive sites and intramolecular interactions in taxol and its four analogues
A density-functional study of the paclitaxel (Taxol) molecule and its four analogues has been performed. The theory of Bader's atoms in molecules (AIM) was applied to examine the electronic structure of these molecules at their ground state. Topological analysis reveals that the esterification of hydroxyl group attached to the oxetane ring results in great change of conformation of the taxane ring, and thus is responsible for bioactivity of the oxetane oxygen atom. It was found that there exists some intramolecular interactions in the molecule, including normal hydrogen bonds (HBs) and double HBs. Visualization of the molecule shows that the central bodies (the four fused rings) of the molecules are wrapped by the intramolecular interactions. It is supposed that these intramolecular interactions lower the aqueous solubility and protect the flexible oxetane ring, which is regarded as the dominating bioactivity site of the drug, from being opened. Our results provide an extended and consistent set of data to gauge classical force fields in view of the atomistic investigations of the interaction of the bioactive molecules. © 2008 Wiley Periodicals, Inc. Int J Quantum Chem, 2009
Does Møller-Plesset perturbation theory converge? A look at two-electron systems
We study the convergence or divergence of the Møller-Plesset Perturbation series for systems with two electrons and a single nucleus of charge Z > 0. This question is essentially to determine if the radius of convergence of a power series in the complex perturbation parameter [lambda] is greater than 1. The power series is centered at [lambda] = 0, so we try to find whether or not the singularity closest to [lambda] = 0 is inside the closed unit disk in the complex plane. We give a description of possible causes for divergence in the general problem and then examine two Helium-like models. The first model is a simple one-dimensional model with delta functions in place of Coulomb potentials. The second is the realistic three-dimensional model. For each model, we show rigorously that if the nuclear charge Z is sufficiently large, there are no singularities for real values of [lambda] between -1 and 1. Using a finite difference scheme, we present numerical results for the delta function model. The numerics are consistent with proven results and also suggest that the closest singularity occurs where [lambda] is real and negative. © 2008 Wiley Periodicals, Inc. Int J Quantum Chem, 2009
1D lead iodide crystals encapsulated within single walled carbon nanotubes
The structure and binding energies of lead iodide crystals encapsulated within single-walled carbon noanotubes are studied using density functional theory. Calculations were performed on the simulated PbI2 structure encapsulated within a (12,12) single-walled nanotube, to investigate the perturbations on the PbI2 crystal and tube structure and electronic structure, and to estimate the binding energy. The calculation confirms the structure as a single chain of PbI6 octahedra bound by two chains of PbI5 square pyramids. The calculated binding energy shows that the encapsulation is noncovalent. Minimal charge transfer is observed between nanotube and the PbI2 crystals. The band gap is shown to increase from the bulk to the encapsulated structure. © 2008 Wiley Periodicals, Inc. Int J Quantum Chem, 2009
Transfer of electron density as a result of hydrogen bond formation
It has been found that the amount of charge transfer between donor and acceptor molecules in four sets of hydrogen-bonded complexes may be adequately described as an exponential function of the equilibrium distance between the hydrogen atom and the nearest atom of the acceptor molecule. The exponential factors of the transfer are of the same order but somewhat larger than the factors found otherwise in the investigations of dynamic electron transfer. © 2008 Wiley Periodicals, Inc. Int J Quantum Chem, 2009
Modeling enzymatic transition states by force field methods
The SEAM method, which models a transition structure as a minimum on the seam of two diabatic surfaces represented by force field functions, has been used to generate 20 transition structures for the decarboxylation of orotidine by the orotidine-5[prime]-monophosphate decarboxylase enzyme. The dependence of the TS geometry on the flexibility of the system has been probed by fixing layers of atoms around the active site and using increasingly larger nonbonded cutoffs. The variability over the 20 structures is found to decrease as the system is made more flexible. Relative energies have been calculated by various electronic structure methods, where part of the enzyme is represented by a force field description and the effects of the solvent are represented by a continuum model. The relative energies vary by several hundreds of kJ/mol between the transition structures, and tests showed that a large part of this variation is due to changes in the enzyme structure at distances more than 5 Å from the active site. There are significant differences between the results obtained by pure quantum methods and those from mixed quantum and molecular mechanics methods. © 2008 Wiley Periodicals, Inc. Int J Quantum Chem, 2009
Theoretical electronic structure of the molecule ScI
Theoretical investigation of the 18 lowest electronic states of the molecule ScI in the representation 2S+1[Lambda](±) has been performed via CASSCF and MRCI (single and double excitation with Davidson correction) calculations. To the best of our knowledge these calculated electronic states are the first ones from ab initio methods. Thirteen electronic states between 4,500 cm-1 and 21,000 cm-1 have been studied for the first time and have not yet been observed experimentally. The harmonic frequency [omega]e, the internuclear distance Re, the electronic transition energy with respect to the ground state Te, and the rotational constant Be have been calculated for the considered electronic states. By using the canonical functions approach the eigenvalues E[upsi] and the rotational constants B[upsi] have also been calculated for the six lowest-lying electronic states. The comparison of these results with the theoretical and the experimental data available in the literature shows a good agreement. © 2008 Wiley Periodicals, Inc. Int J Quantum Chem, 2009
Proton transfer at the carboxylic sites of amino acids: A single water molecule catalyzed process
Ab initio calculations at MP2 level of theory were used to study the proton transfer at the carboxylic sites of amino acids, in the isolated, mono- and di-hydrated forms. In the case of water dimer, two interaction modes with glycine neutral structures (see Fig. ) were explored, corresponding to the concerted and stepwise reaction pathways. Their transition states can be described as (H2O[bond]H[bond]OH2)+ [Fig. (a)] and (H2O---H[bond]OH2)+ [Fig. (b)], respectively. The energy analysis indicated that the concerted pathway is preferred. In the isolated, mono- and di-hydrated glycine complexes, the activation barriers of the proton transfer at the carboxylic sites were calculated to be 34.49, 16.59, and 13.36 kcal mol-1, respectively. It was thus shown that the proton transfer is significantly assisted and catalyzed by water monomer so that it can take place at room temperature. Instead, the further addition of water molecules plays solvent effects rather than catalytic effects to this proton transfer process. The above results obtained with discrete water molecules were supported by the solvent continuum calculated data. It was also observed that the heavy dependence of the solvent continuum models on dipole moments may produce misleading results. © 2008 Wiley Periodicals, Inc. Int J Quantum Chem, 2009
Theoretical study on the mechanism for the thermal rearrangement reactions of 2-silylethyl acetate H3SiCH2CH2OCOCH3
The thermal rearrangement mechanisms of 2-silylethylacetate H3SiCH2CH2OOCCH3 were investigated by ab initio molecular orbital theory for the first time. All structures of reactant, transition states, and products were located and fully optimized at the B3LYP/6-311+G(d, p) levels, and harmonic vibrational frequencies for the involved stationary points on the potential energy surface were obtained. The reaction pathways were analyzed and confirmed by intrinsic reaction coordinate (IRC) calculations. Furthermore, atomic charges were determined by using the natural bond orbital (NBO) analysis. The calculational results show that H3SiCH2CH2OOCCH3 can rearrange thermally in two ways. One is [1,3] rearrangement (Reaction A), in which silyl group transfers from carbon to oxygen(in C[bond]O[bond]C) via a four-membered ring transition state, forming silyl acetate and ethylene, the other way, [1,5] rearrangement (Reaction B), happens with transferring of silyl group from carbon to oxygen (in C[double bond]O) via a six-membered ring transition state, forming the same products as in Reaction A. The energy barriers of the Reactions A and B were calculated to be 188.9 and 191.6 kJ/mol at the B3LYP/6-311+G(d,p) levels, respectively. Changes in thermodynamic functions ([Delta]S, [Delta]H, and [Delta]G), equilibrium constant K(T), as well as preexponential factor A(T), and reaction rate constant k(T) in Eyring transition state theory were calculated over a temperature range of 200-1600 K, and then thermodynamic and kinetic properties of the reactions were analyzed. It can be suggested that Reactions A and B are noncompetitive, and both happen only at elevated temperature. © 2008 Wiley Periodicals, Inc. Int J Quantum Chem, 2009
A DFT study of transformation of nitrosothiol isomers and their decomposition to nitric oxide in gas phase
The structures of nitrosothiol isomers were obtained by geometry optimizations using the density functional theory calculations. Two cis-isomeric, 2 trans-isomeric, and 1 zwitterionic species of the HSNO isomers were found and the most stable species is the trans-isomer. Energetics, thermodynamic properties, rate constants, equilibrium constants of all transformation reactions, and their energy profiles were obtained. Decomposition of the HSNO trans-isomer to nitric oxide was investigated and its rate constant was obtained. © 2008 Wiley Periodicals, Inc. Int J Quantum Chem, 2009
Ab initio analysis of monomers and dimers of trialkylphosphine oxides: Structural and thermodynamic stability
Structural and thermodynamic stabilities of monomers and dimers of trialkylphosphine oxides (TRPO) were studied using quantum chemistry calculations. Density functional theory calculations were carried out and the structures of four TRPO have been determined: TMPO (methyl; R = CH3), TEPO (ethyl; R = CH3CH2), TBPO (n-butyl; R = CH3(CH2)3), and TOPO (n-octyl; R = CH3(CH2)7). TRPO homodimers were investigated considering two isomeric possibilities for each dimer. Relative binding energies and the enthalpic and entropic contributions to the Gibbs free energy were calculated for all dimers. The formation of dimers from the individual monomeric TRPO species as a function of temperature was also analyzed. © 2008 Wiley Periodicals, Inc. Int J Quantum Chem, 2009
New approach for the correction of Ab initio molecular force fields in cartesian coordinates
This paper presents a scaling technique applicable to molecular force constants in Cartesian coordinates. In the course of spectroscopic and structural research, it is a common practice to achieve better correspondence between measured and calculated vibrational frequencies of a molecule by appropriate scaling of the force constant matrices obtained by quantum chemical methods. However, the existing scaling procedure requires introduction of the complete system of internal coordinates, which may be tedious and time-consuming for complex molecules. Scaling scheme suggested in this paper is applied directly to Cartesian force constant matrices and thus does not require introduction of the internal coordinates. Practical implementation of the proposed approach is illustrated with the examples of simple molecules. The scheme allows imposing special constraints on the determined Cartesian scale factors, which may be used for obtaining scale factors possessing transferability properties. These properties are very important for vibrational spectra calculations of related compounds, especially for large biological molecules, associates and nanostructures. © 2008 Wiley Periodicals, Inc. Int J Quantum Chem, 2009
Discrete sets of many-body sturmians
In this work, we present a method to obtain two-particle Coulomb Sturmians Functions (CSF) with an expansion in a set of L2 basis functions. In the two-body case, we recover the exact (discrete) spectrum of the CSFs for negative energies and a discretized approximation for positive ones. Besides, we make use of this method to analyze the two-independent electron problem as a Generalized Sturmian problem. We propose a discretized version of the wave function in terms of the CSF states, and show that the problem reduces to find numerical coincidences between energy-dependent eigencharges of the mutually independent one-electron systems. This expansion methodology includes the continuum information which is lost in the sets used previously in the literature, and is complete when the size of the basis goes to infinity. © 2008 Wiley Periodicals, Inc. Int J Quantum Chem, 2008
Theoretical analysis of vibrational spectra and scaling-factor of 2-aryl-1,3,4-oxadiazole derivatives
In this study, the direct molecular structure implementations for calculating vibrational spectra and scaling factors, and infrared intensities at both the Hartree-Fock (HF) and density functional (B3LYP) levels of theory with 6-31G(d), 6-311G(d), 6-31++G(d,p), and 6-311++G(d,p) basis sets are presented. Also, vibrational frequencies have been investigated as dependence on the choice of method and basis set. The parameters of molecular geometry and vibrational frequencies values of 2-aryl-1,3,4-oxadiazoles 5a-g in the ground state have been calculated. Theoretical determination of vibrational frequencies is quite useful both in understanding the relationship between the molecular structures and scaling factor. The data of 2-aryl-1,3,4-oxadiazoles 5a-g display significant electronic properties provide the basis for future design of efficient materials having the oxadiazole core and theoretical IR studies. © 2008 Wiley Periodicals, Inc. Int J Quantum Chem, 2009
Theoretical study on structures and stabilities of N4X (X = O, S, Se, Te) series
The stable and transition structures of N4X (X = O, S, Se, Te) series with singlet state are optimized with the ab initio (MP2) and density functional theory (B3LYP) methods using the 6-311+G(d) basis set. The ring isomers are found to be the global minima for N4O, N4S, N4Se, and the chain isomer is the minimum for N4Te. The stabilities are studied by evaluating the dissociation barriers with respect to dissociation. The reactants and products connected by transition structures are determined by applying the intrinsic reaction coordinate (IRC) calculations. The C2v, C3v and ring isomers decompose into linear NNX and N2 molecules, however, the chain isomers decompose into cyclic N2X and N2 firstly. A new possible isomerization mechanism between the cyclic and linear structures of N2X series is studied. The cyclic structures of N2X convert into linear structures easily with the very low barriers. © 2008 Wiley Periodicals, Inc. Int J Quantum Chem, 2009
Computational studies on the spectroscopic properties of the 2-pyridylpyrazolate-based platinum(II) complexes with modified pyrazolate fragment
Electronic structures and spectroscopic properties of a series of platinum(II) complexes based on the 2-pyridylpyrazolate ligand with modified pyrazolate fragment have been studied by the time-dependent density functional theory (TD-DFT) calculations. The ground- and excited-state structures were optimized by the DFT and single-excitation configuration interaction (CIS) methods, respectively. The calculated structures and spectroscopic properties are in agreement with the corresponding experimental results. The results of the spectroscopic investigations revealed that the lowest-energy absorptions have 1,3MLCT/1,3ILCT mixing characters. When the electron-withdrawing groups ([bond]CF3, [bond]C3F7) are introduced into the pyrazolate fragment, the lowest-energy absorptions are blue-shifted compared with that without substituents on the pyrazolate fragment, while the opposite case is observed for the electron-donating groups ([bond]Me, [bond]tBu, etc.). Otherwise, the phosphorescent emissions of these complexes have the 3MLCT/3ILCT character and should be originated from the lowest-energy absorptions. When the pyrazolate fragment is replaced by the indazole group, the HOMO and LUMO orbitals of the pyridyl-indazolate ligand platinum(II) complexes have obvious [pi] and [pi]* orbital characters. Therefore, there is no evident MLCT character in the lowest energy absorption and emission. © 2008 Wiley Periodicals, Inc. Int J Quantum Chem, 2009
Investigations on spectroscopic properties of ND(X3[Sigma]-) radical using coupled-cluster theory in combination with the correlation-consistent quintuple basis set augmented with diffuse functions
The coupled-cluster singles-doubles-approximate-triples [CCSD(T)] theory in combination with the correlation-consistent quintuple basis set augmented with diffuse functions (aug-cc-pV5Z) is used to investigate the spectroscopic properties of the ND(X3[Sigma]-) radical. The adiabatic potential energy curve is calculated over the internuclear separation range from 0.06 to 2.47 nm and fitted to the analytic Murrell-Sorbie function, which is used to determine the spectroscopic parameters, [omega]e[chi]e, [alpha]e, and Be. The present De, Re, [omega]e, [omega]e[chi]e, [alpha]e, and Be values are of 3.57357 eV, 0.10367 nm, 2403.166 cm-1, 42.888 cm-1, 0.25344 cm-1, and 8.90867 cm-1, respectively, which are in excellent agreement with the available measurements. With the potential obtained at the UCCSD(T)/aug-cc-pV5Z level of theory, a total of 21 vibrational states is predicted when J = 0 by numerically solving the radial Schrödinger equation
