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.     

X-ray spectral and quantum chemical analyses of the electronic structure of poly(monofluorocarbon)

The electronic structure of poly(monofluorocarbon) has been studied by X-ray spectral and quantum chemical methods. Calculations were performed in terms of the MNDO method, with the fluorographite layer modeled by clusters of different sizes. The high-resolution CK_a and FK_a spectra have been obtained; the calculated spectra are consistent with the experimental ones. It has been shown that carbon and fluorine are bonded mainly through the σ bonds. The p orbitals of fluorine atoms that are perpendicular to the C-F bond are not involved in the chemical bond, while the transitions from the molecular orbitals consisting of these p orbitals are responsible for the main maximum in the FK_a spectrum. Deceased. Institute of Inorganic Chemistry, Siberian Branch, Russian Academy of Sciences. Translated fromZhurnal Strukturnoi Khimii, Vol. 36, No. 4, pp. 630–635, July–August, 1995. Translated by I. Izvekova     

Molecular and electronic structures of anionic σ-complexes of 9-nitroanthracene and its derivatives studied byab initio HF/6-31G** calculations

The molecular structures of anionic σ-complexes of 9-nitroanthracene and its 10-methoxy and 10-acetonyl derivatives were calculated by theab initio quantum-chemical HF/6-31G^** method. The central ring of the anthracene fragment adopts a boat conformation. The values of the bond lengths and bond orders in the compounds under study indicate that the contribution of theaci-resonance form to the structure of the nitro group is substantially larger than that estimated for 2,4,6-trinitrobenzene derivatives. The substituents have no substantial effect on the geometry of the anion. The negative charge is localized mainly on the oxygen atoms of the nitro group and of the substituents. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 11, pp. 2142–2145, November, 1998.     

Theoretical studies on the substitution effect of fluorine on ethylene

The substitution effect of fluorine on ethylene is investigated by means of studyingthe properties of the charge distribution at the bond critical points with the theory of atomsin molecules.It is found that fluorine atom acts not only as a σ electron acceptor,but also asa π electron donor,and these double effects are reflected in the quantity of ellipticity,Lap-lacian and the charge density of charge distribution at the bond critical points.For C—C,C—Fbonds,the major axis of elliptical contours is perpendicular to the molecular plane,but forC—H bond,it is parallel to the molecular plane.Other effects originating from the substi-tution have also been discussed.     

Investigation of azomethine group vibrations in aromatic schiff bases and their N-oxides by vibrational spectroscopy methods

The effects of fluorine substitution in benzene rings and quaternization of the nitrogen atom in Schiff bases of the general formula R−CH=N−R′ on the vibration characteristics of the azomethine group are analyzed. Normal vibrations and intensities of IR bands are calculated. It is shown that the spectroscopic behavior of the C=N bond is almost independent of the electronic effects of substituents in the benzene rings of the molecules studied, and the changes in the intensities of ν_C=N bands are caused by interactions between vibrations. This points to the stability of this bond in the series of molecules under consideration. For diphenylnitrones, N→O vibrations are identified. L. M. Litvinenko Institute of Physical Organic Chemistry and Carbon Chemistry. Translated fromZhurnal Strukturnoi Khimii, Vol. 36, No. 2, pp. 310–315, March–April, 1995. Translated by L. Smolina     

Molecular structure and electronic effects of chlorophosphorane from mndo calculations and35Cl NQR data

The molecular geometries of eleven compounds of pentacoordinated phosphorus are optimized and charges on the central phosphorus atom and on the ligands are calculated by the semiempirical MNDO method. In cases of chlorine atoms as ligands, correlations between the chlorine-35 NQR frequencies and charges on the chlorine atoms of P-Cl bonds, on the one hand, and these bond lengths, on the other, are established. Parameters of the corresponding correlation equations are obtained and discussed. The efficiency of this calculation procedure and NQR method for analyzing the geometrical structure and electron distributions in chlorophosphorane molecules is shown. Translated fromZhurnal Struktumoi Khimii, Vol. 38, No. 6, pp. 1074–1082, November–December, 1997.     

Density functional theory and topological analysis on the hydrogen bonds in cysteine–propanoic acid complexes

The complexes formed via hydrogen bonding interactions between cysteine and propanoic acid have been studied at the density three-parameter hybrid functional DFT-B3LYP/6-311++G(d,p) level regarding their geometries, energies, vibrational frequencies, and topological features of the electron density. The quantum theory of atoms in molecules (QTAIM) and natural bond orbital (NBO) analysis was employed to elucidate the interaction characteristics in cysteine–propanoic acid (Cys–Prop) complexes. More than 10 kinds of hydrogen bonds (H-bonds) including intra- and inter-molecular H-bonds have been found in Cys–Prop complexes. The results show that both the strength of H-bonds and the deformation are important factors for the stability of Cys–Prop complexes. The strongest H-bonds (O2H^A···O1^B and O2H^A···O1^B) exist in the most stable Cys–Prop complex. The stronger H-bonds formed between hydroxyl and O (or N) atom usually stronger than those involve C (or S) atom. Relationships between the electron density (ρ) of BCP and H-bond length as well as the Fock matrix element (F _ij) has also been investigated and used to study the nature of H-bonds. Moreover, the results show that the change of the bond length linearly correlates with the corresponding frequency shift.     

Structure of N-chloromethyllactams and features of the interaction of atoms in them as a result of ab initio calculations

Quantum-chemical calculations have been carried out by the RHF/6-31G(d) and MP2/6-31+G(d) methods of molecules of N-chloromethylpyrrolidone, N-chloromethylcaprolactam, N-chloromethyl-succinimide, and N-chloromethylphthalimide with full optimization of their geometry, and also N-chloromethylpyrrolidone molecule by the RHF/6-31G(d) method at various angles of rotation of the CH₂Cl group around the C―N bond. It was shown that the lower frequencies of the ³⁵Cl NQR of the first two molecules in comparison with the later are mainly determined by the high populations of the p _σ -orbitals of their Cl atoms. The population of the orbitals of the unshared electron pair of the N atom is practically unchanged on rotating the CH₂Cl group, but the N atom polarizes the C―Cl bond in the indicated molecule. This does not confirm the supposed p,σ*-conjugation in the Cl―C―N grouping. Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 10, pp. 1537–1544, October, 2008.     

Dipolar spirocyclic σ-complexes based on of 4,6-dinitrobenzofuroxan

The first dipolar spiro-σ-complexes with a superelectrophilic dinintrobenzofuroxan fragment and tropolone systems with diastereotopic substituents were synthesized. The kinetics of their enantiotopomerization, which occursvia cleavage-formation of the C_spiro-heteroatom bond, was studied by dynamic¹H NMR. The stereorigidity of dinitrobenzofuroxan spiro-complexes in this degenerated process increases in the series: tropolone<aminotropone<aminothiotropone≈aminotroponimine. The two last posses the highest kinetic stability compared to all known zwitterionic spiro-complexes. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 8, pp. 1509–1512, August, 1997.     

The conjugation and effective charges of the atoms at the triple bond in germylacetylenes

The integrated extinction coefficients (A) of the C≡C stretching modes in the IR spectra of 12 germylacetylenes Me₃GeC≡CR are determined by the resonance interactions of substituents with the triple bond. TheA ^1/2 values change linearly with change in the difference between the effective π-electron charges on the atoms at the triple bond and σ⁰ _R constants of organic substituents R. The average value of the σ⁰ _R constant of the Me₃Ge substituent in the compounds studied is +0.06. The resonance acceptor effect of the Me₃Ge substituent toward the triple bond (d,π-conjugation) is stronger than the donor effect (σ,π-conjugation). Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 8, pp. 1569–1574, August, 1998.     

Natural bond orbital analysis of some para-substituted N-nitrosoacetanilide biological molecules

Theoretical study of several para-substituted N-nitrosoacetanilide biological molecules has been performed using density functional B3LYP method with 6-31G(d,p) basis set. Geometries obtained from DFT calculation were used to perform natural bond orbital analysis. The p characters of two nitrogen natural hybrid orbital (NHO) σ _N3–N2 bond orbitals increase with increasing σ _p values of the para substituent group on the benzene, which results in a lengthening of the N₃–N₂ bond. The p characters of oxygen NHO σ _O1–N2 and nitrogen NHO σ _O1–N2 bond orbitals decrease with increasing σ _p values of the para substituent group on the benzene, which results in a shortening of the N₂=O₁ bond. It is also noted that decreased occupancy of the localized σ _N3–N2 orbital in the idealized Lewis structure, or increased occupancy of s_N3-N2^* \sigma_{\rm N3-N2}^{\ast} of the non-Lewis orbital, and their subsequent impact on molecular stability and geometry (bond lengths) are also related with the resulting p character of the corresponding nitrogen NHO of σ _N3–N2 bond orbital.     

Density functional calculation for ozone-ethylene complexes

The density functional method (exchange correlation functional Becke3LYP) is used to calculate the potential energy profiles for the reaction of ozone with ethylene. It is shown that “direct” epoxidation of the C = C double bond demands high activation energy and is unlikely for both themwchemical and photochemical reactions of ozonolysis. Translated fromZhumal Struktumoi Khimii, Vol. 41, No. 2, pp. 240–247,     

X-ray photoelectron,emission, and absorption studies of the electronic structure of molybdenum and tungsten trisulfides and their intercalates

The electronic structure of molybdenum and tungsten trisulfides and their lithium intercalates is investigated by X-ray emission, absorption, and photoelectron spectroscopy. The electron density of lithium atoms intercalated into molybdenum and tungsten trisulfides is distributed over both the metal and sulfur atoms, with more over the atoms of the disulfide pair. These compounds are assumed to have an additional donor-acceptor bond involving a lone pair of disulfide sulfur atom electrons and the vacant levels of the metal. A qualitative scheme of the electronic structure of MoS₃ is constructed based on experimental results and assuming the presence of a donor-acceptor bond. Institute of Inorganic Chemistry, Siberian Branch, Russian Academy of Sciences. Translated fromZhurnal Strukturnoi Khimii, Vol. 35, No. 6, pp. 81–93, November–December, 1994. Translated by L. Smolina     

Electronic effects of Me3SiOCR2 substituents in acetylene derivatives

The σR⁰ and σ_p parameters of Me₃SiOCR₂ and HOCR₂ substituents at the triple bond were determined using the IR spectra of individual acetylene derivatives and their H-complexes. These parameters vary as the effective charge on the atoms of the C≡C fragment of terminal acetylenic alcohols and their trimethylsilyl ethers changes due to intermolecular interaction. The most reliable values of σR⁰ and σ_p parameters (−0.02 and −0.03, respectively) for the Me₃SiOCH₂ substituent were established; they indicate a sharp decrease in σ,π-conjugation of the Me₃SiOCH₂ substituent with the triple bond as compared to the Me₃SiCH₂ substituent. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 9, pp. 1759–1762, September, 1998.     

Intramolecular S···O chalcogen bond in thioindirubin

Results of X-ray diffraction study and quantum-chemical calculations revealed that molecular conformation of thioindirubin molecule creates suitable conditions for formation of intramolecular C–H···O and S···O interactions. Analysis of molecular electrostatic potential (MEP) demonstrates existence of two areas of positive MEP (σ-holes) in the outermost part of the sulfur atom on the continuation of the lines of the C–S bonds. One of these σ-holes is oriented toward region of negative MEP around the oxygen atom of carbonyl group. Such situation corresponds to formation of σ-hole or chalcogen bond. Existence of both types of bonding interactions is confirmed by topological analysis of electron density distribution using “Atoms in Molecules” (AIM) theory. Energies of the C–H···O hydrogen bond and the S···O σ-hole bond derived from AIM and NBO theories are very close.     

Electronic and Spatial Structure of Piperidine and Its Substituted Derivatives from the Results of ab initio Calculations

The results of non empirical quantum-chemical calculations using the RHF/6-31G(d) and MP2/6-31G(d) methods do not agree with proposals for the axial position of the H atom on the N atom in the piperidine molecule. According to RHF/6-31G(d) calculations for the N-methylpiperidine molecule and its chloro-substituted derivatives an equatorially placed methyl group is energetically more favored than an axial. The axial C-Cl and C-H bonds in these molecules are longer than the equatorial. The ³⁵ Cl NQR frequencies for the axial Cl atoms are lower than the equatorial. The ³⁵ Cl NQR frequency of the axial chlorine atom in 2-chloro-1-methylpiperidine is anomalously low. This is chiefly due to the high population density of its p σ-orbital and this is a result of the polarization of the C-Cl bond via the N atom unshared electron pair directly through the field. The effect of a similar unshared electron pair on the parameters of the C-Cl bond in the ClCH₂NH₂ molecule has been studied by the RHF/6-31(g) method for different angles of rotation of the ClCH₂ group around the C-N bond. __________ Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 7, pp. 1044–1052, July, 2005.     

Quantum-chemical investigation of the mechanism of the sulfonation of pyrrole

A detailed quantum-chemical study of the sulfonation of pyrrole with regard to the effect of the solvent (the model of overlapping spheres) on the energy characteristics of the formation of the σ-complexes produced during attack on the α-and β-positions of the heterocycle and their possible transformation paths was made by density functional theory [the B3LYP/6-31G(d) method]. The possibility of mutual transformations between the isomeric σ-complexes by α/β-migration of the SO₃ is examined. The formation of pyrrolesulfonic acids was studied for the case of the intramolecular rearrangement of the complexes. Comparison of the activation energies shows that in contrast to the gas-phase reaction the formation of the β-sulfonic acid is preferred in methylene chloride: the solvation energy of the α-isomer of the σ-complex is higher than the energy for the transition state of its rearrangement and its product, α-pyrrolesulfonic acid, leading to an increase in the kinetic barrier and to a decrease of the energy gain on the path to the formation of the latter. The opposite variation of the energy characteristics on the path to the β-isomer with regard to solvation leads to agreement between the calculated data and the experimentally observed preferred formation of the β-pyrrolesulfonic acid. Dedicated to Mikhail Grigor’evich Voronkov. __________ Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 11, pp. 1647–1654, November, 2006.     

The role of the negatively charged (Si3−Si−F2)− complexes in interactions of fluorine with the (111) surface of silicon

We used the AM1 quantum chemical and cluster models to study the mechanism of formation of a SiF₂-like layer and dissociation of the Si−Si bond during the interaction of atomic fluorine with the (111) surface of silicon. It is shown that the negatively charged (Si₃−Si−F₂)⁻ complex with the five-coordinated centered silicon atom plays an important part in these processes. The above complex participates in the interaction of atomic fluorine with silicon to form a SiF₂-like layer and break the subsurface Si−Si bonds without penetration of fluorine atoms into the subsurface silicon layers. Institute of Semiconductor Physics, Siberian Branch, Russian Academy of Sciences. Translated fromZhurnal Strukturnoi Khimii, Vol. 37, No. 1, pp. 14–21, January–February, 1996. Translated by I. Izvekova     

Molecular and electronic structure of some binary polyhedral clusters

The molecular and electronic structures of some polyhedral alternate molecules X_nY_n, where n=12, 16; X=B, Al, Si; Y=N,P,C, and of homoatomic clusters C₂₄, Si₂₄, C₃₂, and Si₃₂ are calculated in a valence approximation by the MNDO method. It is suggested that the σ-frameworks of these molecules are formed of four- and six-membered rings, with each X atom having only Y atoms as neighbors. The singlet states of all these systems have local minima on the corresponding potential energy surfaces with T_h symmetry for n=12 and T_d symmetry for n=16. The main structural parameters, heats of formation, ionization potentials, and effective charge distributions are given. It is concluded that the X_nY_n heteroatomic clusters can exist when X and Y are atoms of Group III and V elements, respectively, or both are atoms of Group IV elements. A. N. Nesmeyanov Institute of Organoelement Compounds, Russian Academy of Sciences. Translated fromZhurnal Strukturnoi Khimii, Vol. 36, No. 6, pp.976–982, November–December, 1995. Translated by L. Smolina     

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.