Conformational stability of peroxynitric acid molecule and the barriers to internal rotation about the O−O and N−O bonds

The structure of conformers and potential curves of the internal rotation (PCR) about the O?O and N?O bonds in peroxynitric acid (PNA) were calculated by the unrestricted Hartree-Fock-Roothaan method. The standard valence-split 6–31G and 6–31G^* basis sets were used. The presence of two maxima on every curve has been shown. To refine the values of barriers to the internal rotation in the regions of minima and maxima of PCR, calculations taking into account the electron correlation energy have been carried out at the second- and fourth-order Møller-Plesset level of perturbation theory (MP2 and MP4, respectively). At the MP4/6-61G^* level of approximation, the barriers to the rotation about the O?O bond are equal to 8.6 kJ mol^?1 and 14.7 kJ mol^?1, and both barriers to the rotation about the N?O bond are equal to 33.5 kJ mol^?1. The results are compared with those published for PCR in hydrogen peroxide and peroxynitric acid.     

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.     

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 investigation of tautomeric stability,molecular structure,and internal rotation of methylphosphonic dicyanide,methoxydicyanophosphine, and their isocyano analogs

The equilibrium geometric parameters and structures of the transition states of internal rotation for MeP(O)(CN)₂, McOP(CN)₂, and their isocyano analogs, MeP(O)(NC)₂ and MeOP(NC)₂, have been calculated by theab initio SCF method and with inclusion of electron correlation effects according to the second-order Muuller-Plesset perturbation theory (MP2). At both levels the 6-31G* basis set has been used. The estimation of relative stability of these tautomeric forms depends largely on the calculation level. The total energies of the cyanides calculated by the MP2 method are 25–30 kcal mol^–1 lower than those of the corresponding isocyanides. The oxo-tautomeric forms containing four-coordinate phosphorus are 15–25 kcal mol^–1 more stable than the three-coordinate phosphorus aci-derivatives. The internal rotation potential curves of the aci-forms are characterized by a deep minimum for thetrans-arrangement of the methoxy group and phosphorus lone electron pair. Two additional less clearly pronounced minima are located symmetrically on both sides of the weak maximum, which corresponds to thecis-arrangement. The equilibrium oxo-form structures have a staggered configuration of the methyl group with respect to the phosphorus atom bonds.Translated from izvestiyaAkademii Nauk. Seriya Khimicheskaya, No. 5, pp. 1104–1115, May, 1996.     

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).     

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.     

Ab initio calculations of methionines and their protonated forms

Ab initio calculations of molecular and electronic structures of neutral molecules and protonated forms of methionine and its derivatives in the gaseous phase were carried out by the Hartree-Fock method using the 6–31G^* basis set with full geometry optimization. Proton affinities of methionine (1), methionine sulfoxide (2), and methionine sulfone (3) were calculated for different modes of coordination of the proton. The results of calculations demonstrated that in protonated forms of 1 and 3, bonding between the proton and the N atom is most favorable, while in protonated form of 2, bonding between the proton and the O atom of the SO group is most favorable. The proton affinities of the amino acids are as follows: 223.2 (1), 241.2 (2), and 221.5 (3) kcal mol⁻¹,i.e., methionine sulfoxide 2 exhibits the highest proton affinity in the series of the amino acids under consideration. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 8, pp. 1487–1490, August, 1998.     

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.     

Anab initio quantum-chemical study of proton addition to F-, Ch3-, and CF3-substituted ethylene derivatives

Protonated forms of the molecules of ethylene derivatives with the general formula C₂X₂Y₂ (X=Y=H) (1), F (2), CH₃ (3) CH₃ (4); X=F, Y=H:cis-(5)trans- (6)) were calculated by theab initio MP2/6-31 G^* method with full geometry optimization. The minima and saddle points located on the potential energy surface (PES) of the protonated ethylene molecule correspond to the stationary states and transition states of proton migration, respectively. The stationary states are characterized by a nonclassical geometry of carbocations similar to that of π-complexes, whereas the transition states have a classical structure. Unlike1, the carbocations of molecules2–6 have the classical structure. The saddle points on the PES of the ethylene derivatives correspond to the structures of the π-complex type, which are the transition states of proton migration between the C atoms of the ethylene bond. The barrier to rotation about, the C−C bond depends essentially on the substituent nature. Published inIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 8, pp. 1333–1337, August, 2000.     

Molecular structure and conformational transitions of dichloroacetylchloride

RHF and MP2 techniques in 6–31G(d) basis set have been used to determine the structure of the isolated molecule CHCl₂COCl in two stable conformations (cis-and gosh-), as well as in transition states arising due to the rotary motion of CHCl₂ group around the C—C bond. The energy gap between the conformers and the relevant potential barriers has been calculated using the obtained potential dependence of the internal rotation. Plausible conformation of dichloroacetylchloride is discussed on the basis of ³⁵Cl NQR.     

Theoretical model of internal rotation in monosubstituted derivatives of furfural

The present work explores the effect of substitution in all free positions of furfural on conformational preferences of formyl group by using ab-initio calculations at the MP2/6-31G(p,d) level of theory. Theoretical modeling was made in vacuo. The selected substituents were -CH(3), NH(2), NO(2) and F groups in 3, 4, 5 and ipso carbonyl positions. Geometries of all derivatives were analyzed and it is ascertained that substitution has not important consequences on furan ring geometry. Differences of energy between OO-cis and trans conformers and energy barriers between them are described and extreme cases are explained. Interesting features appear in the cases of -NH(2) and -NO(2) groups, and particularly when the 3 and ipso carbonyl positions are substituted. Variations in energy barriers are correlated with variations in C2-C6 distances for the transition states and planar forms. Substitution effect on Mülliken charges are analyzed and related with internal rotation energy barriers and differences between conformers.     

Conformational Study of Naringenin in the Isolated and Solvated States by Semiempirical and Ab Initio Methods

Naringenin is a natural widespread flavanone occurring in different foodstuffs that presents several important biological activities. Although its properties are well documented, its mechanisms of action are still controversial. The present article reports a conformational analysis of naringenin, using the semiempirical AM1 and ab initio methods, at the Hartree–Fock level of theory. The 3-21G, 3-21G*, 6-31G, and 6-31G** basis sets were used. The electron correlation effects were included through the Møller–Plesset second-order perturbation theory. The solvation of naringenin has been investigated through the standard SCRF, the supermolecule (SM), and the combined SM/SCRF models. The results have shown that there are two degenerate forms of naringenin, differing mainly by the orientation of a hydroxyl group (C4—OH). The energy barrier for the interconversion between them is ca. 6 kcal.mol^–1, suggesting some conjugation between the -system of the aromatic B ring and the hydroxyl group (C4—OH).     

Structure and Internal Rotation of Molecules in Chlorodifluoroacetic Acid

The potential energy profile of an isolated CF₂ClCOOH molecule with a CF₂Cl group rotating around the C–C bond was determined by the Hartree–Fock method using the 6-31G(d) basis set. Barriers to internal rotation were estimated for this molecule; its geometrical parameters were found for the equilibrium and transition states that are due to the torsion potential with unequal wells. Crystal effect on CF₂Cl reorientations in solid chlorodifluoroacetic acid has been evaluated.     

An ab initio CI study of the ground and excited states of p-quinodimethane

Configuration interaction (CI) studies of the ground, electronically excited singlet and triplet states and of the ionized states (cations) are reported for p- quinodimethane (p-xylylene). The calculated ionization potentials are compared with the experimental photoelectron spectrum for the low-energy ionization region. The two high-energy low-intensity flanks of the second and third band observed in the photoelectron spectrum are assigned to be due to the two non-Koopmans' cation states, ascribing to shake-up ionizations.The calculated singlet-singlet and singlet-triplet excitation energies are compared with previous semiempirical MO results and experimental data.     

An Ab Initio Study of the Relative Stabilities and Molecular Structures of 3-Substituted 2,5-Dihydrofurans and 4-Substituted 2,3-Dihydrofurans

The geometry optimized structures and total energies of 3-substituted (R) 2,5-dihydrofurans (a) and their isomers, 4-substituted 2,3-dihydrofurans (b), have been determined by ab initio calculations at the MP2/6-31G^*//HF/6-31G^* level. The nature of the moiety R has a marked effect on the relative total energies of the isomeric forms: at the calculation level cited, the reaction enthalpies for the a b isomerization range from +4.7 kJ mol^–1 for R = MeO to –30.5 kJ mol^–1 for both R = COOMe and R = NO₂. The reaction enthalpies appear to be controlled by the electronic effect of R on the strength of p- conjugation in b. The a isomer has a planar ring, independent of R (excluding NH₂), whereas the planarity of b depends on the electronic nature of R: the 2,3-dihydrofuran ring is planar for both R = COOMe and R = NO₂, but nonplanar for less conjugation-enhancing substituents.     

Intramolecular Hindrances to CCl3 Reorientations in CCl3PCl4 and (CCl3)2PCl3 Crystals

The trigonal bipyramidal structure of trichloromethyl-tetrachlorophosphorane CCl₃PCl₄ and bis(trichloromethyl)trichlorophosphorane (CCl₃)₂PCl₃ molecules with apical CCl₃ groups is responsible for steric hindrances to reorientations of these groups around the C–P bond. The intramolecular barriers to CCl₃ reorientations calculated for the two phosphoranes (Hartree–Fock method using the 6-31G(d) basis set) are 105.9 kJ/mole and 106.6 kJ/mole, respectively. These internal barriers are high enough to block the reorientational motion of the CCl₃ groups, as found previously by ³⁵Cl NQR for the stated crystals.     

Ab initio study of the conformational equilibrium of ethylene glycol

Summary The conformational equilibrium of ethylene glycol (CH₂OHCH₂OH) has been examined by performing geometry optimizations at the 6-31G*, MP2/6-31G* and 6-31G** levels. Final energies have been calculated at the MP3 level with the optimized geometries. The two most stable conformers are atGg andgGg but it is verified that the inclusion of electronic correlations reduces their energy difference of 0.6 kcal/mol at the HF level to less than 0.2 kcal/mol. The possible coexistence of the two most stable conformers is in agreement with some previous studies of Frei et al. For thetXg conformer a detailed analysis of the intramolecular potential as a function of rotation around the C-C bond is also reported.     

Electronic structure and reactivity of S—S dications

The electronic structure and reactivity of some S—S dications were studied at the MP2/6-31G* level of theory. The results obtained indicate a stepwise electrophilic addition of disulfonium dication moiety to the double C=C bond to be the preferable mechanism.     

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.     

The structure of CX2YNO (X, Y=F, Cl) molecules in the ground and lowest excited singlet electronic states

Ab initio quantum-chemical calculations of equilibrium geometric parameters, vibrational frequencies, and potentials of internal rotation for CCIF₂NO and CCl₂FNO molecules in the ground (S₀) and lowest excited singlet (S₁) electronic states were performed. The results of calculations were compared with experimental data. A new interpretation of experimental spectra of the CCIF₂NO molecule was suggested. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 8, pp. 1453–1458, August, 1999.