Ab initio calculations, electronegativity equalisation and group electronegativity

A correspondence betweenab initio calculations, the principle of electronegativity equalisation and group electronegativity has been established within the framework of Mulliken population analysis. Using this we have calculated electronegativities of some 37 groups/atoms. These electronegativities show excellent linear correlation with¹ J _CC coupling constants in monosubstituted benzenes and Inamoto’si scale and a satisfactory one with Wells’ group electronegativity data. The correspondence however required a scaling of charge (obtained byab initio calculations) and a proportionality between the electronegativity of the neutral group and its hardness. It is shown that using these electronegativity values it is possible to calculate group charges in molecules where groups under consideration interact with each other through σ bond only.     

Ab initio molecular orbital calculations on ion pair-water complexes of metal halides and oxides

Ab initio MO calculations are performed on a series of ion-molecular and ion pair-molecular complexes of H₂O + MX (MX = LiF, LiCl, NaCl, BeO and MgO) systems. BSSE-corrected stabilization energies, optimized geometrical parameters, internal force constants and harmonic vibrational frequencies have been evaluated for all the structures of interest. The trends observed in the geometrical parameters and other properties calculated for the mono-hydrated contact ion pair complexes parallel those computed for the complexes of the individual ions. The bifurcated structures are found to be saddle points with an imaginary frequency corresponding to the rocking mode of water molecules. The solventshared ion pair complexes have high interaction energies. Trends in the internal force constant and harmonic frequency values are discussed in terms of ion-molecular and ion-pair molecular interactions.     

Ab initio, density functional theory and structural studies of 4-amino-2-methylquinoline

The Fourier transform infrared (FTIR) and FT-Raman spectra of 4-amino-2-methylquinoline (AMQ) have been recorded in the range 4000–400 and 4000–100 cm⁻¹, respectively. The experimental vibrational frequency was compared with the wavenumbers obtained theoretically by ab initio HF and DFT–B3LYP gradient calculations employing the standard 6-31G** and high level 6-311++G** basis sets for optimised geometry of the compound. The complete vibrational assignment and analysis of the fundamental modes of the compounds were carried out using the experimental FTIR and FT-Raman data, and quantum mechanical studies. The geometry and normal modes of vibration obtained from the HF and DFT methods are in good agreement with the experimental data. The potential energy distribution of the fundamental modes was calculated with ab initio force fields utilising Wilson's FG matrix method. The NH-π interactions and the influence of amino and methyl groups on the skeletal modes are investigated.     

钒邻苯二酚双齿配合物从头算研究

运用Gaussian 94W量子化学程序包,采用LanL2DZ基组,对钒邻苯二酚双齿配合物结构单元V(cat)₃^n-(n=1,2)进行从头算研究。探讨这些配合物的稳定性、分子轨道能量、原子净电荷布居规律以及处于前沿的一些分子轨道的组成特征,为过渡金属双齿配合物的合成、分子组装及其在酶和蛋白质中的活性部位研究提供理论参考。     

Crystal structure and ab initio calculations of a cyano-carbamimidic acid ethyl ester

The structure of one tautomer (amine form) of cyano-carbamimidic acid ethyl ester or (amino-ethoxy-methylidene)aminoformonitrile (CAS: 13947-84-7) was determined by single crystal X-ray diffraction. Ab initio quantum chemical calculations at the B3LYP, MP2 and G3 levels were performed to investigate the stability and the formation of the different tautomers and conformers. The calculations indicate that the amine form is the more stable tautomer, showing a high degree of electron conjugation. The most stable amine conformer located by the calculations corresponds to the crystallized structure. On the contrary, in the less stable imine form, the conjugation is separated by a N2–C2 single bond.     

Ab initio and DFT calculations for the structure and vibrational spectra of cyclopentene and its isotopomers

Ab initio calculations using the MP2/cc-pVTZ basis set do an excellent job of predicting the inversion barrier (247 vs. 232 cm⁻¹) and dihedral angle (26°) of cyclopentene. DFT calculations also do an excellent job of predicting the vibrational frequencies of the d₀, d₁, d₄, and d₈ isotopomers. They have also allowed the reassignments of several of the vibrational frequencies.     

Ab initio prediction of the low-temperature phase diagrams in the systems CsX–LiX (X = F, Cl, Br, I)

We have calculated the low-temperature phase diagrams for the ternary alkali halides CsX–LiX (X = F, Cl, Br, I) at an ab initio level without any recourse to experimental information. The starting point of our general approach is the global exploration of the enthalpy landscapes for many different compositions in these systems. Candidates for both ordered stoichiometric modifications and crystalline solid-solution phases are identified, and their free enthalpies are computed at an ab initio level. From this the low-temperature phase diagrams are derived. We find that in all systems under investigation only crystalline ordered phases should be present, in agreement with available experimental data. Furthermore, we predict several new thermodynamically stable and metastable phases in these systems.     

Ab initio evaluation of oxidation numbers in some substituted benzenes

Giambiagi’s definition of oxidation number of an atom in a molecule has been applied successfully in theab initio SCF theory to calculate oxidation numbers using STO-3G and 4–31G basis sets for some substituted benzenes involving -F, -OH, -CH₃ and -NH₂ as substituents. The present study suggests that the oxidation numbers also seem to be indicative of their orientational behaviour like the net atomic charges.     

Ab initio study on the mechanism of reaction HNCO+NH2

Ab initio UMP2 and UQCISD(T) calculations, with 6-311G** basis sets, were performed for the titled reactions. The results show that the reactions have two product channels: NH₂+ HNCO?NH₃+NCO (1) and NH₂+HNCO?N₂H₃+CO (2), where reaction (1) is a hydrogen abstraction reaction via an H-bonded complex (HBC), lowering the energy by 32.48 kJ/mol relative to reactants. The calculated QCISD(T)//MP2(full) energy barrier is 29.04 kJ/mol, which is in excellent accordance with the experimental value of 29.09 kJ/mol. In the range of reaction temperature 2300–2700 K, transition theory rate constant for reaction (1) is 1.68×10¹¹–3.29×10¹¹ mL·mol^-1·s^-1, which is close to the experimental one of 5.0×10¹¹mL·mol^-1·s^-1or less. However, reaction (2) is a stepwise reaction proceeding via two orientation modes,cis andtrans, and the energy barriers for the rate-control step at our best calculations are 92.79 kJ/mol (forcis-mode) and 147.43 kJ/mol (fortrans-mode), respectively, which is much higher than reaction (1). So reaction (1) is the main channel for the titled reaction.     

Ab initio direct dynamics studies on the hydrogen abstraction reactions of CF2H2 and CF3H with F atom

The dynamic properties of the hydrogen abstraction reactions of CF₂H₂ and CF₃H with F atom are investigated in the temperature range of 182–2000 K. The minimum-energy path (MEP) is optimized at MP2/6-311 G(d, p) level, then the energy profiles are refined at the CCSD(T)/6-311++G(3df, 2pd) level (single-point). The theoretical rate constants, which are calculated by the variational transition state theory (VTST) including the small curvature tunneling (SCT) correction, are in good agreement with the experimental ones. It is found that the rate constant of the CF₂H₂ + F reaction are larger than that of the CF₃H + F reaction and the activation energies exhibit in the just opposite order. This phenomenon can be rationalized by the hardness η of the halomethane molecules. The comparison of the two reactions with the CFH₃ + F reaction is made. It is found that the rate constants decrease in the order of CFH₃ + F > CF₂H₂ + F > CF₃H + F. The effect of fluorine substitution leads to a dramatic increase in the activation energy and a decrease in the preexponential factor. We hope that present theoretical studies for these compounds can give further information concerning how fluorine substitution affects the rate constants of hydrogen abstraction reactions.     

Electrochemical study on the keto-enol tautomerization of p-hydroxyphenylpyruvic acid in aqueous solution

The keto-enol tautomerization of p-hydroxyphenylpyruvic acid (pHPP) in aqueous solutions and the complexation reaction between enolic pHPP and boric acid have been studied by electrochemical techniques including linear sweep voltammetry (LSV), pulse voltammetry, and cyclic voltammetry (CV), combining with UV spectrometry. Electrochemical techniques reveal that in aqueous solution, there are two tautomers of pHPP: enolic form and ketonic form; the former exists mainly in freshly prepared pHPP solution, and the latter exists mainly in equilibrium solution. Both enolic and ketonic pHPP are electroactive. The electrochemical oxidation of enolic pHPP gives rise to two anodic waves, I(a) and II(a), while the electrochemical oxidation of ketonic pHPP only results in the observation of the second wave II(a). The oxidation process I(a) is revealed to be associated with the quasi-reversible, two-electron two-proton oxidation of "C=C"group at the side chain of enolic pHPP, and the oxidation process II(a) is proposed to result from the irreversible oxidation of phenolic hydroxyl group. It is observed that in aqueous solution, enolic pHPP can quickly complex with boric acid to yield enol-borate complex that can also oxidize at a glassy carbon electrode to yield an anodic wave.     

Ab initio molecular orbital study of dialuminum-ethylene complexes

The geometries of the various isomeric dialuminum-ethylene complexes Al₂C₂H₄ have been optimized using the STO-3G, 3-21G^(*) and 6-31G^(**) basis sets. Vibrational frequency calculations show that only two out of the six structures examined are thermodynamically stable. Unlike the case of Li₂C₂H₄, 1,2-dialuminoethane is most stable and more ethane-like than 1,2-dilithioethane perhaps due to the larger covalent character of the C---Al bond. Besides, 1,2-dialuminoethane has been found, in contrast to ethane and 1,2-dilithioethane, to have no stable gauche conformer.     

Ab initio study of ladder-type polymers: Polythiophene and polypyrrole

An ab initio study of four polymers, polythiophene, polypyrrole, ladder-type polythiophene, and ladder-type polypyrrole is presented. Upon an analysis of the variation of the band gap when comparing the non-ladder and the ladder-type polymers, a discrepancy is found between the thiophene and the pyrrole polymer families. For polythiophene, the ladder-type polymer has a larger band gap than the non-ladder polymer whereas the opposite is found for the pyrrole polymers. This difference is explained in terms of the competition between the bond length alternation and the effect of the charge density distribution in the carbon backbone.     

Hyperpolarizabilities of hydrogen-bonded complexes of phenol derivatives with ammonia: PM3 and ab initio studies

Polarizability and first hyperpolarizability values of the hydrogen-bonded complexes formed by nitrosubstituted phenols with ammonia have been calculated using PM3 and ab initio (STO-3G) methods. It has been shown that enhancement of the polarizability (Δα) as well as the first hyperpolarizability (Δβ) of the complex arises from the hydrogen bond interaction between the phenol derivative and ammonia.     

Ab initio study of atropisomers of derivatives of 1,8-di-pyridine 9H-fluorene, dibenzo[b,d]furan, 9H-carbazole and dibenzo[b,d]thiophene

Ab initio calculation at HF/6-31G^* and MP2/6-31G^* levels of theory for geometry optimization of some syn- and anti-1,8-di-pyridine 9H-fluorene, dibenzo[b,d]furan, 9H-carbazole and Dibenzo[b,d]thiophene are reported. The rotational barrier energy, heat of formation and Gibbs energy are determined for the conversion of the anti-(syn) to the syn (anti)-isomers at 25 °C in the gas phase. The models are chosen as isomers of 9H-fluorene, dibenzo[b,d]furan, 9H-carbazole and dibenzo[b,d]thiophene as scaffold with pyridine as module. Results obtained show that (at equilibrium) for most of atropisomers the syn- is favored over the anti-isomer. Moreover, the ground state structures show that the modules are not parallel to each other but are tilted away in order to increase separation and there by minimize electrostatic repulsion. In atropisomers of 9H-carbazole the isomers are showing an attraction due to the presence of nitrogen atom. Influence of the position of nitrogen atom on the magnitude of the rotational barriers in these atropisomers is also studied.     

Density functional theory study of tautomerization of 2‐aminothiazole in the gas phase and in solution

The amino/imino tautomeric equilibrium in the isolated, mono‐, di‐, and trihydrate forms and dimer of 2‐aminothiazole, and the effects of hydration or self‐assistance on the transition state structures corresponding to proton transfer from the amino to imino form, have been investigated by the B3LYP method in conjunction with 6‐31+G(d,p) and 6‐311+G(3df,2p) basis sets in the gas phase and in solution. The amino form has been found to be the predominant tautomer. The tautomeric barrier heights for water‐ and self‐assisted tautomerization reactions are significantly lower than that from the amino to imino form by the intramolecular proton transfer, showing the catalytic effect of water molecule(s) and the important role of 2‐aminothiazole itself for intermolecular proton transfer. Comparison between the tautomeric barriers demonstrates that the self‐association tautomerization through the dimerization is the most favorable pathway. Bulk solvent effects have been taken into account using the polarizable continuum model (PCM) of water and CCl₄. The polar medium is favorable for the population of the imino form. The amino/imino equilibrium is also analyzed using the aromaticity index nucleus‐independent chemical shift (NICS); the NICS values for the amino form (about ?10 ppm) are more negative than the imino species (about ?8 ppm), showing that the amino form is more stable. The time‐dependent density functional theory (TDDFT) calculations of electronic absorption spectra suggest that the λ_max of dimer is 255 nm. The oscillator strength of the imino forms is less than the amino form, and increases with the polarity of the solvents. All calculations for the tautomerization of 2‐aminothiazole are in reasonable line with the available experiments. © 2006 Wiley Periodicals, Inc. Int J Quantum Chem, 2007     

The tautomerization and ring closure in the Claisen rearrangement: A DFT study

Elementary processes of the aromatic Claisen rearrangement were investigated by DFT calculations. First, rearrangements of four substrates Ph—O—CH₂—CHCH₂ [A], Ph—O—CH₂—CHCH(OMe) [B], Ph—O—CH₂—CHCH₂····BF₃ [C], and Ph—O—CH—CHCH(OMe)····BF₃ [D] were examined. In these systems, the tautomerization is initiated by the intermolecular proton transfer involving the transient ion‐pair intermediate. An ignition‐propagation chain‐reaction mechanism in the tautomerization was suggested. For [A], the (ortho‐allyl phenol → α‐methyl‐dihydrobenzofuran (α‐methyl‐cumarane)) process was found to be ready and the product of the Claisen rearrangement seems to be the cumarane rather than the phenol. In [D] (activated both by the terminal methoxy group and by the BF₃ catalyst), not the [3,3]‐sigmatropic shift but the tautomerization is the rate determining step. Second, the parent system, Ph—O—CH₂—CHCH₂, was investigated with (H₂O) _n (n = 2, 4, 6, and 10) systematically. The tautomerization takes place by the proton transfer via the water dimer or trimer. Except n = 2, similar changes of Gibbs free energies were obtained from the ether substrate to the cumarane.     

Gas phase ionization of 1,3-propanedial tautomeric forms: A theoretical study

Two stable 1,3-propanedial tautomers and three their anions have been studied theoretically at MP2 and DFT levels of theory. The energies, structural parameters, ionization potentials, and vibration frequencies have been calculated at the two theoretical levels in order to compare the accuracy of the methods used. The ionization potential of the end form of 1,3-propanedial enol form was estimated to be 752 kJ mol^?1; the first and second potentials of the diketo form of 1,3-propanedial are 661 and 1239 kJ mol^?1, respectively.     

Implementation of ab initio multiple spawning in the Molpro quantum chemistry package

The ab initio multiple spawning (AIMS) method has been developed to solve the electronic and nuclear Schrodinger equations simultaneously for application to photochemical reaction dynamics. We discuss some details of the implementation of AIMS in the Molpro program package. A few aspects of the implementation are highlighted, including a new multiple timescale integrator and a scheme for solving the coupled-perturbed multiconfiguration self-consistent field (CP-MCSCF) equations in the context of ab initio molecular dynamics. The implementation is very efficient and we demonstrate calculations on the photoisomerization of ethylene using more than 5000 trajectory basis functions. We have included the capability for hybrid quantum mechanics/molecular mechanics (QM/MM) simulations within AIMS, and we investigate the role of an argon solvent in the photoisomerization of ethylene. Somewhat surprisingly, the surrounding argon has little effect on the timescale of non-adiabatic quenching in ethylene.     

Ab initio study on alkyl radical decomposition and alkyl radical addition to olefins

The β bond dissociation of alkyl radicals and their reverse reactions, the addition of alkyl radicals to olefins were studied by G3MP2 level of theory to obtain a consistent kinetic data set. Both reaction families can be classified depending on the type of radical formed by β bond scission, namely the CH₃, primary, secondary tertiary radical formed. The kinetics of the reaction classes were described by only a limited number of Arrhenius parameters. The unified A factor of 10^13.7 s⁻¹ was found for all β bond dissociations. The Arrhenius activation energies are 125, 121, 113 and 103 kJ mol⁻¹, for methyl, primary, secondary, and tertiary radicals, respectively. The activation energies of 32, 25 and 18 kJ mol⁻¹ are calculated for the terminal addition of primary (including methyl), secondary, and tertiary radicals to olefins, respectively. The biologically important nonterminal radical additions to olefins have higher barriers of 37, 31 and 35 kJ mol⁻¹, respectively. At room temperature both strongly exothermic additions can compete with H-atom abstraction. New groups for Benson’s group additivity rules were defined to describe activation parameters for the β bond dissociation reactions. The group values were calculated by using the ab initio heats of formation of transition state structures.