Hydration and protonation ofO-vinylacetoxime in the gas phase: anab initio study

Ab initio HF/6–31G^* calculations ofO-vinylacetoxime monohydrates and cations were performed. Each conformer forms two stable H-complexes with participation of N and O atoms. The former have planar heavy-atom skeletons, whereas the water molecule in the latter is located above the plane of the proton-acceptor complex. The complexes stabilized by N...HO and O...HO bonds have different dipole moments and frequencies of the OH stretching vibrations. The most energetically favorable cation is formed by adding a proton to the C_β atom of the vinyl group ofO-vinylacetoxime. Theap,ap-conformer (ap is antiperiplanar) of this cation is 6.5 and 34.9 kcal mol⁻¹ more stable than the onium cations with the NH⁺ and OH⁺ fragments, respectively, and is characterized by polarization and appreciable lengthening of the N−O and C=C bonds. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 4, pp. 597–600, April, 2000.     

<Emphasis Type="Italic">Ab initio</Emphasis> quantum-chemical calculations of the energies and structures of 1,2-acetylenedithiol isomers

Ab initio quantum-chemical calculations of 1,2-acetylenedithiol isomers were carried out. The MP2(full), DFT(B3PW91, MPW1PW91), G3, G3B3, and CBS-Q methods were used. According to the calculations, the most stable isomers were 1,2-dithiet, thiiranethione, and trans-1,2-dithioglyoxal. The necessity of including basis set functions with a large angular momentum in calculations was confirmed. The relatively high stability of 1,2-dithiet was attributed to the aromaticity of its four-membered ring. It was noted that the carbon-carbon bond in the three-membered rings of the cis- and trans-isomers of thiirenethiols was unusually short.     

Ab initio study of the structure of 2,2-difluoroethanal in the ground and lowest excited triplet electronic states

The molecular structure of 2,2-difluoroethanal (DFE) in the ground (S₀) and lowest excited triplet (T_i) electronic states was investigated byab initio quantum-chemical methods. In the S₀ state, the DFE molecule exists as the only stablecis conformer. The T_i↓S₀ electronic excitation is accompanied by the rotation of the top and the deviation of the carbonyl fragment from planarity. For the DFE molecule in the T_i state, six minima corresponding to three pairs of enantiomers were found on the potential energy surface. Based on this potential energy surface, the problems on torsion and inversion nuclear motions were solved in the one- and two-dimensional approximations, and the interaction between these motions was revealed. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 6, pp. 989–995, June, 2000.     

Ab initio calculations ofp-Cl2C6H4 molecule and its35Cl NQR parameters

Ab initio calculations ofp-dichlorobenzene molecule were carried out using the Hartree-Fock method in the 6–31 G* valence-split basis set. The molecule was also calculated by the MNDO method in the valence sp-basis set for comparison. The populations of the valent p-orbitals of the C and CI atoms were analyzed. The optimized geometry of the molecule as well as its³⁵Cl NQR frequency and the asymmetry parameter of the electric field gradient at the³⁵CI nuclei calculated using the populations of the less diffuse components of the valent p-orbitals of the Cl atoms are in agreement with the corresponding experimental values for the -modification of 1,4-Cl₂C₆H₄.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 9, pp. 2177–2179, September, 1996.     

Ab initio study of the ground state and conformational stability of peroxyacetyl nitrate in internal rotation about the peroxide bond

The molecular and electronic structure of the ground state of peroxyacetyl nitrate (PAN) was calculated by the unrestricted Hartree-Fock-Roothaan method with the use of the standard 3–21G and 6–31G basis set. The potential curve of the internal rotation about the peroxide bond of PAN was calculated with the 6–31G basis set. The curve contains two maxima. The ground state of PAN is characterized by a structure in which groups of atoms adjacent to the peroxide bond lie in planes that are perpendicular to each other (the dihedral angle ϱ(COON) is 89.9°). The calculated barriers to rotation are 19.6 and 66.8 kJ mol⁻¹. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 4, pp. 600–604, April, 1998.     

Ab initio calculations ofo- andm-Cl2C6H4 molecules and their35CI NQR parameters

The MNDO andab initio (Hartree—Fock method, split 6-31G^* basis set, full optimization of geometry) calculations for theo- andm-dichlorobenzene molecules have been performed. The populations of the valent p-orbitals of the carbon and chlorine atoms have been analyzed. Estimations of the³⁵Cl NQR frequencies and the asymmetry parameters of the electric field gradient at the³⁵CI nuclei have been made. It was confirmed that these values are not determined by the contribution of the whole valent p-shell, but only by its less diffuse component.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 9, pp. 2180–2183, September, 1996.     

Ab initio calculation of 4-ClC6H4CH2Cl and peculiarities of electron density distribution in this molecule

Ab initio calculation of the 4-ClC₆H₄CH₂Cl molecule was performed by the restricted Hartree-Fock method in the split valence 6–31 G* basis set with complete optimization of its geometry. Populations of p-orbitals of atoms of this molecule were analyzed.³⁵Cl NQR frequencies and asymmetry parameters of the electric field gradient on³⁵Cl nuclei were estimated on the basis of the populations of valent p-orbitals of CI atoms and their components. Good conformity with the experimental values was obtained when only less diffuse components of p-orbitals were used in calculations of populations.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 4, pp. 823–826, April, 1996.     

Ab initio calculations of activation energy of the reaction of hydrogen exchange on strongly acidic centers

Ab initio calculations of fragments of the potential energy surfaces of hydrogen exchange reactions between H₂, CH₄, and alanine molecules and the H₃O⁺ ion were performed by the restricted Hartree-Fock method, at the second-order Møller-Plesset level of perturbation theory, and by the method of coupled clusters using the 6–31G^* and aug-cc-pVDZ basis sets. The one-center synchronous mechanism of hydrogen exchange reaction was studied and the activation energies and structures of transition states were determined. It was found that the geometric parameters of the H₂ and CH₄ molecules in the transition states are close to those of the H₃ ⁺ and CH₅ ⁺ ions. The higher the proton affinity of the reacting molecule in the reaction studied the lower the activiation energy of hydrogen exchange. The one-center mechanism studied can be used to describe the high-temperature solid-state catalytic isotope exchange (HSCIE) reaction. The results ofab initio calculations of synchronous hydrogen exchange between the H₃O⁺ ion and hydrogen atoms in different positions of the alanine molecule are in good agreement with experimental data on the regioselectivity and stereoselectivity of the HSCIE reaction with spillover-tritium.     

Ab initio calculations of atomic interaction in the ClCH2OCH3 molecule

Ab initio calculations of both ClCH₂OCH₃ and ClCH₂CH₃ molecules and various ClCH₂OCH₃ structures with fixed angles of rotation of the methoxy group about the C−O bond were performed by the restricted Hartree-Fock method in the valence-split 6–31 G^* basis set with full optimization of the geometry. The populations of the valent p-orbitals of the chlorine atoms in these molecules have been analyzed. The³⁵Cl NQR frequencies and the asymmetry parameters of the electric field gradient (EFG) at the³⁵Cl nuclei have been calculated. Good agreement with experimental NQR frequencies was obtained for the calculations where only the populations of the less diffuse 3p-components of these orbitals were used. The³⁵Cl NQR frequency in ClCH₂OCH₃ is lower than that in ClCH₂CH₃ due to the higher population of the less diffuse component of the p_σ-orbital of the Cl atom in the former molecule. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 3, pp. 431–434, March, 1997.     

Ab initio study of molecular structure and internal rotation in methyldicyanophosphine and methyldiisocyanophosphine. The Raman spectrum of methyldicyanophosphine and its interpretation with the use of scaling ofab initio force fields

The equilibrium geometric parameters and structures of transition states of internal rotation for the molecules of methyldicyanophospine MeP(CN)₂ and its isocyano analog MeP(NC)₂ were calculated by the RHF and MP2 methods with the 6–31G^* and 6–31G^** basis sets. At the MP2 level, the total energy of cyanide is −35 kcal mol⁻¹ lower than that of isocyanide and the barriers to internal rotation of methyl group for MeP(CN)₂ and MeP(NC)₂ are 2.2 and 2.7 kcal mol⁻¹, respectively. For both molecules, the one-dimensionalab initio potential functions of internal rotation approximated by a truncated Fourier series were used to determine the frequencies of torsional transitions by solving direct vibrational problems for a non-rigid model. The Raman spectrum of crystalline MeP(CN)₂ was recorded in the range 3500–50 cm⁻¹. The vibrational spectra of this compound were interpreted by scalingab initio force fields calculated by the RHF and MP2 method. The vibrational spectrum of methyldiisocyanophosphine was predicted with the use of the obtained scale factors. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 9, pp. 1703–1714, September, 1998.     

Structure and isomerization of arenonium ions of dichlorobenzenes in the gas phase. A theoretical study

Ab initio MP2 calculations of all isomeric arenoium ions (AI) ofortho-, meta-, andpara-dichlorobenzenes in the gas phase were carried out with full optimization of geometry with the 6–31 G^* basis set. The calculated proton affinities depend substantially on the position of geminal center in the corresponding dichlorobenzenonium ion and decrease in the series 1,2-dichloro-4H-benzenonium>1,2-dichloro-3H-benzenonium>1,2-dichloro-2H-benzenonium; 1,3-dichloro-4H-benzenonium>1,2-dichloro-3H-benzenonium >1,3-dichloro-5H-benzenonium>1,3-dichloro-3H-benzenonium; 1,4-dichloro-2H-benzenonium >1,4-dichloro-4H-benzenonium. The structures of transition states and activation energies (E _a) of almost all 1,2-shifts of H and Cl atoms in Al were determined. The activation energies of migrations of H atoms are about 6 kcal mol⁻¹ less than those of migrations of Cl atoms in similar structures. The isomerization routes and relations between the rate constants for isomerization of dichlorobenzenes through Al were established. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 9, pp. 1726–1731, September, 1998.     

Ab initio studies of tetrafluoroethylene and tetratrifluoromethylethylene molecules in the ground and “twisted” states

Ab initio calculations with full optimization of geometry have been carried out with the 6–31 G^* basis set on tetrafluoroethylene (with the unrestricted Hartree-Fock method—UHF and the second-order Moller-Plesset perturbation theory—MP2) and tetratrifluoromethylethylene (with UHF) molecules in the singlet ground and triplet biradical states. The symmetry of the tetrafluoroethylene molecule in the triplet biradical state was demonstrated to differ from that of ethylene (D _2d ) due to the deviation of fluorine atoms from CCFF plane. The MP2 optimized geometries of ethylene and tetrafluoroethylene were used for higher level calculations (MP3, MP4, CCSD). The energy of the ground state singlet-biradical triplet splitting decreases in the series: ethylene>tetrafluoroethylene> tetratrifluoromethylethylene. These data on energy splitting explain the increase in reactivity toward the [2+2]-cycloaddition on going from ethylene to tetrafluoroethylene. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 4, pp. 605–607, April, 1998.     

Theoretical study on H2 elimination pathways of silylenoid H2SiLiF with CH4, NH3, H2O, HF, SiH4, PH3, H2S, and HCl

Ab initio molecular orbital calculations have been performed to explore the reaction potential energy surfaces of silylenoid H₂SiLiF with XH _n hydrides, where XH _n = CH₄, NH₃, H₂O, HF, SiH₄, PH₃, H₂S, and HCl. We have identified a previously unreported reaction pathway on each reaction surface, H₂SiLiF + H-XH _{n − 1} → H _n XSiLiF + H₂, which involves H₂ elimination following the initial formation of an association complex via a four-membered ring transition state to form the substituted three-membered ring silylenoid H _n XSiLiF and a H₂ molecule. This theoretical calculations suggest that (i) for H₂ eliminations there is a very clear trend toward lower activation barriers and more exothermic interactions on going from left to right along a given row in periodic table, and (ii) for the second-row hydrides, the H₂ elimination reactions are less exothermic than for the first-row hydrides and the reaction barriers are lower for X–S and Cl. Compared to the insertions of H₂SiLiF into XH _n , the H₂ elimination pathways should be unfavorable with higher barrier and lower exothermic.     

Molecular structure,conformational mobility,vibrational spectra,and thermochemistry of peroxyacetic acid: an ab initio and density functional study

The structure of the peroxyacetic acid (PAA) molecule and its conformational mobility under rotation about the peroxide bond was studied by ab initio and density functional methods. The free rotation is hindered by the trans-barrier of height 22.3 kJ mol^–1. The equilibrium molecular structure of AcOOH (C _s symmetry) is a result of intramolecular hydrogen bond. The high energy of hydrogen bonding (46 kJ mol^–1 according to natural bonding orbital analysis) hampers formation of intermolecular associates of AcOOH in the gas and liquid phases. The standard enthalpies of formation for AcOOH (–353.2 kJ mol^–1) and products of radical decomposition of the peroxide — AcO^· (–190.2 kJ mol^–1) and AcOO^· (–153.4 kJ mol^–1) — were determined by the G2 and G2(MP2) composite methods. The O—H and O—O bonds in the PAA molecule (bond energies are 417.8 and 202.3 kJ mol^–1, respectively) are much stronger than in alkyl hydroperoxide molecules. This provides an explanation for substantial contribution of non-radical channels of the decomposition of peroxyacetic acid. The electron density distribution and gas-phase acidity of PAA were determined. The transition states of the ethylene and cyclohexene epoxidation reactions were located (E _a = 71.7 and 50.9 kJ mol^–1 respectively).     

Pathways of the reactions of nucleophilic addition of H2O and HF molecules to formaldehyde in the gas phase and in the complex with formic acid:ab initio calculations

The gradient pathways of the reactions of nucleophilic addition of H₂O and HF molecules to formaldehyde in the gas phase and in the XH…H₂CO…HC(O)OH complex (X=OH, F) were calculated by theab initio RHF/6-31G^**, MP2(fc)/6-31G^**, and MP2(full)/6-311++G^** methods. Both reactions proceed concertedly. The formation of H-bonded bimolecular pre-reaction complexes is the initial stage of the gas-phase reactions; at the same time, no indications of the formation of stable π-complexes were found on the potential energy surfaces of systems under study. The calculated energy barriers to the gasphase reactions exceed 40 kcal mol⁻¹, while those to reactions in the complex XH…H₂CO…HC(O)OH (X=OH, F) become more than halved. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 11, pp. 2146–2154, November, 1998.     

Pentacoordination of boron, carbon, aluminum, and silicon atoms in organic compounds: Anab initio study

Pentacoordination of boron, carbon, aluminum, and silicon atoms in bicyclic organic compounds of the pentalene type was studied using theab initio RHF/6-31G^** and MP2(full)/6-31G^** methods. It was shown that the ability of the atom to form pentacoordinate structures increases on going from B to Al and from C to Si atom,i.e. as the number of the element of Groups IIIA and IVA of the periodic system increases. At the same time, the reverse tendencies are observed in the 2nd and 3rd periods of the periodic system,viz., the ability of the atom to form pentacoordinate structures increases on going from C to B and from Al to Si atom. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 7, pp. 1238–1245, July, 1999.     

Ab initio study of the structure and conformations of 2-fluoroethanal in the ground and lowest excited electronic states

The structure of the conformationally flexible 2-fluoroethanal molecule (CH₂FCHO, FE) in the ground (S₀) and lowest excited triplet (T₁) and singlet (S₁) electronic states was investigated by ab initio quantum-chemical methods. The FE molecule in the S₀ state was found to exist as two conformers, viz., as cis (the F—C—C—O angle is 0°) and trans (the F—C—C—O angle is 180°) conformers. On going both to the T₁ and S₁ states, the FE molecule undergoes substantial structural changes, in particular, the CH₂F top is rotated with respect to the core and the carbonyl CCHO fragment becomes nonplanar. The potential energy surfaces for the T₁ and S₁ states are qualitatively similar, viz., six minima in each of the excited states of FE correspond to three pairs of mirror-symmetrical conformers. Based on the potential energy surfaces calculated for the FE molecule in the T₁ and S₁ states, the one-dimensional problems on the torsion and inversion nuclear motions as well as the two-dimensional torsion-inversion problems were solved.     

Pathways of the reaction of nucleophilic addition of ammonia to formaldehyde in the gas phase and in the complex with formic acid:ab initio calculations

The gradient pathways of the reaction of nucleophilic addition of ammonia to formaldehyde were calculated for free molecules and in the NH₃...H₂CO...HC(O)OH complex by theab initio RHF/6-31G^**, MP2(fc)/6-31G^**, and MP2(full)/6-311++G^** methods. Both reactions proceed concertedly. In the first case, the reaction successively passes through two transitional states with an energy barrier exceeding 35 kcal mol⁻¹. In the case of the complex with formic acid, the reaction follows a conventional pathway, although its activation barrier calculated by the RHF/6-31G^** and MP2(fc)/6-31G^** methods decreases to 12.6 and 3.8 kcal mol⁻¹, respectively. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 1, pp. 13–20, January, 1998.