Quantum-chemical study of chlorophosphoranes: I. Structure of the (C<Subscript>6</Subscript>H<Subscript>5</Subscript>)<Subscript>2</Subscript>PCl<Subscript>3</Subscript> molecule in gaseous and crystalline states

The quantum-chemical calculations of the (C₆H₅)₂PCl₃ molecule were performed by the methods RHF/6-31G(d) and MP2/6-31G(d) with complete and partial optimization of its geometry. The results of calculations were used for evaluating the NQR frequencies of ³⁵Cl and the parameters of asymmetry of the of electric field gradient on the ³⁵Cl nuclei of this molecule. According to the results of calculations with the partial optimization of geometry and to the experimental data of ³⁵Cl NQR the bond lengths were found of central atom P in the solid state of this compound. The mutual influence of its axial and equatorial bonds was studied. It is shown that in chlorophosphoranes the ³⁵Cl NQR frequencies of the axial and equatorial chlorine atoms decrease with the decrease in the bond lengths of phosphorus atom.     

Quantum-chemical study of chlorophosphoranes: II. Structure of molecules of (Cl2)aP(ClnX3?n)e series, and mutual influence of axial and equatorial substituents

The quantum-chemical calculations of the (C₆H₅)₂PCl₃ molecule were performed by the methods RHF/6-31G(d) and MP2/6-31G(d) with complete and partial optimization of its geometry. The results of calculations were used for evaluating the NQR frequencies of ³⁵Cl and the parameters of asymmetry of the of electric field gradient on the ³⁵Cl nuclei of this molecule. According to the results of calculations with the partial optimization of geometry and to the experimental data of ³⁵Cl NQR the bond lengths were found of central atom P in the solid state of this compound. The mutual influence of its axial and equatorial bonds was studied. It is shown that in chlorophosphoranes the ³⁵Cl NQR frequencies of the axial and equatorial chlorine atoms decrease with the decrease in the bond lengths of phosphorus atom.     

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.     

Features of the molecular structure of compounds of (Cl3PNAr)2 series (Ar = C6H4Z, Z = H, Cl) by quantum-chemical calculations and the data of 35Cl nuclear quadrupole resonance

A series of dimeric molecules (Cl₃PNAr)₂ where Ar = C₆H₄Z, Z = H or Cl in 2–4 positions, was calculated by the RHF/6-31G* method with the full geometry optimization. A specific nature of the intramolecular steric orientation of aryl groups was revealed, therewith in the case Ar = C₆H₄Cl-2 short nonvalent contacts were founf between the ortho-chlorine atom and PCl₃ group manifested in the peculiarity of geometric parameters and NQR spectroscopy data of this compound. The correlation of the ³⁵Cl NQR frequencies of P-Cl bonds with the values of the charges on the chlorine atoms obtained by quantum-chemical calculations of the molecules was analyzed.     

Molecular structure of trichlorophosphazo compounds from ab initio calculation and chlorine-35 NQR spectroscopy data

The molecular structures of trichlorophosphazo compounds Cl₃P=NR with R = C(CH₃)₃, C(C₂H₅)₃, C(CF₃)₃, CCl(CF₃)₂, CCl₂CF₃, CCl₃, CCl₂CCl₃, and CCl(CCl₃)₂ were analyzed by combined consideration of the results of ab initio MP2/6-31G* calculations and previously measured ³⁵Cl NQR frequencies. The conformational peculiarity of these molecules caused by the relative spatial orientations of the P-Cl and N-C bonds is reflected in the calculated geometric parameters of the Cl₃P=NC fragments and in the pattern of ³⁵Cl quadrupole resonance spectra for PCl₃ groups. For these atomic groups, the ³⁵Cl NQR frequencies were brought in correlation with the charges of the chlorine atoms found by quantum-chemical calculations.     

Special Features of the Geometry of the 2,2,2,4,4,4-Hexachloro-1,3-dimethyl-1,3-diaza-2,4-diphosphetane Molecule and Its Electron Distribution According to Data of ab initio Calculations

The results of a nonempirical calculation of the 2,2,2,4,4,4-hexachloro-1,3-dimethyl-1,3-diaza-2,4-diphosphetane (Cl₃PNCH₃)₂ molecule by the RHF 6-31G(d) method are in agreement with the data of X-ray structural analysis of this compound. Calculated ³⁵Cl NQR frequencies for axial and equatorial chlorine atoms are close to the experimental values. The population of the orbitals of the lone electron pairs and the p orbitals of the equatorial Cl atoms were significantly lower than those of the axial atoms. Among the MO there was no MO corresponding to a three-center bond involving a P atom and axial Cl and N atoms.     

Characteristic features of interaction between atoms in chloro-substituted 1,4-dioxane fromab initio calculations

Ab initio calculations in a split-valence shell 6-31G(d) basis for the 2-chloro-1,4-dioxane (with axial and equatorial Cl atom) and 2,3-cis-dichloro-1,4-dioxane molecules with full optimization of their geometry were performed. The major contribution to the lowering of the NQR frequencies of the axial Cl atoms compared with the equatorial Cl atoms comes from the higher p-electron population of the former in these molecules. The populations of the orbitals for the unshared electron pairs of the axial and equatorial Cl atoms are identical in these molecules. But these orbitals are polarized differently, which also has some effect on the ratio of the NQR frequencies of these atoms. The differences in the electron populations of the axial and equatorial Cl atoms for the studied molecules are due to the asymmetry of the electron distribution in the Cl atoms geminal to them, which is responsible for the different polarization of the valence p-orbitals of the axial and equatorial Cl atoms.Institute of Technical Chemistry, Urals Branch, Russian Academy of Sciences, Perm' 614000, Russia; e-mail: chemist@mail.psu.ru. Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 11, pp. 1485–1490, November, 1999.     

35Cl NQR spectra, structure, and mutual influence of substituents in the series of chlorine-containing organic compounds of pentavalent phosphorus

Regularities of transmission of the effect of equatorial and axial substituents were established for a number of chlorine-containing organic compounds of pentatalent phosphorus using³⁵Cl NQR spectra. The field constants of³⁵Cl NQR frequencies of the chlorine atoms participating in the P−Cl bond were estimated for a series of tetrahedral phosphorus ions. The³⁵Cl NQR frequencies of chlorine atoms in tetracoordinated ions of pentavalent phosphorus and in analogous isoelectronic silicon compounds are related by a linear dependence. Deceased Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 5, pp. 915–922, May, 1999.     

Effect of the Carbonyl Group on the C-Cl Bonds in 2,2-Dichlorocyclohexanone and Some Model Molecules, According to ab initio Calculations

Ab initioRHF/6-31G(d) calculations of 2,2-dichlorocyclohexanone, chloropropanone, and chloroacetaldehyde molecules were performed. In 2,2-dichlorocyclohexanone, the carbonyl group exerts different effects on the axial and equatorial C-Cl bonds. At rotation of the C(O)CH₃ group in chloropropanone and C(O)H group in chloroacetaldehyde around the C-C bond, the population of the 3p component of the chlorine valence p _z orbital varies in the opposite direction with the calculated ³⁵Cl NQR frequency and, in the case of chloroacetaldehyde, with the charge on the oxygen atom.     

35Cl, 93Nb, 181Ta,and 121Sb NQR in Cl5MV · OPCl3 (MV = Nb,Ta, Sb,or P) Complexes

The behavior of ³⁵Cl NQR spectra in Cl₅M · OPCl₃ complexes is affected by the nature of the metal center and the distortion of its surrounding geometry. In all cases considered, the ³⁵Cl NQR frequencies in the OPCl₃ fragment shift toward higher frequencies upon complex formation, whereas the frequencies of axial chlorine atoms in the Cl₅M group in the phosphorus and antimony complexes lie below those for the equatorial atoms, and in the niobium and tantalum complexes, the frequencies of the axial atoms are higher than those of the equatorial ones.     

Spatial and electronic structure and 35Cl NQR parameters of ethyl(trichlorogermyl) propionate according to Ab initio calculations

Two stable structures of ethyl(trichlorogermyl) propionate have been studied by RHF/6-31G(d) and MP2/6-31G(d) quantum-chemical caclulation with full geometry optimization. The structure with pentacoordinated Ge atom has been more stable than that with tetracoordinated Ge atom. Based on the computation results, the frequencies of ³⁵Cl nuclear quadrupole resonance of the studied compound with pentacoordinated Ge atom has been estimated, it has been in satisfactory agreement with the experimental data. Additionally, the calculations by means of the RHF/6-31G(d) method have been performed at various fixed Ge…O interatomic distances. When the Ge and O coordination centers get closer, both the positive charge on Ge and the negative charge on O increase. The electron density shifts from Ge atom to the axial Cl atom, and the electron density shifst from the carbonyl C atom to the carbonyl oxygen atom. The electron density charge trasfer from O to Ge does not occur.     

Structure of the CCl3CCl=NCH2C6H5 molecule and reorientations of its CCl3 group, according to ab initio calculations

The optimal geometry of the isolated CCl₃CCl=NCH₂C₆H₅ molecule and the intramolecular barrier to reorientations of its trichloromethyl group are calculated by the RHF/6-31G* and B3LYP/6-31G* methods. The barrier found (14.1 kJ mol^?1) is compared to that determined previously by ³⁵Cl NQR for a crystal of this compound, which allows estimation of the contribution of intermolecular interactions to braking of this motion of the CCl₃ group. The structural features of the molecule of this compound are consistently manifested in quantum-chemical calculations and NQR spectra.     

Conformational Features of Monochloro-substituted Tetrahydropyrans According to Data from <Emphasis Type="Italic">ab initio</Emphasis> Calculations and <Superscript>35</Superscript>Cl NQR

Nonempirical quantum-chemical calculations of 2-, 3-, and 4-chloro-substituted tetrahydropyrans by the RHF/6-31G(d) and MP2/6-31G(d) methods showed that their conformational energy increases in the transition from the 4- to the 3-chloro-substituted compound, while in the transition to the 2-substituted compound the sign changes. In each of these isomers the axial C-Cl and C-H bonds, situated in the geminal position in relation to the oxygen atom, are longer, while the electron density at their Cl and H atoms is higher than in the corresponding equatorial bonds.__________Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 2, pp. 196–201, February, 2005.     

Peculiarities of atomic interaction in organic molecules of 3- and 4-coordinated phosphorus using ab initio calculations and Cl NQR

The results of ab initio calculations at the MP2/6-31G(d) level of molecules of the series ClPXX′ (X, X′=C₂H₅, N(CH₃)₂, OCH₃) and ClP(M)XX′ (M=O, S; X=CH₃, ?CH₃; X′=C₂H₅, OCH₃) with total optimization of their geometry were presented. They were compared with the obtained earlier results of such calculations at the RHF/6-31G(d) level and with experimental ³⁵Cl nuclear quadrupole resonance (NQR) spectra for these compounds. MP2/6-31G(d) calculations confirm non-inductive influence of heteroatoms on the geminal Cl atom in the non-linear three-atomic Cl–P–M groups. They agree to the conclusion that the abnormal correlation of the ³⁵Cl NQR frequencies for the compounds studied at different X, X′ and M is caused, in general, by the P–Cl bond polarization under the action of the geminal atom partial charges directly through the field. The satisfactory conformity between the experimental ³⁵Cl NQR frequencies and those estimated from 3p-components of the Cl atom valence p-orbitals calculated at the MP2/6-31G(d) level was obtained.     

Geminal interaction and “anomeric effect”

Quantum-chemical calculations by the methods of RHF/6-31G(d) and MP2/6-31+G(d) show equal ratio of the lengths of axial and equatorial bonds and electron density on the atoms forming them in cyclohexane and its mono derivatives and in six-membered heterocyclic molecules as well. This feature is due to the interaction of atoms in the chair form of these molecules regardless of the presence in them of a heteroatom. Introducing heteroatom to a cyclohexane ring leads only to increase in the difference between axial and equatorial bonds. This equation excludes the possibility to ascribe the elongation of axial bonds and increase in the electron density on atoms forming them in the heterocyclic molecules to the p,σ*-conjugation of the lone electron pair of the ring heteroatom with the antibonding orbital of the axial bond. Features of molecular geometry defined by the mutual influence of atoms in them, including inductive and non-inductive interaction of geminal atoms in triatomic groups Y-Z-M, result in energetic preference of gauche conformations of these molecules and “anomeric effect” in them.     

Ab initio calculations of complexes of group IVA tetrachlorides: VI. Structure and dynamics of formation of a complex of GeCl<Subscript>4</Subscript> with tetramethylurea

RHF/6-31G(d) calculations of the GeCl₄←OC[N(CH₃)₂]₂ system were done with full geometry optimization and at varied Ge←O distances. The calculated structure of the complex GeCl₄←OC[N(CH₃)₂]₂ and its ³⁵Cl NQR parameters were consistent with the experimental data not at the fully optimized geometry of the system but at the Ge←O distance fixed at 2.0 Å, at which the total energy of the system is higher by 0.264 eV. With a decrease in the Ge←O distance from ∞ to 1.9 Å, the electron density of the Cl atoms increases as a result of the electron density transfer from the H atoms and polarization of the Ge-Cl bonds under the action of the electron-donor fragment. The O, C, and N atoms of this fragment are merely conductors of the electron density from the H atoms of the methyl groups to the Cl atoms of the electron acceptor.     

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.     

Electronic and Steric Structure of ClAsX<Subscript>2</Subscript> Molecules: Evaluation by <Superscript>35</Superscript>Cl NQR and Quantum-Chemical Calculations

Molecules of the series ClAsX₂ [X = C₂H₅, N(CH₃)₂, OCH₃] were studied by RHF/6-31G(d) and MP2/6-31G(d) calculations. Their ³⁵Cl NQR frequencies were calculated from the populations of the 3p constituents of the chlorine valence p orbitals. The features of interaction of the geminal atoms in the molecules and the effect of this interaction on the electron distribution in them were analyzed.     

Nonempirical approach to the conformational analysis of 2,4-dimethyl-1,3,2-dioxaborinane and its oxonium ions

Using nonempirical quantum-chemical approximations RHF//STO-3G, 3-21G, 6-31G(d) and MP2//6-31G(d,p) a conformation isomerism of 2,4-dimethyl-1,3,2-dioxaborinane and its oxonium ions was studied. It was shown that the potential energy surface of the studied molecules has minima corresponding to equatorial (main minimum) and axial sofa forms and maxima corresponding to equatorial and axial conformations of 2,5-twist forms. Calculated values of the barriers of internal rotation of methyl group at the ring C⁴ atom were found. It was also established that the heat of protonation of the cyclic boric ester was smaller than of the non-boric analog, cis-2,4-dimethyl-1,3-dioxane, owing to decrease in basicity of the oxygen atoms in the cyclic boric ester due to partial double bond character of B-O bond.     

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.