PROFILE: Early Excellence in Physical Organic Chemistry

Posted on 24 May 2012 | 12:14 pm

Ring opening/closure reactions of novel diheteroarylethenes derivatives. Solvent effects

Three novel diarylethenes derived from benzothiophene were synthesized and their photochemical optical properties studied in four solvents (n-hexane, toluene, acetonitrile and methanol). It was determined that these compounds dramatically increase the light absorption in the visible region upon exposure to UV light, showing two absorption maxima at 420?nm and 520?nm, respectively. The observed change is completely reverted upon exposure to visible light. The three compounds demonstrate negligible fatigue, excellent stability in the four solvents, and absence of photodegradation and thermal back reactions. This paper also describes a complete study of solvent effects on the kinetics of the ring opening and cycloreversion. While, the UV spectra are little sensitive both to the substitution pattern and to the solvent influence, the kinetics of ring opening/cycloreversion reactions exhibit a more interesting behavior that differs from other diarylethenes reported in the literature. The cyclization kinetics shows a complex order, well fitted by an exponential equation, while the cycloreversion presents first-order kinetics. The whole complex reaction scheme can be explained by the unusual presence of two conformers of the open form, a parallel and an antiparallel conformation. Though other alternative mechanism could explain the kinetics, additional evidence is consistent with the conformational equilibrium proposed: i.e. the solvent influence on the kinetics of the nitro-substituted substrate, and the double signals for the methyl protons and for the methoxy protons (when present) observed in the nuclear magnetic resonance spectra. These observations are relevant in relation to the application of these novel diarylethenes as potential optical materials. Copyright © 2012 John Wiley & Sons, Ltd.Three novel diarylethenes were synthesised and their photochemical properties examined in four different solvents. While the UV spectra are almost insensitive to the substitution pattern and to the solvent effects, the kinetics of ring opening/cycloreversion reactions exhibit a more complex behavior. The whole reaction scheme can be explained by the presence of two conformers of the open form, a parallel and an antiparallel conformation.

Posted on 18 May 2012 | 6:04 am

The enthalpy of formation of methionine revisited

Methionine (2-amino-4-(methylthio)butanoic acid) is a sulfur-containing essential ?-amino acid. Despite its importance, its enthalpy of formation is contentious. From the analysis of its polymorphism confirms that the previously reported calorimetric experiments were made in the temperature interval in which a phase transitions occurs. A new value for the enthalpy of formation is suggested based on these results, as well as G4 quantum chemical calculations and thermochemical estimates. From the analysis of experimental enthalpy of formation measurements, the group increment temperature adjusted enthalpy of sublimation, quantum chemical calculations, and difference quantity derived estimates, our recommended value for the gas phase enthalpy of formation of methionine is –(430?±?10) kJ·mol?1. Copyright © 2012 John Wiley & Sons, Ltd.Methionine (2-amino-4-(methylthio)butanoic acid) is a sulfur-containing essential ?-amino acid. Despite its importance, its enthalpy of formation is contentious. From the analysis of experimental enthalpy of formation measurements, the group increment temperature adjusted enthalpy of sublimation, quantum chemical calculations, and difference quantity derived enthalpy estimates, our recommended value for the gas phase enthalpy of formation of methionine is -(430 ± 10) kJ·mol-1.

Posted on 17 May 2012 | 5:01 am

Additivity of current density patterns in altan-molecules

Four molecules of the recently introduced class of altan-molecules have been studied. They are the altan homologues of naphtalene, pyrene, pyracylene, and kekulene. Their optimized geometries are bowl shaped. The smaller three have current density patterns with an outer paratropic current, as expected for isolated [4n]annulenes, whereas the larger displays an outer weak diatropic circulation. Computed magnetizabilities are consistent with the current density pattern. Copyright © 2012 John Wiley & Sons, Ltd.Conjugated molecules with outgoing C-H bonds can be enlarged to their altan-homologues, replacing H atoms with acetylenic groups C-C-H and bonding them in such a manner that the carbon atoms of the parent system are bound to alternating carbon atoms only of an external annulene. These molecules can be expected to preserve the paratropic character of the external 4n loop, as exemplified here in three cases.

Posted on 17 May 2012 | 4:36 am

Issue Information

No abstract is available for this article.

Posted on 16 May 2012 | 11:40 am

Effects of acidity and emulsifier concentration on the distribution of vitamin C in a model food emulsion

We have analyzed the effects of acidity and emulsifier concentration on the distribution of ascorbic acid or vitamin C (VC) in a model emulsion prepared by mixing octane, acidic (HCl) water and the non-ionic surfactant hexaethyleneglycol monododecyl ether (C12E6). VC is oil insoluble, and only the partition constant between the interfacial and aqueous regions (PWI) is needed to describe its distribution within the emulsion. The PWI values are calculated from the kinetic analyses of the variation of the observed rate constant (kobs) determined electrochemically for the reaction between the hydrophobic 4-hexadecylbenzenediazonium (16-ArN2+) ions and VC with the emulsifier concentration. The determined PWI values (PWI?=?4–25) are low in comparison with those obtained for more hydrophobic antioxidants like vitamin E, and pH dependent, decreasing upon decreasing the acidity of the medium. These low values are reflected in the distribution of VC so that a large fraction (>70%) is located in the aqueous region at low emulsifier concentrations. At a given pH, the %VC in the aqueous region decreases upon increasing the emulsifier concentration; meanwhile, at a given emulsifier concentration %VC increases upon lowering the acidity. The intrinsic rate constants in the interfacial region kI for the reaction between 16-ArN2+ and VC have also been determined at different acidities. Their variation with the pH follows an upward bend curve, suggesting an inverse dependence of kI with the acidity of the medium, in keeping with the proposed mechanism for the reaction. The results may be of some interest to the food and pharmaceutical industries because of the extensive use of VC as antioxidant or preservative and because of the health benefits of VC. Copyright © 2012 John Wiley & Sons, Ltd.The distribution of ascorbic acid (vitamin C [VC]) in an emulsion composed of octane, acidic water and the non-ionic surfactant hexaethyleneglycol monododecyl ether (C12E6) has been determined by employing a kinetic method that exploits the reaction between a hydrophobic arenediazonium, 16-ArN2+ and VC. The fraction of VC in the interfacial region increases upon increasing the surfactant volume fraction and on increasing the acidity of the medium.

Posted on 16 May 2012 | 4:40 am

Facile preparation of ruthenium nanoparticles with activity in hydrogenation of aliphatic and aromatic nitriles to amines

The thermal decomposition of [Ru3(CO)12] allowed the production of ruthenium nanoparticles (NPs) with a unique activity for this metal in the hydrogenation of aliphatic nitriles, yielding the corresponding tertiary amines (selectivity >80%) and aromatic nitriles to secondary amines and imines at 140?°C, employing a moderate hydrogen pressure. The use of several stabilizing agents such as sodium dodecylsulfate and sodium 2-ethylhexanoate allowed the stabilization of ruthenium NPs (Ru-NPs), and further reactivity in the hydrogenation process was assessed. The Ru-NPs were characterized by transmission electron microscopy and exhibited an average size of 2–3?nm. Copyright © 2012 John Wiley & Sons, Ltd.Ruthenium NPs of 2–3?nm size are active in the nitrile hydrogenation of alkyl and aryl nitriles, to selectively yield tertiary amines for the first and secondary amines and imines for the latter.

Posted on 15 May 2012 | 6:15 am

The basicity of substituted N,N-dimethylanilines in solution and in the gas phase

Basicities of a number of ring-substituted N,N-dimethylanilines (DMA) and some related bases in water, acetonitrile, and THF and in the gas phase have been experimentally determined. Gas-phase basicities of DMAs and related bases were calculated at DFT B3LYP 6-311+G** and G3(MP2) levels. Structure–Basicity relationships in these four media were discussed. By comparison of gas-phase basicity shifts induced by stepwise substitution, starting from ammonia and ending at triphenylamine, it was observed that solvent effects of water and acetonitrile exceed the structural effects on intrinsic basicity. It was shown that the influence of substituents in the phenyl ring on DMA basicity is reduced by two to three times when going from the gas phase into the abovementioned condensed media. In the gas phase, 4-NO, 4-NO2, 4-CN, 4-COMe, and 4-CHO substituted DMAs protonate on substituent, whereas in solvents, only 4-NO-DMA probably protonates on substituent. The sensitivity of DMA basicity toward substitution in the phenyl ring was compared with the related family of phenylphosphazene bases, and it was found that the substituent effect is stronger in DMAs by 1.35 times in acetonitrile, 1.45 times in the gas phase, 2.0 times in THF, and 2.6 times in water. Copyright © 2012 John Wiley & Sons, Ltd.Basicities of ring-substituted N,N-dimethylanilines (DMA) and some related bases in water, acetonitrile, and THF and in the gas phase have been experimentally determined. By comparison of gas-phase basicity shifts induced by stepwise substitution, starting from ammonia and ending at triphenylamine, it was observed that solvent effects of water and acetonitrile exceed the structural effects on intrinsic basicity. In the gas phase, 4-NO, 4-NO2, 4-CN, 4-COMe, and 4-CHO substituted DMAs protonate on substituent, whereas in solvents, only 4-NO-DMA will probably protonate on substituent.

Posted on 11 May 2012 | 12:42 pm

Substituent effects on indirect carbon–carbon couplings, J(CC), in substituted thiophenes, pyrroles, and furans studied by experiment and theory

An excellent linear correlation is found between a large set of experimental spin–spin carbon–carbon coupling constants, J(CC), in thiophene, pyrrole, and furan systems and the corresponding B3PW91/6-311++G(2d,p)//B3PW91/6-311++G(2d,p) calculated estimates. The correlation does not differ significantly from the simplest relationship possible, J(CC)exp.?=?J(CC)calcd., within a small and random spread of about 1?Hz. There are 285 experimental values considered, and 202 out of these are new and come from the present work. The character of the correlation indicates that rovibronic effects on aromatic J(CC)'s, and those of nuclear motions on aromatic J(CC)'s are practically negligible. All of this is in a perfect agreement with our recent extensive studies on aromatic J(CC)'s in pyridine and benzene ring systems. As has been shown by computations, not only large one-bond couplings but also almost all long-range ones occurring between the carbons of the heteroaromatic rings are, with a few exceptions, positive. Significant substituent effects experimentally observed in the one-bond as well as long-range couplings are very accurately reproduced by the computation. The experimental coupling magnitudes vary from ca. 1 to 98?Hz. The J(CC)'s computed for the model variously substituted trimethylsilyl and fluoro derivatives, which are not easily accessible experimentally, span a range of about 130?Hz, from ca. ?2 in up to ca. +125?Hz . Copyright © 2012 John Wiley & Sons, Ltd.The simplest relationship possible, J(CC)exp.?=?J(CC)calcd., is found between a set of 285 experimental spin–spin couplings, J(CC), within thiophene, pyrrole, and furan ring systems and the corresponding B3PW91/6-311++G(2d,p)//B3PW91/6-311++G(2d,p) computations, within a small and random spread of about 1?Hz, over a range of 1 through 98?Hz.

Posted on 10 May 2012 | 8:58 am

Prediction of the experimental regioselectivity of C60 fullerene bis-adducts

Regiochemical control of multiple additions to the fullerene core of C60 is attractive and feasible but not fully understood. Specifically, the second nucleophilic addition of bis-adducts produces unpredicted products. Here, we sought to elucidate the extent to which the first addition influenced, electronically and structurally, the regioselectivity of the second nucleophilic attack on C60 fullerene by exploring three combinations of N-(diphenyl-methylene)glycinate and malonate addends connected through 1,3-benzenedimethanol. Molecular orbitals of C60 and the N-(diphenyl-methylene)glycinate and malonate mono-adducts were determined at the B3LYP/6-31G**/B3LYP/3-21G* level of theory. We observed that employing only the lowest unoccupied molecular orbital (LUMO) coefficients was not sufficient to understand the regioselectivity of the tethered second addend. Also, structural perturbations due to the presence of the first addend were largely local. These findings suggested that the selectivity should be explained by the combination of the orbital coefficient and the thermodynamic factors. Thus, the structure and the stability of 21 bis-adducts of C60 fullerene, containing bis-methano, bis-dihydropyrrole, or methanodihydropyrrole, were optimized at the PCM/B3LYP/6-31G*//PCM/B3LYP/3-21G*//B3LYP/3-21G* level of theory. In all cases, the relative energies and Boltzmann populations of the bis-adducts were in agreement with the main regioisomers obtained experimentally. This study showed that the regioselectivity of the second addition is governed by electronic contributions (LUMO coefficients) and by the strong effect of the strain produced by the linker and the addends (three-membered and/or five-membered rings). This insight should be taken into consideration when designing the synthesis of functionalized fullerenes. Copyright © 2012 John Wiley & Sons, Ltd.Regiochemical control of multiple additions to the fullerene core of C60 is attractive and feasible but not fully understood. Specifically, the second nucleophilic addition of bis-adducts produces unpredicted products. This study shows that the regioselectivity of the second addition is governed by electronic contributions and by the strong effect of the strain aroused by the linker and the addends (three-membered and/or five-membered rings). This insight should be taken into consideration when designing the synthesis of functionalized fullerenes.

Posted on 7 May 2012 | 5:18 am

Synthesis and characterization of new derivatives of azulene, including experimental and theoretical studies of electronic and spectroscopic behavior

Four new azulene derivatives featuring two ?-conjugated and sulfur terminated rigid side arms in the 1,3-positions of the azulene core were designed, synthesized, and characterized. The syntheses were carried out on the basis of a palladium cross-coupling procedure in the key reaction step. The crystal structures of the target compound 3 and of the intermediate compounds 7 and 9 have been studied. Supporting density functional theory calculations reveal insights into the electronic properties of the particular azulenes. The comparison of measured UV–Vis, infrared, and Raman data to calculated values allowed assignment of major spectral features to the particular structural differences of the new compounds and comparison to plain azulene. Cyclic voltammetry measurements were performed showing the redox characteristics of the new azulene derivatives. Copyright © 2012 John Wiley & Sons, Ltd.New derivatives of azulene featuring ?-conjugated and sulfur terminated rigid side arms in the 1,3-positions were synthesised, crystal structures were elucidated, and the electronic and photophysical properties were studied experimentally and by density functional theory calculations.

Posted on 4 May 2012 | 3:43 am

Theoretical study of substituent effects on the gas-phase stabilities of phenoxide anions

Relative gas-phase stabilities of 25 kinds of ring-substituted phenoxide anions were determined theoretically using proton transfer reactions. The energies and geometries of phenoxide anions and their corresponding phenols, which are involved in the reactions, were calculated at the B3LYP/6-311+G(2d,p) level of theory. The comparison of the obtained substituent effects with those of benzylic anions revealed that the stabilities of phenoxide anions were governed by the inductive effect and two additive effects of ring substituents in a manner similar to that of benzylic anions. Statistical analyses elucidated that the behavior of the substituent effects of phenoxide anions as well as benzylic anions can be described quantitatively by three terms using an extended Yukawa-Tsuno equation: ??EX=??0+r???¯R?+s??¯S. Through examinations of the structural changes and charge distributions in the anions, these additional effects were identified as the resonance and saturation effects, which stabilize anions by the para?+?R groups and the electron-releasing groups, respectively. Copyright © 2012 John Wiley & Sons, Ltd.The substituent effects on the gas-phase stabilities of phenoxide anions were compared with those of benzylic anions. It was found that the stabilities of all these anions were governed by the inductive, resonance, and saturation effects, and the substituent effects can be correlated with an extended Yukawa-Tsuno equation.

Posted on 3 May 2012 | 4:05 am

Acidities of strong neutral Brønsted acids in different media

Acidities of different families of acids are examined in media of different physical and chemical nature: water, acetonitrile (AN), 1,2-dichloroethane (DCE) and the gas phase, with special emphasis on strong acids. Included are OH (carboxylic acids, alcohols, and phenols), NH (sulfonamides, imides), and CH (phenylmalononitriles, etc.) acids as well as HCl, HBr, and HI. Dependence of the acidity trends on moving from water to the gas phase on the nature of the acidity center, and the molecular structure are analyzed. The acidity orders are different in water, AN, DCE, and the gas phase. In some cases the differences are dramatic. AN and DCE display similar acidity order in the set of the investigated acids. It is demonstrated that the decisive factor for the behavior of the acids when transferring between different media is the extent of charge delocalization in the anion and that the recently introduced weighted average positive sigma parameter in spite of its simplicity enables interpretation of the trends in the majority of cases. Copyright © 2012 John Wiley & Sons, Ltd.Acidity orders of acids are different in water, acetonitrile, 1,2-dichloroethane, and gas phase. The acidity order trends of acids depend on the extent of charge delocalization in their anions and can be interpreted using the weighted average positive sigma parameters of the anions.

Posted on 3 May 2012 | 3:53 am

Theoretical study of keto–enol tautomerism by quantum mechanical calculations (the QM/MC/FEP method)

In this study, the tautomeric equilibrium between the keto and enol forms has been studied for five typical ketones and aldehydes: i-butanal, acetaldehyde, acetone, acetylacetone, and dimedone. The level of theory used in the gas-phase calculation was Becke, three-parameter, Lee–Yang–Parr/6-311G(d,p)//Becke, three-parameter, Lee–Yang–Parr/6-31G(d). The free energies of solvation were included in the calculation by using the free-energy perturbation method based on Monte Carlo simulation, that is, the quantum mechanical/Monte Carlo/free-energy perturbation method. Three different models, incorporating no-water, one-water, and two-waters, were adopted. The results showed that in the gas phase the addition of water molecules to the reaction mechanism caused the activation barriers (?G‡gas) to decrease by half relative to the water-free mechanism, but there was no effect on the relative difference in free energy, ?Ggas. The solvation effects (?Gsol), based on quantum mechanical/Monte Carlo/free-energy perturbation calculations, were added to those of the gas-phase results of the one-water and two-waters models. The two-waters model produced values that were very consistent with the experimental data for all of the tautomers. The differences in the relative Gibbs free energy (?Grxn) were less than 1.0?kcal?mol–1. In summary, the inclusion of solvent molecules in gas-phase calculations plays a very important role in producing results consistent with experimental data. Copyright © 2012 John Wiley & Sons, Ltd.The tautomeric equilibrium between the keto and enol forms has been studied for five typical ketones and aldehydes. The free energies of solvation were included in the calculation by using quantum mechanical/Monte Carlo/free-energy perturbation method. The two-water model using quantum mechanical/Monte Carlo/free-energy perturbation method produced values that were very consistent with the experimental data for all of the tautomers.

Posted on 3 May 2012 | 3:23 am

1,3,5-tricorannulenylbenzene: stereochemistry, reduction and supramolecular dimerization of a branched oligocorannulene

Oligocorannulenes are polyarenes composed of several corannulene units that are covalently linked. Their behavior arises both from the diverse properties of each corannulenyl unit and from the interactions between them. In this paper, we present the synthesis, stereochemistry, reduction, and self-assembly properties of a novel type of oligocorannulene: branched 1,3,5-tricorannulenylbenzene. Several stereodynamical elements combine to give rich stereochemistry: bowl-to-bowl inversion, rotation about the aryl-aryl single bonds, and residual stereoisomerism of molecular propellers. Reduction with lithium metal yields an intermediate hexaanion and ultimately produces a highly charged dodecaanion. Self-diffusion NMR demonstrates that the dodecaanion undergoes supramolecular dimerization through charged polyarene stacking, wherein two molecules are linked at all three contact points. The preference for dimerization over dendrimerization is attributed to an entropic effect. The dimer is found to undergo complex structural dynamics, as well as ion-pairing dynamics, as revealed by variable-temperature 1H- and 7Li-NMR. Copyright © 2012 John Wiley & Sons, Ltd.A novel branched oligocorannulene is synthesized, and its redox and supramolecular chemistry are elucidated using experimental and theoretical methods.

Posted on 3 May 2012 | 3:15 am

Remote substituent effects on homolytic Fe-N bond energies of p-G-C6H4NHFe(CO)2(?5-C5H5) and p-G-C6H4(COMe)NFe(CO)2(?5-C5H5) studied using Hartree–Fock and density functional theory methods

The nature and strength of metal–ligand bonds in organotransition–metal complexes is crucial to the understanding of organometallic reactions and catalysis. The Fe-N homolytic bond dissociation energies [?Hhomo(Fe-N)?s] of two series of para-substituted Fp anilines p-G-C6H4NHFp [1] and p-G-C6H4N(COMe)Fp [2] were studied using the Hartree–Fock (HF) and the density functional theory methods with large basis sets. In this study, Fp is (?5-C5H5)Fe(CO)2 and G are NO2, CN, COMe, CO2Me, CF3, Br, Cl, F, H, Me, MeO and NMe2. The results show that BP86 and TPSSTPSS can provide the best price/performance ratio and accurate predictions of ?Hhomo(Fe-N)?s. B3LYP can also satisfactorily predict the ? and remote substituent effects on ?Hhomo(Fe-N)?s [??Hhomo(Fe-N)?s]. The good correlations [r?=?0.96 (g, 1), 0.99(g, 2)] of ??Hhomo(Fe-N)?s in series 1 and 2 with the substituent ?p+ constants imply that the para-substituent effects on ?Hhomo(Fe-N)?s originate mainly from polar effects, but those on radical stability originate from both spin delocalization and polar effects. ??Hhomo(Fe-N)?s(1,2) conform to the captodative principle. Insight from this work may help the design of more effective catalytic processes. Copyright © 2012 John Wiley & Sons, Ltd.The homolytic Fe-N bond dissociation energies of 12 para-substituted anilinyldicarbonyl(?5-cyclopentadienyl)iron and 12 para-substituted ?-acetylanilinyldicarbonyl(?5-cyclopentadienyl)iron were studied using HF and density functional theory methods with large basis sets. The results show that BP86 and TPSSTPSS can provide the best price/performance ratio and more accurate predictions in the study of ?Hhomo(Fe-N)?s. The B3LYP method satisfactorily predicts the ? and remote substituent effects on ?Hhomo(Fe-N)?s.

Posted on 2 May 2012 | 8:58 am

One-pot three-component preparation of novel selenium-containing spiroketals

The one-pot three-component preparation of novel selenium-containing spiroketals with potential biological activity is described. When in situ prepared selenium dichloride is reacted with either propargyl or homopropargyl alcohols in the presence of acetone, two competitive reactions take place, namely 1,2-addition to the triple bond with the formation of the corresponding functionalized divinyl selenides in a completely regio- and stereospecific manner and selenylation at the ?-position of the ketone, leading to the novel selenium-containing spiroketals. An alternative approach to the synthesis of the selenium-containing spiroketals with substantial increase of the chemical yield involves the reaction of alkynyl acetals with SeCl2. The structure of (4E,11E)-4,11-bis(chloromethylene)-1,8-dioxa-5,12-diselenaspiro[6.6]tridecane was unambiguously confirmed by X-ray analysis. These novel selenium-containing spiroketals were tested for regulation of glucose uptake in L6 myotubes. Copyright © 2012 John Wiley & Sons, Ltd.The one-pot three-component preparation of novel selenium-containing spiroketals with potential biological activity which involves the reaction of selenium dichloride with either propargyl or homopropargyl alcohols in the presence of acetone is described.

Posted on 2 May 2012 | 8:45 am

PROFILE: Early Excellence in Physical Organic Chemistry

Posted on 2 May 2012 | 4:46 am

Isokinetic relationships for SNAr reactions in different solvents

On the basis of the changes of the activation parameters ??Hint? and ??S? and the statistically significant isokinetic and compensation temperatures, Tiso and Tcomp, for the SNAr reactions of the benzene derivatives with anionic and neutral nucleophiles at their structural variations in different solvents, the general isokinetic relationship ??Hint? versus (Tiso???Tcomp)?????S? was found. The existence of this isokinetic relationship for a series of SNAr reactions suggests the possibility to predict the Tiso value or to interpret it when it is experimentally determined. Copyright © 2012 John Wiley & Sons, Ltd.The general isokinetic relationship was found for SNAr reactions: ??Hint??=?(?0.49?±?0.29)?+?(0.97?±?0.02)(Tiso???Tcomp) ??S? (r?=?0.995, s?=?1.35, n?=?20) where Tiso and Tcomp are the isokinetic and compensation temperatures. The ??Hint? and ??S? values are the changes of the internal part of the activation enthalpy and activation entropy, respectively.

Posted on 27 April 2012 | 10:39 am

?LG values in mechanistic studies on the transesterification of RNA models and their application in a metal ion complex promoted transesterification

Studies on the mechanism of transesterification reactions of nucleoside 3?-phosphoesters using ?LG values as a mechanistic tool are discussed in this mini review. Our recent observation that a dinuclear metal ion complex enhances both the cleavage and isomerisation is discussed in the light of previous studies. Copyright © 2012 John Wiley & Sons, Ltd.?LG values reported for the transesterification of phosphodiester bonds ribonucleoside 3?-alkyl and 3?-dialkyl phosphates under various conditions, as well as the mechanistic interpretation of the values, is discussed in this review.

Posted on 27 April 2012 | 10:35 am

Mechanism for the reactions of sulfides and sulfoxides with hypochlorites: racemization and oxygen exchange of oxysulfonium salts and sulfoxides

Density functional theory computations have been performed on the oxidations of sulfides and sulfoxides with hypochlorite ion (OCl?), hypochlorous acid, and alkyl hypochlorites to study the mechanism of the reactions. The OCl? anion transforms sulfides to sulfoxides and sulfoxides to sulfones in oxygen transfers. The oxygen atom of QOCl hypochlorites (Q?=?H, Me, t-Bu) attacks at the sulfur atom of the substrates, and oxysulfonium cation intermediates are formed; the departure of the leaving Cl? is catalyzed by soft Lewis acids. The structures of the early transition states are determined by highest occupied molecular orbital–lowest unoccupied molecular orbital interactions. The sulfur compounds are the electron acceptors in the reaction with OCl?, but they are the electron donors in the reactions with QOCl. The attack of Cl? at the oxygen atom of oxysulfonium cation intermediates leads to the sulfide and QOCl precursors and can result in racemization, oxygen exchange, and reduction of oxysulfonium salts in reversible reactions. The attack of Cl? at the sulfur atom of oxysulfonium salts produces ?4-sulfane intermediates. Oxysulfonium cations can be transformed into sulfoxide products with the attack of Cl? or water at the ?-carbon atom of the O-alkyl group. The attack of water at the sulfur atom of oxysulfonium cation leads to hydrolysis or oxygen exchange reactions. Racemization and oxygen exchange of sulfoxides proceeds in similar reactions, through the formation of hydroxysulfonium cation intermediates in acidic media in the presence of Cl?. Chlorosulfonium cations are of very high energy; their intermediacy can be ruled out in the reactions of sulfides with hypochlorites. Copyright © 2012 John Wiley & Sons, Ltd.The density functional theory calculations supported that the oxygen atom of hypochlorites attacks at the sulfur atom of sulfides and sulfoxides, resulting in the formation of oxysulfonium cation intermediates. The reactions of alkoxysulfonium cations and protonated sulfoxides with Cl? or water can lead to racemization, oxygen exchange, reduction, and hydrolysis. The intermediacy of chlorosulfonium cations of very high energy can be ruled out.

Posted on 26 April 2012 | 6:05 am

Synthesis of sterically hindered antipyrines for intramolecular charge transfer facilitated sensing

A new series of sterically hindered antipyrine derivatives have been synthesized by Schiff base condensation for potential fluorescence sensing applications. Their optical, thermal, and electrochemical properties were fully characterized. Broad absorption spectra were observed as a result of the strong intramolecular charge transfer between antipyrine and sterically hindered conjugated moiety. Reversible one electron oxidation couple was observed for most compounds, with carbazole, phenothiazine, and bithiophene functionalized derivatives exhibiting extra reversible one one-electron oxidation process at higher positive potentials. Cabazole functionalized antipyrine derivative APCZ presents good fluorescence sensing for Co2+ and Cu2+, with 3.41-fold enhancement in fluorescence intensity and 2-fold enhancement in fluorescence quantum yield observed. Copyright © 2012 John Wiley & Sons, Ltd.Cabazole functionalized antipyrine derivative APCZ presents good fluorescence sensing for Co2+ and Cu2+, with 3.41-fold enhancement in fluorescence intensity and 2-fold enhancement in fluorescence quantum yield observed.

Posted on 26 April 2012 | 5:55 am

Three-photon absorption properties of a novel symmetrical Carbazole derivative having terminal 1,10-phenanthroline rings via carbon–nitrogen (C?=?N) double bond

Three-photon absorption (3PA) properties of symmetric-type carbazole derivatives show great potential for application in light-activated therapy and optical limiting. A novel symmetrical carbazole derivative (abbreviated as POCP) with end-groups of 1,10-phenanthroline rings as the donor moieties, chained via carbon–nitrogen (C?=?N) double bond, has been synthetized and its three photon absorption properties has been also determined by using a Q-switched Nd: YAG laser pumped with 30?ps pulses at 1064?nm in dimethylformamide. The measurement of 3PA cross-section of this compound is performed by open aperture Z-scan and ?3PA is 481?×?10–78?cm6 • s2/photon2 for the transition S0???S1. The influence of the molecular structure of this compound on three-photon absorption cross-sections is discussed micromechanically by Austin model 1 and Zerner's Intermediate Neglect of Differential Overlap/S method. Copyright © 2012 John Wiley & Sons, Ltd.3,6-Bisphenanthrolinimidazole-N-n-octylcarbazole (POCP) with phenanthroline end-group, chained via C?=?N, has been first synthesized and the three-photon absorption (3PA) effect of POCP was demonstrated using a ps pulse at 1064?nm by Z-scan. 3PA properties of POCP were theoretically investigated by Austin model 1 and Zerner's Intermediate Neglect of Differential Overlap method. Phenanthroline as end-group and chained via C?=?N influence 3PA of POCP.

Posted on 23 April 2012 | 1:55 pm

A computational study of the thermolysis of ?-hydroxy ketones in gas phase and in m-xylene solution

Theoretical calculations at the M05-2X/6-31+G(d) level of theory have been carried out in order to explore the nature of the mechanism of the thermal decomposition reactions of the ?-hydroxy ketones, 4-hydroxy-2-butanone, 4-hydroxy-2-pentanone, and 4-hydroxy-2-methyl-2-pentanone in gas phase and in m-xylene solution. The mechanism proposed is a one-step process proceeding through a six-membered cyclic transition state.A reasonable agreement between experimental and calculated activation parameters and rate constants has been obtained, the tertiary?:?secondary?:?primary alcohol rate constant ratio being calculated, at T?=?503.15?K, as 5.9:4.7:1.0 in m-xylene solution and 44.1:5.0:1.0 in the gas phase, compared with the experimental values, 3.7:1.3:1.0 and 13.5:3.2:1.0, respectively.The progress of the thermal decomposition reactions of ?-hydroxy ketones has been followed by means of the Wiberg bond indices. The lengthening of the O1–C2 bond with the initial migration of the H6 atom from O5 to O1 can be seen as the driving force for the studied reactions. Calculated synchronicity values indicate that the mechanisms correspond to concerted and highly synchronous processes. The transition states are “advanced”, nearer to the products than to the reactants. Copyright © 2012 John Wiley & Sons, Ltd.Theoretical calculations at the M05-2X/6-31+G(d) level of theory have been carried out in order to explore the nature of the mechanism of the thermal decomposition reactions of the ?-hydroxy ketones in the gas phase and in m-xylene solution. The mechanism proposed is a one-step process proceeding through a six-membered cyclic transition state.

Posted on 13 April 2012 | 8:00 am

Comparative studies on reaction of bis(p-nitrophenyl) phosphate and ?-nucleophiles in cationic micellar media

We studied the cleave of bis(p-nitrophenyl) phosphate (BNPP) over a pH range of 7.0–12.0 in the presence of cationic micelles of cetyldiethylethanolammonium bromide, cetyldimethylethanolammonium bromide, cetylpyridinium bromide, cetyltrimethylammonium bromide, and cetylpyridinium chloride by using different ?-nucleophiles, viz acetohydroxamate, benzohydroxamate, salicylhydroxamate, butane-2,3-dione monooximate, and ?-benzoin oximate ions. With the use of ?-nucleophiles in cationic micellar media, the hydrolytic cleavage of BNPP was found to be approximately 105-fold faster than its spontaneous hydrolysis. All reactions followed pseudo-first-order kinetics. The effect of various concentrations of cationic micelles for the reaction of BNPP and ?-nucleophiles has been studied. The variation of kobs values of the reactions depends on the micellar structure, that is, head groups, hydrophobic tail length, and counter ion. Copyright © 2012 John Wiley & Sons, Ltd. Cationic micellar media accelerate the cleavage of bis(p-nitrophenyl) phosphate with ?-nuleophiles than its spontaneous hydrolysis. Kinetic aspects of micellar catalysis represent a significant contribution to the observed rate in this case of hydrolysis. Surfactant-based system provides better understanding of the hydrolytic cleavage of phosphate diester.

Posted on 3 April 2012 | 11:58 am

Incorporation of steroidal biomarkers into petroleum model compounds

A series of fully-substituted 5,6-benzoquinolines covalently fused to the chlolestane framework has been synthesized to serve as model compounds for the poorly characterized components found in the heaviest fractions of petroleum, namely the asphaltenes. Acid-catalyzed cyclocondensation of an aromatic imine with 5-?-cholestan-3-one gives 5,6-benzoquinoline cycloadducts incorporating the steroidal biomarker fused to the benzoquinoline system at the 3,4-positions, with various pendent substituents in the 2-position. X-ray crystallography of three such derivatives shows that the solid-state geometries of these molecules are quite similar, and packing in the solid state appears to be dominated to a great extent by the nature of the pendent substituent. X-ray crystallography and solution phase circular dichroism spectroscopy together suggest that the conjugated benzoquinoline moiety adopts a helical conformation. Copyright © 2012 John Wiley & Sons, Ltd.2, 3, 4-Substituted 5,6-benzoquinolines have been synthesized via the condensation reaction of an imine with 5-?-cholestan-3-one (Kozlov's procedure). With a steroid biomarker fused to the benzoquinoline core, the products serve as model compounds for components found in the heaviest fractions of petroleum (the asphaltenes). X-ray crystallography shows that packing appears dominated by the nature of the pendent substituent, while X-ray crystallography and circular dichroism spectroscopy suggest that the conjugated benzoquinoline group adopts a helical conformation.

Posted on 3 April 2012 | 11:53 am

New guests for the cucurbit[8]uril host. Formation of G2H ternary complexes

On the basis of the highly stable G2H (2?:?1) ternary complex formed by two methyl viologen cation radicals inside the cavity of cucurbit[8]uril, we prepared three monocationic 4-phenylpyridinium derivatives: 1-(hydroxyethyl)-4-phenyl-pyridinium (1+) bromide, 1-(octaethyleneglycol)-4-phenyl-pyridinium (2+) chloride, and 4-[4-(methoxymethoxy)phenyl]pyridinium (3+) iodide, as possible guests for 2?:?1 complexation inside cucurbit[8]uril. We also investigated a fourth monocationic guest (4+), in which a central vinylidene group is inserted to elongate the 4-phenyl-pyridinium residue. Using 1H NMR and UV–Vis spectroscopic data and mass spectrometric data, we obtained unequivocal evidence for the formation of G2H (2?:?1) ternary complexes in all cases. The stoichiometry of the complexes was further verified by continuous variation (Job) plots, and in some cases, high resolution ESI-MS spectrometric data. Diffusion coefficient measurements, using 1H NMR pulse gradient spin echo techniques, yielded values consistent with the formation and expected structures of the ternary complexes. Copyright © 2012 John Wiley & Sons, Ltd.A set of 4-phenyl-pyridinium derivatives were prepared and characterized as guests for the cucurbit[8]uril molecular receptor. These guests were found to form stable ternary supramolecular complexes in which two identical guests share the space inside the host cavity.

Posted on 3 April 2012 | 11:48 am

Trapping intermediates and comparing relative reactivities: a DFT M06-2X study on Diels–Alder cycloadditions of butadiene to C60 and C70

All possible types of Diels–Alder cycloadditions of 1,3-cis-butadiene to C60 (2 in total) and to C70 (8 in total) were theoretically investigated by the M06-2X density functional method in gas phase and solutions. An intermediate between the reactant and the transition state was located for each reaction. These intermediates except one have not been experimentally or theoretically reported before. The reactivities of the 10 reactions in both the gas phase and solutions were systematically compared based on the calculated results. The present conclusion agrees with the experimental observations and partly disagrees with the previously theoretical conclusion. Copyright © 2012 John Wiley & Sons, Ltd.All possible types of Diels-Alder (DA) cycloadditions of 1,3-cis-butadiene to C60 and C70 were theoretically investigated by the M06-2X density functional method in gas phase and solutions. An intermediate between the reactant and the transition state was located for each reaction. The present conclusion agrees with the experimental observations and partly disagrees with the previously theoretical conclusion.

Posted on 7 March 2012 | 3:20 pm

Structure prediction and aromaticity of acenes and phenylenes series. A combination method between asymmetric Kekulé structures

The combination method between asymmetric Kekulé structures (CMAK) was expanded to the structure prediction of a series of acenes and phenylenes. The structures predicted by the CMAK correspond to those calculated by the ab initio molecular orbital (MO) and B3LYP density functional methods. The CMAK can characterize the structures. The aromaticity for the whole ring and each ring in the treated compounds was estimated by the index of deviation from aromaticity (IDA). The estimated aromaticity corresponds fairly well to the characterization of the structure predicted by the CMAK. The predicted structures of linear acenes reveal the aromaticity of the central ring and also explain its reactivity. The side ring in [n]phenacene shows high aromaticity. The critical difference in the structure and aromaticity for the central six-membered ring of the isomers benzo[1,2:4,5]dicyclobutene and benzo[1,2:5,6]dicyclobutene was explained. Copyright © 2012 John Wiley & Sons, Ltd.The stable structures of polycyclic conjugated hydrocarbons were predicted by the combination method between asymmetric Kekulé structures (CMAK). The predicted structures are consistent with the aromaticity estimated by IDA. The structure prediction by CMAK explained the aromaticity and reactivity of acenes.

Posted on 7 March 2012 | 1:28 pm

Construction of phthalocyanine-terminated polystyrene nanoarchitectures

Nanostructuring of phthalocyanine-based materials is a powerful tool towards the preparation of new materials with outstanding properties. It has been previously shown that porphyrin-functionalized and phthalocyanine-functionalized polymers give rise to nanosized aggregates. With the goal in mind of searching new phthalocyanine-containing polymeric materials that are able to self-organize into stable supramolecular nanostructures, we have prepared unsymmetrically functionalized Zn(II) phthalocyanines that are able to behave as initiators in the atom transfer radical polymerization of styrene. Hybrid phthalocyanine–polystyrene materials of different tail lengths have been prepared, and their self-organization behavior was studied by means of UV–Vis spectroscopy and transmission electron microscopy. Copyright © 2012 John Wiley & Sons, Ltd.Polymerization of styrene by ATRP using a series of Zn(II) phthalocyanine initiators proceeds in a controlled fashion and yields well-defined Zn(II)Pc-polystyrene hybrids with low polydispersities which form well-organized nanoarchitectures.

Posted on 29 February 2012 | 6:20 am

In Silico study of carcinogenic o-Quinone metabolites derived from polycyclic aromatic hydrocarbons (PAHs)

A computational density functional theory study on the structural and electronic properties of several polycyclic aromatic hydrocarbon (PAH) ortho-quinones was performed and the possible mechanism of DNA-adduct formation was analyzed to evaluate its thermodynamic viability. Molecular docking techniques were applied to examine the noncovalent interactions developed when a model PAH ortho-quinone intercalates between the DNA double helix. Quantum-chemical ONIOM (our Own N-layer Integrated molecular Orbital molecular Mechanics) calculations within the structure of a DNA fragment were carried out to evaluate the significant steps of noncovalent complex and covalent adduct formation. The solvent effect was also considered by employing a continuum solvation model. The present calculations suggest that initial noncovalent interactions of the PAH o-quinone within the DNA double helix could determine the feasibility of benzo[a]pyrene-7,8-dione-DNA covalent adduct formation, and that dispersion-corrected functionals are more suitable for locating the noncovalent complex. Copyright © 2012 John Wiley & Sons, Ltd.Structural and electronic properties of several PAH o-quinones were studied computationally by DFT methods. The mechanism of DNA-adduct formation was also analyzed. Molecular docking and ONIOM quantum chemical calculations were performed within the structure of a DNA fragment.

Posted on 21 February 2012 | 5:51 am

Analysis of benzodiaza-15-crown-5 ether derivative binding properties by potentiometric and optical methods

This research concerns the analysis of the binding properties of benzodiaza-15-crown-5 ether derivatives towards different metal ions (Mg(II), Cd(II), Ni(II), Cu(II), Zn(II), Pb(II), Hg(II) and Ag(I)) in acetonitrile and water by potentiometric and optical methods. Benzodiaza-15-crown-5 ether demonstrates high binding affinity towards Hg2+ (lg K11?=?12.7), whereas the stability constants of complexes with other studied cations varied from 3 to 6 logarithmic units. Copyright © 2012 John Wiley & Sons, Ltd.Detailed potentiomentic and optical analysis showed the ability of benzodiaza-15-crown-5 ether derivatives to form complexes with wide series of metal ions (Mg(II), Ca(II), Ba(II), Cd(II), Ni(II), Cu(II), Zn(II), Pb(II), Hg(II) and Ag(I)) of different structures, compositions and stability in water and acetonitrile.

Posted on 17 February 2012 | 10:31 am

Synthesis, structure and solid state properties of benzannulated phenalenyl based neutral radical conductor

We report the preparation, crystallization, and solid state characterization of a new benzannulated phenalenyl radical; a distinctive feature of this molecular solid is the absence of any well defined conducting pathway(s). In the solid state, the radicals exist as ?-dimer and the requisite C···C contacts between dimers are all larger than the sum of the van der Waals distances. The magnetic susceptibility measurement indicates that, in the solid state, the radical remains paramagnetic and the fraction of Curie spins is 0.55 per molecule. Pressed pellet conductivity measurements indicate a value of ?RT?=?0.5?×?10?6?S/cm. The conductivity value is comparable with those of similarly packed phenalenyl based neutral radicals. Copyright © 2012 John Wiley & Sons, Ltd.A stable benzannulated phenalenyl based organic neutral radical has been reported in which we identify the features that limit the conductivity of organic molecular solids. The radical exists as a ?-dimer and the requisite C···C contacts between dimers are all larger than the sum of the van der Waals distances. The radical remains paramagnetic and the fraction of Curie spin available 0.55 per molecule. Pressed pellet conductivity measurements indicate a value of ?RT?=?0.5?×?10?6?S/cm.

Posted on 16 February 2012 | 9:01 am

Influence of the hydrogen bonding on the basicity of selected macrocyclic amines

The optimized minimum-energy geometries of different macrocyclic amines and their protonated structures were determined by using ab initio and density functional theory (DFT) calculations. All the gas phase optimizations and energy calculations were performed at the DFT/B3LYP/6-311++G(d,p) level of theory. The HF/6-31?+?G(d,p) level was used for all single point calculations in the solution phase. Geometry optimizations indicate that the most stable structures are stabilized by intramolecular hydrogen bonds. The proton affinity (PA) of macrocyclic amines is controlled by the strength of intramolecular hydrogen bonds of macrocyclic amines. These hydrogen bonds strongly influence the basicity of heteroatoms in macrocycles. The highest PA value among the studied macrocyclic amines was found to be 264.9?kcal?mol?1 for structure?7. This is comparable with PA of proton sponges such as 1,8-bis(dimethylamino)naphthalene. The solution phase calculations were carried out in the dimethyl sulfoxide solution as a commonly used solvent in organic reactions. Natural bond orbital analysis was performed to calculate the charge transfers and the second-order interaction energies (E(2)) between the donor and acceptor. Quantum theory of atoms in molecules (QTAIM) was also applied to determine the nature of hydrogen bonds. QTAIM studies showed that the intramolecular hydrogen bonds in these structures are electrostatic (closed-shell) interactions as well as partially covalent and partially electrostatic in nature. Copyright © 2012 John Wiley & Sons, Ltd.1) Macrocyclic amines and their protonated structures are stabilized by intramolecular hydrogen bonds.The proton affinity values of studied macrocyclic amines are comparable to those of proton sponges such as 1,8 - bis (dimethylamino) naphthalene. The intramolecular hydrogen bonds in protonated macrocyclic amines are partially covalent and partially electrostatic in nature.

Posted on 14 February 2012 | 4:28 am

From density functional steric analysis and molecular electrostatic potential to the estimation of etherification rate constant

This research extends our more recent work on the application of molecular electrostatic potential as an effective approach in describing the influence of substituent on etherification reaction rate constant of phenol derivatives. Here, in addition to electronic factor, the steric effects have also been considered for our purpose. To analyze steric effects on etherification rate constant, we use the novel energy partition scheme proposed recently by Liu [S. B. Liu, J. Chem. Phys. 2007, 126, 244103], where the total electronic energy is decomposed into three independent components: steric, electrostatic, and fermionic quantum. In this scheme, the steric potential has also been introduced. We first derive a relationship on the basis of density functional theory to show that the etherification rate constant should be proportional to the electrostatic potential on the atomic sites. Then, a bilinear function of molecular electrostatic potential and steric energy or steric potential is proposed for estimation of etherification reaction rate constants. Taking the experimental kinetics data of 30 substituted phenols, the validity of the proposed approach has been verified in position and momentum spaces. It is worth noting that the remarkable good performance of the momentum densities, which for the first time used in calculations of steric energy for a reaction, has been observed. Finally, using the relationship between new energy partition scheme and information theory, applicability of the Shannon entropy as one of the information theoretic measures is also tested for our goal and considerable results were obtained. Copyright © 2012 John Wiley & Sons, Ltd.On the basis of a density-based quantification of the steric effect and the applicability of electrostatic potential at the nucleus, the etherification rate constants of phenol derivatives have been estimated.

Posted on 7 February 2012 | 9:06 am

Substituent effect study on ?c values of the bridge group carbons in disubstituted cinnamyl aniline series

The 13C nuclear magnetic resonance (NMR) chemical shifts ?c of bridge group carbons (C-?, C-?, and C?N) were measured in this work for a wide set of substituted cinnamyl anilines p-XC6H4CH?CHCH?NC6H4Y-p (X?=?NO2, Cl, H, Me, MeO, or NMe2; Y?=?NO2, CN, CO2Et, Cl, F, H, Me, MeO, or NMe2) and were used to study the substituent effect. In the study on 13C NMR chemical shifts of the titled compounds with single substituent changed, for every bridge carbon ?c, the effect of cinnamyl substituent X is opposite to that of aniline substituent Y. That is, the action of the same substituent on different aromatic rings is different from the 13C NMR chemical shifts, and for C-?, C-?, and C?N, the choice of correlation equation depends on the ratio ?F(Y)/?R(Y). When the ratio ?F(Y)/?R(Y) is close to 1, the chemical shifts of bridge carbons can be well correlated with the single-parameter equation; otherwise, it is better to adopt the dual-parameter equation for correlation, and the further the values of ?F(Y)/?R(Y) stray from 1, the more suitable the corresponding ?c values are to be correlated with the dual-parameter equation. In the study on ?c of model compounds with simultaneous variations of substituents X and Y, for ?c(C?N), a multi-parameter correlation equation is obtained, and the substituent cross-interaction item ??2 is suitable to scale the interaction between substituents; however, for ?c(C-? and C-?), the substituent cross-interaction item ??2 is perhaps too small to be observed. The multi-parameter correlation equations can be recommended to predict well the corresponding ?c values of disubstituted cinnamyl anilines. Copyright © 2012 John Wiley & Sons, Ltd.In the study on ?c values of the bridge group carbons in disubstituted cinnamyl aniline series(XPhCH=CHCH=NPhY), with a single substituent changed, the ratio ?F(Y)/?R(Y) decided the choice of correlation equations, and the effect of cinnamyl substituent X is opposite to that of aniline substituent Y; With simultaneous variations of substituents X and Y, the ?c values of all bridge carbons were correlated by multi-parameter correlation equations with parameters ?F and ?R.

Posted on 6 February 2012 | 10:15 am

Photorepair of cyclobutane pyrimidine dimers by 8-oxopurine nucleosides

The 8-oxopurine nucleosides 2?,3?,5?-tri-O-acetyl-8-oxo-7,8-dihydroguanosine (OG) and 2?,3?,5?-tri-O-acetyl-ribosyluric acid (RU) were studied for their ability to mediate the photochemical (??>?300?nm) reversion of cyclobutane pyrimidine dimers to their parent pyrimidines thymine and uracil. The bimolecular reactions of these monomers proceeded at very slow rates compared with recently published work using oligonucleotide contexts; nevertheless, it was possible to make comparisons between the efficacy of OG and RU as photocatalysts as a function of pH. Although RU has a lower redox potential and anionic character, it was only equivalent to OG in facilitating thymine dimer photorepair over a broad pH range. Only OG showed pH-dependent behavior with higher activity at pH 8–9 where the base becomes deprotonated. Despite the overall low activity of OG and RU, the results are instructive with respect to a comparison of the two 8-oxopurines, and support the hypothesis that 8-oxopurine nucleosides may have played primordial roles as precursors to modern-day flavins in redox reactions of the RNA world. Copyright © 2012 John Wiley & Sons, Ltd.The 8-oxopurine nucleosides were studied for their ability to mediate the photochemical (??>?300?nm) reversion of cyclobutane pyrimidine dimers to their parent pyrimidines thymine and uracil. Despite overall low activity, the results are instructive concerning the role of pH and redox potential, and support the hypothesis that 8-oxopurine nucleosides may have played primordial roles as precursors to modern-day flavins in redox reactions of the RNA world.

Posted on 6 February 2012 | 4:15 am

What roles do boron substitutions play in structural, tautomeric, base pairing and electronic properties of uracil? NBO & AIM analysis

The synthesis of modified versions of deoxyribonucleic acid is an area that is receiving much attention. The replacement of the nitrogen atom on the nucleobases with boron atom has provided insight into deoxyribonucleic acid and ribonucleic acid stability, recognition, and replication at the atomic level. In the present research, we investigated a detailed density functional theory study of the structural, tautomeric, base-pairing ability, bond dissociation energy, and electronic properties of two boron analogues (i.e., boron substitutions at 4-position and 5-position of uracil) of uracil nucleobase. The effects of these modifications on theirs acid–base properties have been considered. Our goal is to gather data to help elucidate the structure and electronic properties of boron analogues of uracil.Density functional theory calculations were performed using a nonlocal hybrid B3LYP and 6-311++G (d,p) atomic basis set. The result of calculation revealed that the canonical ‘keto’ form is the most stable tautomeric form of uracil nucleobase and its boronated analogues in the gas phase at density functional theory and second-order Møller–Plesset levels. Hyperconjugation stabilization factors that affect the stability of generated radicals and anions in these molecules were investigated by natural bond orbital analysis. Furthermore, quantum theory of atoms in molecules analysis was performed to extract the bond critical point properties of various base pairs. The electron density can be considered as a good description of the strength of the different types of hydrogen bonding. Copyright © 2012 John Wiley & Sons, Ltd.We investigated a detailed density functional theory study of the structural, tautomeric, base-pairing ability, bond dissociation energy (BDE), and electronic properties of two boron analogues (i.e., boron substitutions at 4-position and 5-position of uracil) of uracil nucleobase and effects of these modifications on theirs acid-base properties.

Posted on 1 February 2012 | 8:36 am

Higher reactivity of 3-pyridinium boronic acid compared with 3-pyridinium boronate ion toward 4-isopropyltropolone in acidic aqueous solution: fundamental reaction analyses for an effective organoboron-based chemosensor

The pKas of 3-pyridylboronic acid and its derivatives were determined spectrophotometrically. Most of them had two pKas assignable to the boron center and pyridine moiety. The pKa assignment performed by 11B nuclear magnetic resonance spectroscopy revealed that both boron centers in 3-pyridylboronic acid [3-PyB(OH)2] and the N-methylated derivative [3-(N-Me)Py+B(OH)2] have strong acidities (pKa?=?4.4 for both). It was found that introduction of a substituent to pyridine-C atom in 3-pyridylboronic acid drastically increased the acidity of the pyridinium moiety, but decreased the acidity of the boron center, whereas the introduction to pyridine-N atom had no influence on the acidity of the boron center. Kinetic studies on the complexation reactions of 3-pyridinium boronic acid [3-HPy+B(OH)2] with 4-isopropyltropolone (Hipt) carried out in strongly acidic aqueous solution indicated that the positive charge on the boronic acid influenced little on its reactivity; 3-HPy+B(OH)2 reacts with Hipt and protonated H2ipt+, and its reactivity was in line with those of a series of boronic acids. Kinetics in weakly acidic aqueous solution revealed that 3-HPy+B(OH)2 reacts with Hipt faster than its conjugate boronate [3-HPy+B(OH)3–], which is consistent with our recent results. The reactivity of 3-(N-Me)Py+B(OH)2 towards Hipt was also examined kinetically; the reactivities of 3-(N-Me)Py+B(OH)2 and 3-(N-Me)Py+B(OH)3– are almost the same as those of their original 3-HPy+B(OH)2 and 3-HPy+B(OH)3–, respectively. Copyright © 2012 John Wiley & Sons, Ltd.A kinetic study on the complexation reaction of cationic 3-pyridinum boronic acid [3-HPy+B(OH)2] having strong acidity with 4-isopropyltropolone (HL) in solution revealed that the trigonal boronic acid has even higher reactivity than the tetragonal zwitterionic boronate ion.

Posted on 30 January 2012 | 12:06 pm

Computational investigations into new fluorescence quenching process induced by complexation of alkali metal ion

A novel fluorescent switchable chemosensor 1, which is composed of an anthracene-modified calix[4]crown in the 1,3-alternate conformation, was calculated by density functional theory and time-dependent density functional theory method. Geometries, molecular orbitals and binding thermal energies were evaluated at the restricted hybrid Becke's three-parameter exchange functional using 6-31?G(d) basis set and relativistic effective core potentials. The metal–ligand and cation–? interactions were investigated acting as two main types of driving force. Our calculations clearly show that solvent effects strongly influence cation selectivity, and K+ selectivity is recovered when even a few waters of hydration are considered. The calculations indicate that because of the photoinduced electron transfer effect, the addition of alkali metal ions have hardly any effect on the fluorescence of ligand 1 under neutral or basic conditions. Also, the high selectivity of ligand 1 for K+ and Rb+, under acidic conditions, the complexed metal ion can result in ammonium ion deprotonation, which leads to quenching of fluorescence of 1•H+. Copyright © 2012 John Wiley & Sons, Ltd.A novel fluorescent switchable chemosensor 1, which is composed of a anthracene-modified calix[4]crown in the 1,3-alternate conformation, was calculated by density functional theory (DFT) and time-dependent density functional theory (TDDFT) method. The results reported here will be helpful for the future design of more sophisticated and elaborate fluorescent chemosensors for molecular recognition.

Posted on 26 January 2012 | 11:26 am

Excited state mixed valence in a dual-bridged three-chromophore system

The optical and resonance Raman spectra of the 2,2?: 6?,2?:6?,6-trioxytriphenyl-amine cation are measured and interpreted. This molecule contains two simultaneous types of coupling between three chromophores and two types of bridging atoms. The first and conventional coupling involves a single nitrogen bridge that couples all three aryl groups. The second is provided by the three oxygen atoms, each of which bridges two adjacent aryl groups. There are two bands in the visible region of the optical absorption spectrum; their assignment and the interpretation of the contributing orbitals and electronic states are described in terms of the neighboring orbital model that explains the effects of the two types of coupling. The bonding changes that take place in the excited electronic states are probed by resonance Raman spectroscopy intensities and analyzed using the time-dependent theory of resonance Raman spectroscopy. The optical absorption spectrum was fit using the measured vibrational frequencies and excited state distortions. The distortions correlate well with the bonding changes predicted by the neighboring orbital model. The resonance Raman data and neighboring orbital model analysis reveal that the two optical absorption bands correspond to charge transfers from aryl groups with different nodal structures in their pi orbitals. Copyright © 2012 John Wiley & Sons, Ltd.The three-chromophore cation (left) has a symmetric charge distribution in its ground state and three equivalent charge-bearing units in its lowest excited state. Coupling between the three units occurs through the central nitrogen and pair-wise through the oxygen. The effects of the coupling on the electronic and resonance Raman spectra are interpreted using the neighboring orbital model.

Posted on 26 January 2012 | 11:21 am

Mechanisms and Möbius strips: understanding dynamic processes in annulenes

Exploratory computational studies on annulenes with planar, Möbius, and two-twist topologies have resulted in new mechanisms to explain facile thermal configuration change (cis-trans isomerization) for medium-sized annulenes ([12]- to [16]annulene). Möbius ?-bond shifting through both aromatic and antiaromatic transition states, two-twist ?-bond shifting, and planar nondegenerate ?-bond shifting can all be invoked to explain experimental results. Moreover, a simple bond-shift rule, which is based on the change in number of trans CC double bonds (?trans), was developed that predicts the topology of the transition state(s) necessary to effect the desired cis-trans isomerization. The bond-shift rule was also applied to configuration change in dehydro[12]annulene. Finally, extensive investigation of the [14]annulene hypersurface revealed that numerous Möbius minima exist within 10?kcal/mol of the global minimum. Copyright © 2012 John Wiley & Sons, Ltd.Facile thermal configuration change in annulenes and dehydroannulenes is explained by ?-bond shifting. According to the bond-shift rule, these reactions occur via planar, Möbius, or two-twist conformations.

Posted on 26 January 2012 | 10:54 am

The basicity of sulfonamides and carboxamides. Theoretical and experimental analysis and effect of fluorinated substituent

The basicity of a series of sulfonamides and carboxamides with respect to protonation and hydrogen-bonded complex formation with phenol was investigated by calculations using the Becke three-parameter hybrid functional combined with Lee–Yang–Parr correlation functional with the 6-311G** and 6-311++G** basis sets and by infrared spectroscopy. The effect of fluorinated substituent was studied for the two series. The proton affinity of nitrogen in sulfonamides is higher than oxygen, in contrast to carboxamides, which are protonated at oxygen. The phenyl group in benzenesulfonamide increases the basicity of both heteroatoms, but more strongly of the nitrogen, whereas in benzamide the effect on the two heteroatoms is about the same. The CF3 group equally decreases the basicity of nitrogen and oxygen atoms in sulfonamides and carboxamides. The second fluorinated substituent decreases the basicity of oxygen in (CF3CO)2NH more strongly than of nitrogen. For sulfonamides, the same effect results in the reverse of the center of basicity from nitrogen in (MeSO2)2NH to oxygen in (CF3SO2)2NH. All studied carboxamides give H-complexes via the carbonyl oxygen, whereas for sulfonamides two types of H-complexes, with the OH···N and OH···O=S, were found theoretically, the latter being more stable. The exception is bisimide (CF3SO2)2NH, for which only the OH···O=S complex is stable. Experimentally, only the oxygen-bound complexes are observed. Analysis of the natural charges revealed an ‘abnormal’ increase of the electron density on the NH group by electron-acceptor substituents in CF3SO2NHR, which was explained using the natural bond orbital analysis by loosening of the S–N bond because of orbital interactions with the ?*S?N orbital. Copyright © 2012 John Wiley & Sons, Ltd.The basicity of sulfon- and carboxamides with respect to protonation and H-bonding was studied theoretically and experimentally. spectroscopy. Unlike carboxamides, the proton affinity of nitrogen in sulfonamides is higher than of oxygen. Carboxamides give H-complexes via the carbonyl oxygen, whereas for sulfonamides H-complexes with OH···N and OH···OS H-bonds were found, the latter being more stable. Experimentally, only the O-bound complexes are observed.

Posted on 25 January 2012 | 5:14 am

Facile synthesis and photochromic properties of diarylethene-containing terpyridine and its transition metal (Zn2+/Co2+/Ru2+) complexes

Several transition metal complexes based on 2,3-bis(2,4,5-trimethyl-3-thienyl)maleic anhydride (DTE) bearing terpyridine (TPY) have been designed and synthesized. Furthermore, the photochromism of the free ligand and the influence of transition metal moieties on the photochemical properties have been thoroughly characterized by monitoring the changes in their UVvis spectra. Compounds 3–6 all display excellent response to UV irradiation, especially Co complex 5 with optimum sensitivity that took only 5?s to reach photostationary state. The photochromic properties of DTE unit have been found to be strongly influenced by the bridging transition metals, as complexes 4 (L–Zn–L) and 6 (L–Ru–L) exhibit much better photochromic properties in tetrahydrofuran solution than complex 5 (L–Co–L) and the free ligand 3 (L). It is worth noting that the optical/chemical thermal stabilities of photochromic compounds 3–6 are all greatly improved after the precursor functionalized with TPY and further coordinated with metal ions. Copyright © 2012 John Wiley & Sons, Ltd.Several transition metal complexes based on 2,3-bis(2,4,5-trimethyl-3-thienyl)-maleic anhydride (DTE) bearing terpyridine (TPY) have been designed and synthesized. The photochromic properties of DTE unit have been found to be strongly influenced by the bridging transition metal. It is worth noting that the chemical thermal stabilities of these photochromic compounds were greatly improved after functionalized with TPY and further coordinated with metal ions.

Posted on 24 January 2012 | 8:41 am

Photophysical properties of a series of 4-aryl substituted 1,4-dihydropyridines

In this article, a series of Hantzsch 1,4-dihydropyridines with different substituted aryl groups were synthesized and its spectral data obtained by UV–Vis absorption and fluorescence emission spectroscopies in solution. The dihydropyridines present absorption located around 350?nm and fluorescence emission in the blue–green region. A higher Stokes' shift could be observed for the derivative 3b because of an intramolecular charge transfer in the excited state from the dimethylaniline to the dihydropyridine chromophores, which was corroborated by a linear relation of the fluorescence maxima (?max) versus the solvent polarity function (?f) from the Lippert–Mataga correlation. A comparison between the experimental data and time-dependent density functional theory-polarizable continuum model calculations of the vertical transitions was performed to help on the elucidation of the photophysics of these compounds. For these calculations, the S0 and S1 states were optimized using Becke, three-parameter, Lee–Yang–Parr/6-31?G* and Configuration Interaction Singles/6-31?G*, respectively. The predicted absorption maxima are in good agreement with the experimental; however, the theoretical fluorescence emission maxima do not match the experimental, which means that the excited specie cannot be related to neither a locally excited state nor to an aromatized structure. Copyright © 2012 John Wiley & Sons, Ltd.A series of Hantzsch 1,4-dihydropyridines with different substituted aryl groups were synthesized and its spectral data obtained by UV–Vis absorption and fluorescence emission spectroscopies in solution. The dihydropyridines present absorption located around 350 nm and fluorescence emission in the blue–green region. A higher Stokes' shift could be observed for the derivative 3b because of an intramolecular charge transfer in the excited state from the dimethylaniline to the dihydropyridine chromophores. Time-dependent density functional theory-polarizable continuum model calculations of the vertical transitions were performed to help on the elucidation of the photophysics of these compounds. The predicted absorption maxima are in good agreement with the experimental; however, the fluorescence emission theoretical results do not match the experimental, which means that the excited specie cannot be related to neither a locally excited state nor to an aromatized structure.

Posted on 24 January 2012 | 6:09 am

Mechanism and regioselectivity of the 1,3-dipolar cycloaddition of thiocarbonyl S-imide with cyclopent-3-ene-1,2-dione and methoxyethene: a density functional theory approach

A theoretical study on the regioselectivity of 1,3-dipolar cycloaddition reaction between an uncommon dipole (thiocarbonyl S-imide) with cyclopent-3-ene-1,2-dione (DPh1) and methoxyethene (DPh2) has been carried out by means of several theoretical approaches, namely, activation energy, Houk's rule based on the frontier molecular orbital theory and density functional theory (DFT) reactivity indices. The calculations were performed at the DFT-B3LYP/6-31G(d) level of theory using GAUSSIAN 09. The present analysis shows that the 1,3-dipolar cycloaddition of thiocarbonyl S-imide with DPh1 and DPh2 has normal-electron demand and inverse-electron demand character, respectively. Moreover, the results obtained from energetic point view are in agreement with electronic approaches, and the Houk's rule is capable to predict true regioselectivity. Copyright © 2012 John Wiley & Sons, Ltd.1,3-Dipolar cycloaddition of thiocarbonyl S-imide with cyclopent-3-ene-1,2-dione and methoxyethene has normal-electron demand and inverse-electron demand character, respectively. The results of activation energies, frontier molecular orbital energies, Gazquez-Mendez rule, Chattaraj's polar model and Houk's rule for prediction of favored regioisomers are in agreement with each other.

Posted on 24 January 2012 | 5:54 am

Preparation of onium salts of a reduced anthracenone crown ether macrocycle: a reactivity series involving pyridine, phosphine, thiophene, nitrile and primary amide nucleophiles

Reaction of mineral acids with a cyclic macromolecule containing a secondary alcohol produces ammonium, phosphonium, thiophene, and amide adducts via a carbocation intermediate. X-ray crystallography confirms the structures of the products, including those when two competing nucleophiles are present. A reactivity series that mirrors the nucleophilicity index, where reactivity decreases in the order thiophene >?pyridine >?primary amides >?alkyl nitriles >> aromatic nitriles (unreactive), results. Addition of metal ions to ammonium adducts dissolved in acetonitrile produces secondary amides via the Ritter amide synthesis. Copyright © 2012 John Wiley & Sons, Ltd.Reaction of acid with a cyclic macromolecule containing a secondary alcohol produces ammonium, phosphonium, thiophene and amide adducts where reactivity decreases in the order: thiophene >?pyridine >?primary amides >?alkyl nitriles >>? aromatic nitriles (unreactive).

Posted on 24 January 2012 | 5:45 am

The photochemistry of the self-healing chromophore Disperse Orange 11

Recent interest in the self-healing ability of the laser dye 1-amino-2-methylanthraquinone, Disperse Orange 11, has lead us to investigate the possible alternative mechanisms of action, either intramolecular proton transfer (PT) or twisted intramolecular charge transfer (TICT) formation. AMPAC semiempirical PM3 CI (all single excited configurations) potential energy surfaces searches have been conducted with either reaction mechanism. Based purely on the potential energy surface results, no state, S0, T1, or S1, seems especially likely to be kinetically favorable for PT. The T1 state is favorable thermodynamically for PT. However, the S1 state TICT reaction is both thermodynamically favorable and kinetically preferred over all PT reactions. There is also a favorable T1 TICT reaction, but much slower kinetically on the triplet surface than S1 TICT. The Wentzel–Kramers–Brillouin (WKB) method has been used to ascertain proton tunneling contributions to PT. Even with proton tunneling, S1 TICT is still more highly favored, though proton tunneling could make the T1 PT reaction competitive depending on the rate of intersystem crossing. We also examine spectroscopic properties of PT transfer and TICT reaction path entities in comparison with published experimental evidence. However, this comparison leads to ambiguous findings that suggest that electronic spectral properties alone will not fully clarify the mechanism. Overall, results suggest that the TICT mechanism is the most likely for optical damage and self-repair for Disperse Orange 11, and might be considered for the damage and repair mechanisms for other organic solid state laser materials. Copyright © 2012 John Wiley & Sons, Ltd.Twisted intramolecular charge transfer transitions (TICT) have been identified as the possible mechanism behind optical damage and self-repair for the Disperse Orange laser dye. Semiempirical PM3 calculations of the potential energy surface and WKB approximation proton tunneling calculations suggest S1 state intramolecular proton transfer (PT) is less likely. It is possible that intersystem crossing could lead to a reasonably competitive T1 PT reaction. Shown is the TICT state charge distribution.

Posted on 18 January 2012 | 8:50 am

Degradation of dichloromethane by bispidine

An effective degradation reaction of CH2Cl2 by bispidine (3,7-diazabicyclo[3.3.1]nonane, C7H12(NH)2, 1) is reported. The reaction starts as low as ?20?°C and is quantitative with respect to 1. The overall reaction implies nucleophilic substitution of chloride, followed by a series of cascading acid–base reactions, ending with the formation of two easily separable products, one being soluble and the other insoluble. The starting 1, the intermediates, and the products show a variety of interesting solid-state structures, associated with plasticity, N–H?N and N–H?Cl?H–N hydrogen bonding, and polymorphism. Copyright © 2012 John Wiley & Sons, Ltd.Degradation of CH2Cl2 by the parent bispidine starts as low as ?20?°C. The overall reaction implies nucleophilic substitution, followed by a series of cascading acid–base reactions. The starting bispidine, intermediates, and products show a variety of interesting solid-state structures, associated with plasticity, N–H?N and N–H?Cl?H–N hydrogen bonding, and polymorphism.

Posted on 17 January 2012 | 2:10 pm

An ultrafast time-resolved infrared and UV–vis spectroscopic and computational study of the photochemistry of acyl azides

The photochemistry of pivaloyl, benzoyl, 4-phenylbenzoyl, and 2-anthroyl azides has been studied using femtosecond (fs) time-resolved infrared (TRIR) and UV–vis spectroscopy and interpreted with the aid of computational chemistry. Density functional theory calculations revealed a significant difference in the nature of the lowest singlet excited state for these carbonyl azides. The lowest singlet excited states (S1) of p-phenylbenzoyl and 2-anthroyl azides are (?,?*) in nature, while the pivaloyl and benzoyl azides S1 states involve (n,?*) excitations. Nevertheless, for all acyl azides studied here, a similar, and intense, IR band at about 2100?cm?1 has been detected in the ultrafast TRIR experiments following 270?nm excitation. These bands were shifted to lower energy by about 100?cm?1 relative to the N3 stretching mode for the ground states of these azides. These 2100?cm?1 vibrational bands were assigned to the S1 states of acyl azides in agreement with density functional theory calculations. The decay of the acyl azide S1 states was described by bi-exponential functions. The fast component was attributed to the decay of the hot S1 state and the longer component to the decay of the thermally relaxed S1 state. A strong and broad transient absorption in the 350–650?nm spectral range was observed in the fs UV–vis experiments for p-phenylbenzoyl and 2-anthroyl azides. The carrier of this absorption also decayed bi-exponentially, and the time constants were in excellent agreement with those found in the fs TRIR experiments. The slow component of the S1 state decay was found to be dependent on the solvent polarity. When the lifetime of the acyl azide S1 state is substantially longer than the time constant for vibrational cooling of nascent (hot) isocyanate, the correlation between the S1 decay and isocyanate formation was clear. The 270?nm excitation populates the Sn (n???2) states of these acyl azides. It was established that a hot nitrene is produced more efficiently from both the Sn and hot S1 states than from the relaxed S1 state of these acyl azides. Thus, time-resolved study provides direct experimental evidence that the S1 state is the precursor of nitrene only when the S1 state is pumped directly and when the S1 state lifetime is longer than the time constant of vibrational cooling of the newborn nitrene. All of these results are consistent with the data obtained recently for 2-napththoyl azide. Copyright © 2012 John Wiley & Sons, Ltd.The photochemistry of pivaloyl, benzoyl, 4-phenylbenzoyl, and 2-anthroyl azides has been studied using femtosecond time-resolved infrared (TRIR) and UV–vis spectroscopy and interpreted with the aid of computational chemistry. The density functional theory calculations revealed a significant difference in the nature of the lowest excited state for these carbonyl azides. For all acyl azides studied here, a similar, and intense, IR band at about 2100?cm?1 has been detected in the ultrafast TRIR experiments following 270?nm excitation.

Posted on 17 January 2012 | 10:38 am

Dicyanoaromatic radical anions as mixed valence species

The reduction of nine symmetric dicyanoaromatic radical anions by sodium amalgam in the presence of cryptand[2.2.2] was studied using cyclic voltammetry and using optical and electron paramagnetic resonance (EPR) spectroscopies in two aprotic solvents. All radicals are charge-delocalized (Class III) mixed valence species, as shown by the weak solvatochromism of their low-energy optical bands and by the vibrational structure exhibited by most of the bands. The maximum of the low-energy optical transition decreases logarithmically with the number of bonds (n) between cyano groups in a series of p-phenylene-bridged radicals, from p-phenyl (n?=?5) to p-quaterphenyl (n?=?17), although the energy of the latter lies higher than the value predicted by the linear regression. The energy of this band decreases linearly with the cos2 value of the torsion angle between phenyl rings in the spectra of biphenyl-4,4?-dicarbonitrile radical anion and their methyl-substituted derivatives. The fact that charge delocalization is maintained in radicals with non-Kekulé bridges, with unusually large bridges and with bridges with highly twisted biphenyl systems suggests that cyano charge-bearing units have small reorganization energies and induce high electronic couplings through the bridges. Copyright © 2012 John Wiley & Sons, Ltd.Nine dicyanoaromatic radical anions were studied by visible/near infrared spectroscopy and found to be charge-delocalized mixed valence species, even the ones with bridges that are supposed to induce a small electronic coupling between cyanoaromatic units.

Posted on 17 January 2012 | 5:27 am

Advances in bis(N-heterocyclic carbene) chemistry: new classes of structurally dynamic materials

A major challenge in the pursuit of dynamic materials is the dichotomy between reversible structure and electronic conjugation. Guided by the precedence for N-heterocyclic carbenes (NHCs) to dimerize, couple with electrophiles and bind to transition metals, we proposed that linearly opposed bis(NHC)s could, in a similar manner, afford unsaturated homopolymers, alternating copolymers, and metallopolymers. To realize these goals, we developed synthetic methods for accessing ditopic NHCs and investigated their propensities to undergo homopolymerization via dimerization and copolymerization via coupling with ditopic electrophiles or divalent transition metals. The materials obtained from these reactions were of relatively high molecular weight, exhibited electronic properties that were consistent with extensively delocalized systems, and in many cases, were found to be thermally reversible. In addition to expanding the scope of carbene chemistry, these polymers represent significant advancements towards the realization of reversible, conjugated polymers. Copyright © 2012 John Wiley & Sons, Ltd.The pursuit of structurally dynamic, conjugated materials based upon bis(N-heterocyclic carbenes) (NHCs) is highlighted. Synthetic methods for accessing ditopic NHCs were developed, and their propensities to undergo homopolymerization via dimerization and copolymerization via coupling with ditopic electrophiles or divalent transition metals were investigated. In addition to expanding the scope of carbene chemistry, these polymers represent significant advancements towards the realization of reversible, conjugated polymers.

Posted on 12 January 2012 | 9:28 am

Reactivity of acenes: mechanisms and dependence on acene length

Acenes are polycyclic aromatic hydrocarbons consisting of linearly fused benzene rings. These compounds are currently the subject of great interest from both fundamental and applied perspectives, particularly for use in organic electronics. This review highlights the computational studies carried out to understand acene reactivity, their reaction mechanisms, and the relationship of the latter to acene length. Generally, as acene length increases, the reactivity of acenes increases, and a greater tendency towards a biradical mechanism is observed. For example, an interesting change in mechanism (from concerted to biradical) is observed for the reaction of acenes (benzene through pentacene) with molecular oxygen. A computational study of the addition of HCl (which behaves as an electrophile) and water (which behaves mostly as a nucleophile) to acenes revealed that acene reactivity increases along the series up to hexacene and remains constant from hexacene and above because of the biradical character of the ground state of higher acenes. Although the exothermicity of the addition of water and HCl to acene is similar, the activation barrier for the addition of water is significantly higher than that for HCl. There is a substantial substituent effect on the energy barriers for electrophilic and nucleophilic reactions. Phenyl substitution at the most reactive meso-carbon atoms of the central ring of acene blocks dimerization through this ring but does not efficiently prevent dimerization through other rings. With respect to the self-reactivity (such as dimerization, etc.) of acenes, a computational study showed that the preferred pathways are the formation of acene dimers via a central benzene ring and the formation of acene-based polymers. Interestingly, longer acenes are predicted to self-react to form polymers, rather than acene dimers, as the thermodynamically preferred product. These findings invite experimentalists to reinvestigate the dimerization of long unsubstituted acenes. Given that computational studies directly relate to the gas phase reactivity of acenes, whereas long acenes find application in organic semiconductor devices as solids, the next research challenge is to obtain an understanding of the solid state reactivity of long acenes. Copyright © 2012 John Wiley & Sons, Ltd.The study of acene reactivity is important for basic understanding of aromatic and conjugated systems and can guide efforts to design stable acene derivatives for electronic applications. The current review summarizes recent computational studies of acene reactivity towards nucleophilic and electrophilic additions, oxidation by molecular oxygen, and dimerization. Generally, as acene length increases, reactivity increases, and often, change in mechanism with a greater tendency towards a biradical mechanism is observed.

Posted on 10 January 2012 | 5:02 am

Micellar effects on aromatic nucleophilic substitution by the ANRORC mechanism. Hydrolysis of 2-chloro-3,5-dinitropyridine

The hydrolysis of 2-chloro-3,5-dinitropyridine by sodium hydroxide in the presence of micelles of cetyltrimethylammonium bromide (CTABr), cetyltrimethylammonium chloride (CTACl) and sodium dodecyl sulfate (SDS) has been studied. The reaction follows a consecutive reaction path involving the formation of a long-lived intermediate 4 and finally giving the product, 3,5-dinitro 2-pyridone 2. The mechanism follows an addition of the nucleophile, ring opening and ring closure (ANRORC) reaction path. The rate constant was observed to be first-order dependent on [OH?]. The rate of reaction increased on increasing [CTABr] and, after reaching to the maxima, it started decreasing. The anionic SDS micelles inhibited the rate of hydrolysis. The results of the kinetic experiments were treated with the help of the pseudophase ion exchange model and the Menger–Portnoy model. The added salts, viz. NaBr, Na-toluene-4-sulphonate, and (CH3)4NBr on varying [CTACl] and [SDS] inhibited the rate of reaction. The various kinetic parameters in the presence and absence of salts were determined and are reported herewith. Copyright © 2011 John Wiley & Sons, Ltd.2-chloro-3,5-dinitropyridine is hydrolyzed by sodium hydroxide to give 3,5-dinitro 2-pyridone. The reaction proceeds through ANRORC reaction mechanism with the formation of a long lived anion. Addition of CTABr micelles catalyzed the rate of hydrolysis, whereas SDS inhibited the rate of reaction. The observed results are explained on the basis of Menger–Portnoy model.

Posted on 10 January 2012 | 4:49 am

Experimental study on the reaction pathway of ?-haloacetophenones with nucleophiles: direct substitution or carbonyl addition?

The reaction of PhCOCH2Cl with OH– gave the expected ?-substituted alcohol (PhCOCH2OH) in addition to three dimer products. To clarify whether the substitution product is formed by direct SN2 or via carbonyl addition, the reaction of PhCOCH2Cl and OMe– was examined. The reaction gave two products, PhCOCH2OH as the major product after acid hydrolysis and PhCOCH2OMe as the minor product. An electron-withdrawing substituent on the phenyl ring enhanced the overall reactivity and gave more alcohol than ether. It was concluded that the alcohol was formed via carbonyl addition-epoxidation route, whereas the ether was formed by the direct substitution route. Copyright © 2012 John Wiley & Sons, Ltd.The reaction of PhCOCH2Cl with NaOMe gave two products, PhCOCH2OH as the major product and PhCOCH2OMe as the minor product. An electron-withdrawing sustituent on the phenyl ring enhanced the overall reactivity and gave more alcohol than ether. The alcohol was formed via carbonyl addition-epoxidation route, whereas the ether was formed by the direct substitution route.

Posted on 10 January 2012 | 4:45 am

Theoretical study on the mechanisms of Proline-catalyzed Mannich reaction between acetaldehyde and N-Boc imines

The mechanisms of the single and double Mannich reactions between acetaldehyde and N-Boc imines are clarified by density functional theory calculations. For single addition of Mannich reaction, the energy difference between the transition states of different configurations correspond to an enantiomeric excess value of 90.58% (without solvent) and 98.46% (in acetonitrile) in favor of the (S)-configuration product. For bis-addition of Mannich reaction, the calculated enantiomeric excess value is 95.02% (without solvent) and 98.57% (in acetonitrile) in favor of the (S, S)-configuration product. These calculated results are in good agreement with the experimental results. The calculations clearly demonstrate that the hydrogen-bonding determine the stereochemistry of the reactions. Copyright © 2012 John Wiley & Sons, Ltd.DFT calculations demonstrate that the single and double Mannich reactions between acetaldehyde and N-Boc imines lead to the formation of the (S)-configuration and (S, S)-configuration products, respectively; the hydrogen-bonding determine the stereochemistry of the reactions and the solvent employed play an important role in determining the reaction rate and enantioselectivity.

Posted on 10 January 2012 | 4:21 am

Bowl-shaped carbocations: easy to produce, hard to reduce

Stable nonplanar polyaromatic carbocation, [C20H10CH2Cl]+ (1) having the chloromethyl group covalently bound to the hub site of corannulene was prepared by reaction of C20H10 with CH2Cl2 and GaCl3 in the presence of SnCl2. The X-ray diffraction study of the resulting salt, [1]?[(SnCl)(GaCl4)2], revealed the presence of two rotational isomers for 1 in the crystal lattice. In the solid state, a complex polymeric inorganic anion of the overall composition [(Sn2Cl2)?(GaCl4)4]? is formed with the cationic ?-bowls being attached to its backbone. Both X-ray and nuclear magnetic resonance spectroscopic data of 1 indicate the sp3 hybridization of the C-atom of corannulene at the attachment point. An electrochemical investigation of 1 and the series of related corannulene cations, [C20H10R]+ (R?=?CHxCl3–x (x?=?0–3) and CH2CH2Cl), was carried out to elucidate the electronic consequences triggered by the above changes. This study revealed that the surface decoration of corannulene core makes the reduction of the resulting [C20H10R]+ species significantly more difficult (by about 0.3?V) with respect to the parent bowl. To evaluate the consequences of the observed one-electron reduction and conceivable but not seen second-electron reduction on the corannulene core, theoretical calculations at the density functional theory level have been carried out. Copyright © 2012 John Wiley & Sons, Ltd.Functionalized corannulene cations [C20H10R]+ are prepared and investigated by single crystal X-ray diffraction, nuclear magnetic resonance and UV–Vis spectroscopies. Their first electrochemical study reveals that the reduction of cationic species is more difficult compared with the parent bowl.

Posted on 6 January 2012 | 7:04 am

Evolution of excess electron binding motifs under both internal-push (from exo C–F bonds) and external-push (from endo C–F bonds) electron effects in endohedral metallofullerenes with endo C–F bonds

How does the endo C–F bond influence the excess electron binding motif? For lithium-doped endohedral perfluorofullerenes with endo C–F bonds, under both internal-push (from exo C–F bonds) and external-push (from endo C–F bonds) electron effects, the singly occupied molecular orbital electron cloud of the sphere-like Li···F8@C60F52 (D2) is partially dispersed within the ?p–s antibonding orbital of endo C–F bonds and the space between C?+–Fendo?– double electric layers, which makes Li···F8@C60F52 have partial excess electron (electride characteristics) and partial lithium salt characteristics, while in the tube-like Li···F2@C60F58 (Cs), as the Li is changing from approaching F to keeping away from F and to approaching another one, the singly occupied molecular orbital electron cloud is mainly dispersed from within the p orbital of the short endo C–F bond to within the middle of the two F atoms and again to within the p orbital of the short endo C–F bond, which indicates an evolution from lithium salt characteristic to excess electron characteristic, and again to lithium salt characteristic. Copyright © 2012 John Wiley & Sons, Ltd.Evolution of excess electron binding motifs in different lithium-doped endohedral perfluorofullerenes with endo C–F bonds.

Posted on 6 January 2012 | 4:04 am

Solvent-dependent ground-state distributions in a donor–acceptor redox-active bistable [2]catenane

The solvent dependency of the ground-state distribution as well as the electrochemical switching behavior in a redox-active bistable donor–acceptor [2]catenane, containing bisthiotetrathiafulvalene (STTFS) and 1,5-dioxynaphthalene (DNP) recognition sites incorporated within a macrocyclic polyether encircled by the cyclobis(paraquat-p-phenylene) (CBPQT4+) ring, has been investigated. There are two translational isomers: (i) the ground-state co-conformation (GSCC) in which the CBPQT4+ ring encircles the STTFS unit and (ii) the metastable-state co-conformation (MSCC) in which the CBPQT4+ ring encircles the DNP unit. 1H NMR spectroscopy indicates that the ground-state distribution of GSCC to MSCC varies from approximately 1:1 in MeCN to 7:1 in MeCN?:?H2O (1:1, v/v) at 283?K. The reversible electrochemical switching behavior of the [2]catenane was confirmed by 1H NMR and UV?Vis spectroscopies, as well as by cyclic voltammetry (CV). Additionally, variable scan-rate CV studies were compared with simulated CV data and show that the ground-state distribution of GSCC to MSCC is about 30:1 in MeCN?:?H2O (1:1, v/v) at 298?K. With the assistance of isothermal titration calorimetry of model compounds, it was found that the changing ground-state distribution in differing solvent systems is driven entropically rather than enthalpically. Copyright © 2012 John Wiley & Sons, Ltd.The solvent dependency of the ground state distribution and the electrochemical switching behavior in a redox-active bistable donor-acceptor [2]catenane, containing bisthiotetrathia fulvalene (STTFS) and 1,5-dioxynaphthalene (DNP) recognition sites incorporated within a macrocyclic polyether encircled by the cyclobis(paraquat-p-phenylene) (CBPQT4+) ring, has been investigated. In aqueous solvent, CBPQT4+ prefers to reside upon the STTFS unit, while in organic solvent the cyclophane shows little selectivity between the two stations. Entropy, rather than enthalpy, drives the changing ground-state distribution in differing solvent systems.

Posted on 4 January 2012 | 11:57 am

Ligand-site exchange in intramolecular complexes of silicon: substituent effects

The intramolecular complexes containing coordination bonds Si?N or Si?O are distinguished for their stereochemical nonrigidity resulting in interconversion between isomers, that is, ligand-site exchange. The influence of the substituents bound to the silicon atom on the free energies of activation for ligand exchange ?G? of specific interest is poorly understood. In this work, the literature data on substituent influence on the energies ?G? for 13 series of the complexes have been considered, using the correlation analysis. On the basis of the obedience of the energies ?G? to the linear free energy relationship, it has been established for the first time that the ?G? values depend not only on the inductive and resonance effects but also on the polarizability and steric effects of substituents. The reason for the occurrence of the polarizability effect is the appearance of excess charges on Si and N (or O) atoms as a result of intramolecular coordination consisting in the charge transfer from the donor center (N or O atom) to the acceptor one (Si atom). In some series the contribution of the polarizability or steric effect to the overall change in ?G? because of the influence of substituents is a maximum. An understanding of these effects may give a better insight into the mechanism of nucleophilic substitution, involving organoelement compounds. Copyright © 2012 John Wiley & Sons, Ltd.The free energies of activation for ligand exchange ?G? values depend not only on the inductive and resonance effects but also on the polarizability and steric effects of substituents.

Posted on 2 January 2012 | 3:36 am

Solid-state Bis-hexahapto-metal complexation of single-walled carbon nanotubes

We report the effect of transition metal deposition on the conductivity of single-walled carbon nanotube (SWNT) networks. The slow evaporation of small amounts of highly mobile transition metal atoms significantly reduces the resistance of SWNT thin films; we suggest that Cr, Fe, and Ti atoms form bis-hexahapto complexes at the SWNT sidewalls, thereby bridging adjacent carbon nanotubes and reducing the inter-nanotube junction resistance. Copyright © 2012 John Wiley & Sons, Ltd.We report the effect of transition metal deposition on the conductivity of single-walled carbon nanotube (SWNT) networks. The slow evaporation of small amounts of highly mobile transition metal atoms significantly reduces the resistance of SWNT thin films; we suggest that Cr, Fe, and Ti atoms form bis-hexahapto complexes at the SWNT sidewalls, thereby bridging adjacent carbon nanotubes and reducing the inter-nanotube junction resistance.

Posted on 2 January 2012 | 3:06 am

Spectroscopic and stability studies on unsymmetrical 1,3-dialkyl-1,3-diphosphacyclobutane-2,4-diyls

Air-tolerant 2,4-bis(2,4,6-tri-t-butylphenyl)-1,3-diphosphacyclobutane-2,4-diyl singlet biradicals can be prepared by utilizing the unique reactivity of a kinetically stabilized P?C triple bond compound. In this procedure, we studied the spectroscopic properties of a fundamental unsymmetrical P-heterocyclic biradical containing both PEt and PMe moieties, and the effects of the PCH2OMe group in relation to the stability of the P-heterocyclic biradical skeleton. The experimentally observed nuclear magnetic resonance and photo-absorption parameters of 1-ethyl-3-methyl-2,4-bis(2,4,6-tri-t-butylphenyl)-1,3-diphosphacyclobutane-2,4-diyl were discussed based on our previous findings and density functional theory calculations, suggesting particular structural characteristics of the P-heterocyclic biradical skeleton and aromatic substituent effects on the sp2-C atoms in the 4-membered ring. Introduction of the methoxymethyl group in the P2C2 biradical moiety gave more stabilized 1,3-diphosphacyclobutane-2,4-diyl derivatives. In comparison with considerably unstable biradicals bearing propargyl substituents, relatively higher lowest unoccupied molecular orbital energies suggest reluctant oxidation of the P-heterocyclic skeleton. Copyright © 2011 John Wiley & Sons, Ltd.UV-Vis photo-absorption properties for air-tolerant 1,3-diphosphacyclobutane-2,4-diyls as singlet biradicals indicate that employed sterically bulky aromatic substituents such as Mes* on the sp2-C atoms substantially conjugate with the cyclic skeleton to protect the biradical moiety effectively. The PR groups control the biradical stability by altering the electronic structures of the particular 4-membered heterocyclic system.

Posted on 21 December 2011 | 1:26 pm

GIAO/DFT calculations of relative conformer energies and 1H NMR shifts of unsaturated alcohols in benzene

Density functional theory calculations were performed on alkenols and alkynols at the PBE1PBE/6-311?+?G(d,p) level with the inclusion of solvent (benzene) effects by the integral-equation-formalism polarizable continuum model (IEFPCM). For the smaller molecules, conformers in which the OH group is in the vicinity of the double or triple bond are preferred, but this preference falls as the alkyl chain is lengthened. The solvent effect on the relative Gibbs energies of different conformers is irregular, and in only two cases, 3-buten-1-ol and 3-butyn-1-ol, is there marked levelling. Gauge-including atomic orbital calculations, based on the PBE1PBE/6-311?+?G(d,p) geometries but using a larger basis set, cc-pVTZ, give nuclear magnetic resonance shifts for all the protons in each conformer. Overall shifts are calculated by weighing these according to the conformer population. Calculated values are well correlated with experimental data from high-dilution spectra in the same solvent, ranging from about 0.4 to 5.8?ppm, with a slope of 1.09?±?0.01. Calculations on some alcohols with hetero-atom substituents confirm that these also prefer gauche (synclinal) conformers in the gas phase, with a less marked preference in benzene. The nuclear magnetic resonance shifts, however, are calculated to be on average over 0.8?ppm higher than observed. Copyright © 2011 John Wiley & Sons, Ltd.1H NMR shifts and the relative populations are calculated for all conformers of 10 alkenols and alkynols in benzene to give overall shifts which correlate well with experimental data.

Posted on 5 December 2011 | 8:40 am

1H NMR study of the hetero-association of unsaturated alcohols with pyridine

The 1H NMR titration method is used to investigate the association of some unsaturated alcohols with pyridine in benzene. The association constant for allyl alcohol (2-propen-1-ol) is slightly higher than for alkanols, but putting one methylene group between the OH and vinyl group completely eliminates this enhancement. In alkynols both the OH and??CH protons associate with pyridine. Here, the effects of chain-lengthening on the association constant are irregular. Values for alkynols and some alcohols with hetero-atom substituents are lower than expected on the basis of a Taft polar constant (?*) correlation of alkanol association constants and of a correlation with the pKa’s of the corresponding carboxylic acids. It is suggested that stabilization of the ground state by OH/? interactions is responsible for these low association constants. Small increases in the NMR shifts of the OH protons in the 3-carbon and 4-carbon alkenols and alkynols can also be attributed to OH/? interactions, but the 5-carbon analogues have shifts as low as alkanols. Copyright © 2011 John Wiley & Sons, Ltd.Through-bond enhancement of association constants by electron attractors are offset by through-space OH/p interactions, which show up in the 1H NMR spectra of some alkenols and alkynols.

Posted on 5 December 2011 | 8:27 am

Computational study on the conformational preferences in nateglinide

Ab initio and semi-empirical calculations were performed on the monomers, dimers and tetramers of the antidiabetic drug nateglinide to understand the conformational preferences and to explore their possible relation with polymorphism. The reported crystal structure of bis(nateglinide) hydronium chloride shows one asymmetric unit consisting of four different conformations of the drug nateglinide. The Becke, three-parameter, Lee–Yang–Parr /6-31+G(d,p) optimizations indicate that these conformers are energetically quite comparable and the differences disappear in gas phase. Our analysis shows that ? (phi) torsion angle of this phenylalanine derivative is responsible for the observed differences in stability among the nateglinide conformations. Four different polymorphs of nateglinide (B, H, S and X2) were reported but the structural differences are not available. This quantum chemical study on the dimers of nateglinide helps in proposing the structures of polymorphs. As per the quantum chemical analysis, the dimer N-44 is the structure of the stable polymorph, whereas, the dimers N-AA, N-CC and N-AC are almost isoenergetic, thus proposed to be the structures of metastable state. The dimerization and tetramerization energies are estimated to be about ?9.0 and ?38.67?kcal/mol, respectively. The extra stability in tetrameric state compared with the dimeric form is attributed to additional hydrophobic and van der Waals interactions. Copyright © 2011 John Wiley & Sons, Ltd.The antidiabetic drug nateglinide exhibits conformational polymorphism. Quantum chemical analysis shows that ? (phi) torsion angle of this phenylalanine derivative is responsible for the observed differences in stability among the nateglinide conformations (N-A, N-C and N-4). Ab initio studies indicate that dimer N-44 is the structure of the stable polymorph, whereas, dimers N-AA, N-CC and N-AC are proposed as structures of the metastable state.

Posted on 5 December 2011 | 7:57 am

Photoinduced decarboxylation of 9-oxo-6,9-dihydro[1,2,5]selenadiazolo[3,4-f]quinoline-8-carboxylic acid

Photoinduced reactions of 9-oxo-6,9-dihydro[1,2,5]selenadiazolo[3,4-f]quinoline-8-carboxylic acid (SeQCA) were investigated in alkaline media (aqueous NaOH solutions) by electron paramagnetic resonance (EPR) spectroscopy, following the in situ formation of paramagnetic species. According to UV–Vis and nuclear magnetic resonance investigations, protonation (pH???11) and deprotonation (pH???13) of the imino hydrogen of the 4-pyridone moiety has to be considered, reflected also in the different EPR spectra observed upon irradiation. Photoinduced generation of radicals was found only for carboxylate substituted SeQCA; other studied selenadiazoloquinolone derivatives, together with those substituted at the C(8) position (R?=?H, COOCH2CH3, COOCH3, COCH3 or CN), did not generate paramagnetic species during exposure. Consequently, photodecarboxylation was suggested as the decisive step, accompanied by the decomposition of the selenadiazole ring, resulting in the formation of ortho-hydroxylate anions. EPR parameters elucidated from experimental EPR spectra obtained at pH???11 and pH???13 indicate the formation of oxygen-centered radicals at the decarboxylated 4-pyridone ring. EPR spin trapping experiments with nitromethane confirmed a very effective photoinduced electron transfer from all the selenadiazoloquinolones investigated. Copyright © 2011 John Wiley & Sons, Ltd.Photoexcitation of 9-oxo-6,9-dihydro[1,2,5]selenadiazolo[3,4-f]quinoline-8-carboxylic acid in alkaline aqueous solutions results in the generation of two different radicals as monitored by in situ electron paramagnetic resonance spectroscopy, reflecting the protonation (pH???11) and deprotonation (pH???13) at the imino hydrogen of the 4-pyridone moiety. The reaction mechanism proposes photodecarboxylation, conversion of selenadiazole ring in ortho-hydroxylate substructure, as was evidenced in 17O enriched water, and formation of oxygen-centered radicals on decarboxylated 4-pyridone ring.

Posted on 5 December 2011 | 4:12 am

Thermolysis kinetics of diethyl 2,3-dicyano-2,3-di(p-substituted phenyl)succinates

An experimental approach was developed to determine the intrinsic thermolysis rate constants of the central carbon–carbon bond during the dl/meso isomerization of diethyl 2,3-dicyano-2,3-di(p-substituted phenyl)succinates (G=H, Me, OMe, Cl, and NO2) at temperatures ranging from 80 to 120?°C. The obtained rate constants are significantly affected by the polarity of the para substituents, in sharp contrast to their negligible effects on the dl/meso isomerization equilibrium constants. Moreover, the substituent effects on the activation enthalpies can be linearly correlated with the Hammett substituent resonance constants and the homolytic dissociation enthalpies (bond dissociation energies) of the benzylic C–H bonds of ethyl 2-cyano-2-(p-substituted phenyl)acetates. Copyright © 2011 John Wiley & Sons, Ltd.An experimental approach was developed to determine the intrinsic thermolysis rate constants of the central carbon-carbon bond during the dl/meso isomerization of diethyl 2,3-dicyano-2,3-di(p-substituted phenyl)succinates (G?=?H, Me, OMe, Cl, and NO2) at temperatures ranging from 80 to 120 _C. The obtained rate constants are significantly affected by the polarity of the para substituents, in sharp contrast to their negligible effects on the dl/meso isomerization equilibrium constants.

Posted on 29 November 2011 | 5:13 am

Aza-Diels–Alder reaction between cyclopentadiene and protonated N-phenylethyliminoacetates of 8-phenylmenthol and 8-phenylneomenthol: a density functional theory study

The cycloaddition between glyoxylate imines possessing two chiral auxiliaries, N-(R)- or N-(S)-1-phenylethyl and 8-phenylmenthyl or 8-phenylneomenthyl, and cyclopentadiene is described. Computational calculations using density functional theory with the Becke, three-parameter, Lee–Yang–Parr functional and the 6-31G(d) basis set were performed to better understand the highly diastereoselective mechanism and the exo-selectivity observed experimentally for these ionic aza-Diels–Alder reactions. Copyright © 2011 John Wiley & Sons, Ltd.The cycloaddition between glyoxylate imines possessing two chiral auxiliaries, N-(R)- or N-(S)-1-phenylethyl and 8-phenylmenthyl or 8-phenylneomenthyl, and cyclopentadiene is described. Computational calculations using density functional theory with the Becke, three-parameter, Lee–Yang–Parr functional and the 6-31G(d) basis set were performed to better understand the highly diastereoselective mechanism and the exo-selectivity observed experimentally for these ionic aza-Diels–Alder reactions.

Posted on 29 November 2011 | 2:04 am

Electrochemically induced oxidative cyclization of 2,3-dihydroxypyridine. Synthesis of a novel highly oxygenated heterocyclic compound

Electrochemical oxidation of 2,3-dihydroxypyridine in aqueous phosphate buffer solution at a glassy carbon electrode has been studied using cyclic voltammetry and controlled potential coulometry. The results indicate that oxidation of 2,3-dihydroxypyridine on glassy carbon electrode shows an irreversible feature in aqueous solution. This data indicates that the electrochemically generated pyridindione is unstable and via an oxidative conversion pathway converts to a novel highly oxygenated heterocyclic compound. By means of the obtained electrochemical data, an efficient, one-pot method for the synthesis of this heterocyclic compound based on the oxidative cyclization of 2,3-dihydroxypyridine under green conditions, and in a good yield and purity is described. Copyright © 2011 John Wiley & Sons, Ltd.Electrochemical oxidation of 2,3-dihydroxypyridine in aqueous solutions shows an irreversible process. The electrochemically generated pyridindione via an oxidative conversion pathway converts to a highly oxygenated heterocyclic compound. In this work, a green method for the synthesis of a novel heterocyclic compound is described.

Posted on 18 November 2011 | 8:44 am

Structural and energetic properties of alkylfluoride–BF3 complexes in the gas phase and condensed-phase media: computations and matrix infrared spectroscopy

We have undertaken an experimental and computational study of the structural properties of a few alkylfluoride–BF3 complexes (RF?–BF3), which are proposed intermediates in a certain class of Friedel–Crafts reactions. Using density functional theory and second-order Møller–Plesset calculations, we have obtained gas-phase structures, frequencies, and B–F? bond potentials for CH3F–BF3, (CH3)2CHF–BF3, and (CH3)3CF–BF3. All the complexes are weakly-bonded in the gas phase, with B–F? distances (X3LYP/aug-cc-pVTZ) of about 2.4 Å and binding energies (MP2/aug-cc-pVTZ) ranging from 5.4 and 6.7 kcal/mol. Accordingly, gas-phase bond potentials are relatively shallow and flat for these complexes. However, even though the inner walls of the potentials are rather soft (the energies rise by only about 5 to 10 kcal/mol between 2.4 and 1.6 Å), we observe no global or local minima at short B–F? distances. For the (CH3)2CHF–BF3 and (CH3)3CF–BF3 potentials in dielectric media, we do observe a distinct flattening along the inner wall, which results in shelf-like region near 1.7 Å, but this feature is not a true local minimum. We have also obtained low-temperature infrared spectra of the (CH3)2CHF–BF3 complex in solid neon, and the frequencies agree quite favorably with those obtained via computations, which validates the computational assessment of the gas-phase complexes. Copyright © 2011 John Wiley & Sons, Ltd.The structural and energetic properties of a few Freidel-Crafts intermediates (alkyl-fluoride - BF3 complexes) have been characterized via computations and infrared spectroscopy. These complexes are weakly-bonded, with long intermolecular B-F' distances, and the alkyl substituents exhibit little or no carbocation character. The structures that correspond to points along the inner wall of the B-F' bond potential (1.4 to 1.6 Å) show some additional carbocation character, but these are predicted to lie at significantly higher energies, even in bulk, dielectric media.

Posted on 16 November 2011 | 9:10 am

Matrix isolation and IR spectroscopic characterization of 3,5-difluoropyridyl-2,4,6-trinitrene

The photochemistry of 2,4,6-triazido-3,5-difluoropyridine 21 was investigated by matrix infrared and electron paramagnetic resonance spectroscopies. Ultraviolet irradiation (>260?nm) of 21 results in the formation of 3,5-difluoropyridyl-2,4,6-trinitrene 26 in yields high enough for characterization by infrared spectroscopy. The experimental infrared spectrum is in good agreement with density functional theory calculations. Under similar conditions, a very strong electron paramagnetic resonance spectrum of septet trinitrene 26 was obtained. Shorter irradiation times resulted in more complex product mixtures containing, in addition, mononitrenes and dinitrenes. Surprisingly, azirines and keteneimines, the typical photoproducts of arylnitrenes, were not observed. Copyright © 2011 John Wiley & Sons, Ltd.UV irradiation (>260 nm) of 2,4,6-triazido-3,5-difluoropyridine results in the formation of 3,5-Difluoropyridyl-2,4,6-trinitrene in yields high enough for characterization by IR spectroscopy. The experimental IR spectrum is in good agreement with DFT calculations. Under similar conditions a very strong EPR spectrum of septet the trinitrene was obtained. Shorter irradiation times resulted in more complex product mixtures containing in addition mononitrenes and dinitrenes. Surprisingly, azirines and keteneimines, the typical photoproducts of arylnitrenes, were not observed.

Posted on 11 November 2011 | 11:08 am

Computational study on structures, thermochemical properties, and bond energies of disulfide oxygen (S–S–O)-bridged CH3SSOH and CH3SS(=O)H and radicals

Cleavage of disulfide bonds is a common method used in linking peptides to proteins in biochemical reactions. The structures, internal rotor potentials, bond energies, and thermochemical properties (?fH°, S°, and Cp(T)) of the S–S bridge molecules CH3SSOH and CH3SS(=O)H and the radicals CH3SS•=O and C•H2SSOH that correspond to H-atom loss are determined by computational chemistry. Structure and thermochemical parameters (S° and Cp(T)) are determined using density functional Becke, three-parameter, Lee–Yang–Parr (B3LYP)/6-31++G (d, p), B3LYP/6-311++G (3df, 2p). The enthalpies of formation for stable species are calculated using the total energies at B3LYP/6-31++G (d, p), B3LYP/6-311++G (3df, 2p), and the higher level composite CBS–QB3 levels with work reactions that are close to isodesmic in most cases. The enthalpies of formation for CH3SSOH, CH3SS(=O)H are ?38.3 and ?16.6?kcal?mol?1, respectively, where the difference is in enthalpy RSO–H versus RS(=O)–H bonding. The C–H bond energy of CH3SSOH is 99.2?kcal?mol?1, and the O–H bond energy is weaker at 76.9?kcal?mol?1. Cleavage of the weak O–H bond in CH3SSOH results in an electron rearrangement upon loss of the CH3SSO–H hydrogen atom; the radical rearranges to form the more stable CH3SS·?=?O radical structure. Cleavage of the C–H bond in CH3SS(=O)H results in an unstable [CH2SS(=O)H]* intermediate, which decomposes exothermically to lower energy CH2?=?S?+?HSO. The CH3SS(=O)–H bond energy is quite weak at 54.8?kcal?mol?1 with the H–C bond estimated at between 91 and 98?kcal?mol?1. Disulfide bond energies for CH3S–SOH and CH3S–S(=O)H are low: 67.1 and 39.2?kcal?mol?1. Copyright © 2011 John Wiley & Sons, Ltd.Structures, internal rotor potentials, thermochemical properties (?fHo, So and Cp(T)) and bond energies on the S-S bridge molecules CH3SSOH and CH3SS(=O)H and radicals CH3SS•=O and C•H2SSOH are determined using DFT and CBS-QB3 computational chemistry methods. Disulfide bond energies for CH3S–SOH and CH3S–S(=O)H are 67.1 and 39.2 kcal mol?1. The CH3SS(=O)?H bond energy is only 54.8 kcal mol?1. The C?H bond energy of CH3SSOH is 99.2 and the O?H bond energy is 76.9 kcal mol?1

Posted on 10 November 2011 | 7:44 am

Kinetic study of BH3 reduction of benzaldehydes: identification of effective reducing species

A kinetic study was carried out for the reaction of benzaldehyde and borane (BH3) in tetrahydrofuran. The effect of BH3 concentration on the rate constant showed that the reaction order with respect to BH3 was 1.6. Substituent effects gave a linear Hammett plot with a ? value of ?0.51. It was concluded that the reaction proceeds through a rate-determining hydride-transfer transition state with two BH3 molecules, in which one molecule of BH3 acts as a reducing agent and the other serves as a catalyst. Copyright © 2011 John Wiley & Sons, Ltd.Kinetic study for the reaction of benzaldehyde and BH3 in THF showed that the reaction order with respect to BH3 was 1.6. Substituent effects gave a linear Hammett plot with a ? value of ?0.51. It was concluded that the reaction proceeds through a rate-determining hydride-transfer TS, in which one molecule of BH3 act as reducing agent and the second BH3 molecules serves as catalyst.

Posted on 10 November 2011 | 4:10 am

Electrochemical reduction of benzoyl chloride to benzil in ionic liquid BMIMBF4

A new electrochemical procedure for the electrocatalytic dimerisation of benzoyl chloride in ionic liquid, 1-butyl-3-methylimidazoliumtetrafluoborate, to benzil was investigated for the first time. The electrochemical behavior of benzoyl chlorides in BMIMBF4 was studied by cyclic voltammetry with a reduction peak at ?1.2?V (vs Ag). The electrolysis experiments were carried out in an undivided cell under mild conditions without any toxic solvents, catalysts and supporting electrolytes, creating benzil at a moderate yield (51%). The results showed that the yields were strongly affected by various factors: temperature, working potential, electrode material. Moreover, the ionic liquid was successfully recycled for this reaction. Copyright © 2011 John Wiley & Sons, Ltd.A new electrochemical procedure for the electrocatalytic dimerisation of benzoyl chloride in ionic liquid, 1-butyl-3-methylimidazoliumtetra fluoborate (BMIMBF4), to benzil was investigated for the first time. The synthesis was carried out under mild conditions, without resorting to the use of toxic solvents and catalysts. Benzil could be prepared in moderate yields (51%) under optimized conditions. Moreover, the ionic liquid was successfully recycled.

Posted on 10 November 2011 | 4:07 am

Tautomerism, dynamic properties and level crossing in 2-(pyridin-2-yl) furan-3-ol by density functional theory and natural bond orbital analysis

2-(Pyridin-2-yl)furan-3-ol (PYFO, T1) and (2E)-2-(pyridin-2(1H)-ylidene)furan-3(2H)-one (PYFO, T2) were considered to study their tautomerism interconversions, relative rotations of rings, OH bond rotations, and possibility of crossing between those energy surfaces using density functional theory methods at the Becke, three-parameter, Lee–Yang–Parr/6-311++G** level of theory. The optimized structures of both tautomers and the transition state of tautomerism are completely planar. A study of tautomerism in PYFO shows that T1 tautomer is about 24.38?kJ/mol more stable than T2. The rate constants of tautomerism interconversion for converting T1 to T2 is 1.98?×?108?M–1?s–1 and for converting T2 to T1 is 3.70?×?1012?M–1?s–1 at room temperature that show the possibility of this tautomerism with high rate at ambient temperature. Rotation of OH bond in T1 shows two minimum (at 0° (global minimum) and 180° (local minimum)) and a transition state at 110° (and 265°) with 47.10?kJ/mol barrier energy. Relative rotation of rings shows global minimum at 0° for both tautomers and local minimum at 154° and 206° for T1 and 180° for T2. The barrier energy for ring rotation of T1 was observed at 90° and 270° with 63.69?kJ/mol height and for T2 was observed at 120 with 170.86?kJ/mol height. Interestingly, the energy levels of ring rotations for T1 and T2 are the same and crossing between them was observed. Therefore, although these two potentials do not have the same symmetries, because of the crossing between their energy level, crossing is not avoided. Copyright © 2011 John Wiley & Sons, Ltd.2-(Pyridin-2-yl)furan-3-ol (PYFO, T1) and (2E)-2-(pyridin-2(1H)-ylidene)furan-3(2H)-one (PYFO, T2) were considered to study their tautomerism interconversions, relative rotations of rings, OH bond rotations, and possibility of crossing between those energy surfaces. The energy levels of ring rotations for T1 and T2 are the same and crossing between them was observed. Therefore, although these two potentials do not have the same symmetries, because of the crossing between their energy level, crossing is not avoided.

Posted on 25 October 2011 | 4:42 am

The mechanism for negative photochromism of spiropyran in silica

The mechanism for negative photochromism of spiropyran in silica was investigated. Prior to our study, the chemical origin of the high thermal stability of the photomerocyanine form (PMC-form) dispersed in perhydropolysilazane (PHPS), which is converted to silica at ambient temperature, had been investigated. The high thermal stability of the PMC-form is attributed to the protonated PMC-form (H???PMC-form), which is produced by intermolecular hydrogen bonding between oxide anions generated by the cleavage of the C???O bonds and the partially uncondensed Si???OH and O???H bonds of silica. Furthermore, the H???PMC-form could be thermally isomerized from the SP-form without UV light irradiation. This specific phenomenon is caused by the so-called negative photochromism. In this study, we proposed a mechanism for negative photochromism according to the relationship of the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO). The relationship between the HOMOs was determined using cyclic voltammetry (CV) and ultraviolet photoelectron spectroscopy (UPS). On the other hand, the relationship between the LUMOs was determined from the respective optical bandgap. As a result, the HOMO level of H???PMC-form was ?6.1?eV and that of SP-form was ?5.3?eV. Accordingly, the thermodynamic stabilization of H???PMC-form was attributed to the thermal isomerization through negative photochromism from the SP-form. Copyright © 2011 John Wiley & Sons, Ltd.We proposed a mechanism for negative photochromism according to the relationship of the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO). The relationship between the HOMOs was determined using cyclic voltammetry (CV) and ultraviolet photoelectron spectroscopy (UPS). The LUMOs was determined from the respective optical bandgap. The HOMO level of the H???PMC-form was ?6.1?eV, and that of SP-form was ?5.3?eV.

Posted on 21 September 2011 | 5:22 am