An ab initio study of the p,π interaction: III. Interaction of an ether oxygen atom with a double bond

RHF/6-311G(d) calculations were performed for the H₂C=CHOCH₃ and H₂C=CClOCH₃ molecules with full geometry optimization and at varied angles of rotation of the methoxy group about the C-O bond, with all the other geometric parameters optimized. The first molecule has one energy minimum and one transition state, and the second molecule, two minima. Changes in the populations of the p _y orbitals of the olefinic carbon and oxygen atoms (orbitals whose symmetry axes are perpendicular to the molecular plane) and in the fractional charges on these carbon atoms, occurring upon rotation of the methoxy groups about the C-O bonds, cannot be attributed to changes in the extent of the p,π conjugation between the lone electron pairs of the oxygen atoms and π electrons of the C=C bonds.     

An ab initio study of the p,π interaction: IV. Interaction of an ether oxygen atom with a carbonyl group

RHF/6-311G(d) calculations were performed for the H₃COCOH molecule with full geometry optimization and at varied angles of rotation of the methoxy group about the C-O bond, with all the other geometric parameters optimized. The molecule can exist in two stable conformations with the dihedral angle O¹C¹O²C² of 0.00° and 179.99°. The influence of the rotation angle on the population of the p _y orbital of the carbonyl oxygen atom in compounds with different types of the adjacent bond is essentially similar. The results obtained are inconsistent with the concept of the p,π conjugation involving the p _y orbitals of the planar molecular fragment (orbitals whose symmetry axes are perpendicular to this fragment).     

Nonempirical study on the potential energy surface of 5-methyl-1,3-dioxane

Quantum-chemical study on the potential energy surface of 5-methyl-1,3-dioxane at the nonempirical RHF//STO-3G, RHF//3-21G, RHF//6-31G(d), RHF//6-31G(d,p), and MP2//6-31G(d,p) levels of theory revealed two energy-equivalent paths of conformational transformation of the equatorial and axial chair conformers. Potential barriers to these processes were estimated. The δG° value for the methyl substituent on C⁵ in 1,3-dioxane ring, determined on the basis of the experimental (NMR) and calculated vicinal 1H-1H coupling constants, was very consistent with published data     

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.     

Conformational analysis of 2,4-dialkyl-1,3,2-dioxaborinanes

Computer modeling using the quantum-chemical empirical MM+ method and nonempirical RHF//6-31G(d), MP2//6-31G(d), and, in individual cases, RHF//3-21G methods was employed to study the potential energy surface of 2,4-dialkyl-1,3,2-dioxaborinanes. The optimal geometry and ¹H NMR spectral data gave values for ∆G ⁰ of the methyl and hexyl substituents at C-4 of the heterocyclic ring equal to 0.6 and 1.6 kcal/mol, respectively.     

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.     

Quantum-chemical study of 1,3-dioxane complexes with two water molecules

Methods PM3, RHF/6-31G(d), and MP2/6-31G(d)//RHF/6-31G(d) were used in calculation of the energy of formation of five 1,3-dioxane complexes with two water molecules formed through hydrogen bonds. The study of the conformational properties of the most stable associate revealed two routes of the chair-chair conformation isomerization. It was shown that the difference between the minima on the potential energy surface in this gase increased, and the barriersto the interconversion decreased as compared to the calculated values for the isolated molecule of 1,3-dioxane.     

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.     

Vibrational assignment of the normal modes of 2-phenyl-4-(4-methoxy benzylidene)-2-oxazolin-5-one via FTIR and Raman spectra, and ab inito calculations

Raman and FTIR spectra of 2-phenyl-4-(4-methoxy benzylidene)-2-oxazolin-5-one were recorded in the regions, 100-3300 and 400-4000 cm(-1), respectively. Vibrational frequencies and intensities of the fundamental modes of this hetrocyclic organic molecule were computed using ab initio as well as AM1 semiempirical molecular orbital methods. Ab initio calculations were carried out with basis set up to RHF/6-311G. Conformational studies regarding the effect of moving the methoxy group in the 2-phenyl-4-(4-methoxy benzylidene)-2-oxazolin-5-one molecule to a different position on the ring was also carried out. Observed vibrational wavenumbers were found to be mostly consistent with ab initio values. The most intense mode of vibration observed at 1250 cm(-1) in Raman spectra, also observed as a strong band in FTIR, was assigned as C-O stretching vibration in the methoxy group. Asymmetric stretching vibrations between CC and CN bonds was predicted as most intense mode by our ab initio calculation.     

Conformational analysis of 2-methyl-5-alkyl- and 5-aryl-1,3,2-dioxaborinanes

Quantum chemical study of conformational isomerization of 2-methyl-5-alkyl- and 5-aryl-1,3,2-dioxaborinanes using RHF/6-31G(d) method led to the conclusion that the equilibrium between equatorial and axial sofa conformers is shifted to the latter form. Based on the experimental and theoretically calculated vicinal coupling constants J _HH the quantitative conformational composition and the values of ΔG ⁰ for substituents at the C⁵ ring atom were established.     

Conformational stability of trivinylborane from vibrational spectra and ab initio calculations

The conformational stability, barriers to internal rotation and vibrational frequencies of trivinylborane have been determined from the vibrational spectra and ab initio calculations. The ab initio calculations have been carried out utilizing the RHF/3-21G, RHF/6-31G*, and MP2/6-31G* basis sets and support the vibrational data that there are two stable conformations in the fluid phases separated by a relatively small energy difference. One of the conformations is a near-planar form which has the three vinyl groups twisted out of the BC₃ plane and belongs to the C₃ point group. The other conformer has a non-planar structure and belongs to the C₁ point group. These and other calculated results are compared to the corresponding quantities obtained from the experiment.     

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.     

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 and MNDO calculations of atomic interactions in the CH3OCOCl molecule

The CH₃OCOCl molecule is calculated by ab initio methods using the split-valence basis sets at RHF/3-21G//RHF/3-21G, RHF/6-31G^*//RHF/6-31G^*, and RHF/6-311G^*//RHF/6-31G^* levels of theory and in the MNDO approximation. The optimized geometry of the molecule is consistent with the experimental data. The populations of the p-AOs of this molecule and the MO compositions show that the electron distribution in this molecule should be interpreted without considering the conjugation between the lone electron pairs of the Cl or O atoms and the π-electron system of the carbonyl group. The asymmetry parameters of the electric field gradient on the³⁵Cl nucleus were calculated using the Cl p-AO populations and compared with the corresponding experimental value. Instite of Technical Chemistry, Ural Branch, Russian Academy of Sciences. Translated fromZhurnal Struktumoi Khimii, Vol. 37, No. 4, pp. 646–651, July–August, 1996. Translated by I. Izvekova     

Mutual influence of axial and equatorial P-Cl bonds in the molecular forms of pentachlorophosphorane,their lengths in gaseous and crystalline states

The quantum-chemical calculations by RHF/6-31G(d), MR2/6-31G(d) and MP2/6-31+G(d) methods of molecular forms of pentachlorophosphorane with the complete geometry optimization at the different lengths of P-Cl bonds are performed. The results of the calculations are used for estimation of respective ³⁵Cl NQR frequencies. The lengths of axial (2.05 Å) and equatorial (2.01 Å) P-Cl bonds are found, when the calculated NQR frequencies of Cl atoms practically coincide with the obtained experimentally at 77 K. These bonds are a little shorter than in the gaseous state of the matter. In contrast to the ratio of the lengths of the experimental P-Cl bonds and NQR frequencies of respective Cl atoms, at the use of increased lengths of these bonds in the quantum-chemical calculations of the molecule, the ³⁵Cl NQR frequencies of Cl atoms calculated from the results of these calculations, increase. Therewith, the change in the length of the equatorial P-Cl bonds with unchanged length of axial bonds leads to a significantly greater change in the NQR frequencies of axial chloroine atoms than of the equatorial, and vice versa.     

Molecular Structure of Diphenylamine by Gas-Phase Electron Diffraction and Quantum Chemistry

Geometric parameters of the diphenylamine molecule were determined by gas-phase electron diffraction and quantum-chemical calculations. By gas-phase electron diffraction, the molecule has an asymmetric structure with torsion angles about N-C bonds of ?45.6(23)° and 173.4(46)°, which agrees with RHF/6-31G** calculations. Density functional theory (DFT) calculations at the B3LYP/6-31G** level of theory lead to a C ₂ molecular conformation in the ground electronic state. The principal experimental geometric parameters are as follows: bond lengths: C-N 1.417(1), C-C_av 1.403(1) Å; and bond angles: CNC 123.9(5)°, and NCC 121.5° (assumed) and 116.4°.     

Neutron diffraction analysis at 15 K and ab initio molecular orbital calculations for α-cyanoacetohydrazide: A crystal structure with a high energy conformer

The crystal structure of α-cyanoacetohydrazide, C₃H₅N₃O, is refined using single-crystal neutron diffraction data at 15 K. Nuclear equilibrium geometries of the isolated molecules are calculated using GAUSSIAN-82 for the eight possible conformers having C_s symmetry. The conformation observed in the crystal has the eighth highest calculated energy, 36.6 kJ mol⁻¹ above the lowest energy conformer, and the largest calculated dipole moment, 8.6 D, with a HF/6-31G* basis. In the crystal the molecule is distorted from the ideal C_s symmetry. These distortions add another 20.3 kJ mol⁻¹ in energy, calculated with HF/6-31G*. All the hydrogens, including the CH, are involved in intermolecular hydrogen-bonding which is unusual in that it is formed entirely by three-center bonds. These bonds form a network which includes dimer and chain configurations.     

Proton Migration in Benzene Complexes of Methyl and Silyl Cations

Quantum-chemical calculations at the B3LYP/6-31G(d,p) level of theory were used to optimize stationary points in the C₆H₆ + H₃X⁺ (X = C, Si) systems. At X = Si, the adduct of the cation with benzene is the most stable isomer, whereas at X = C, the para isomer is more stable than the adduct (ipso isomer). This difference is explained in terms of charge distribution in the benzene ring in toluene and phenylsilane: In the latter, the negative charge on the carbon atom attached to silicon is much higher than on the other carbon atoms, unlike toluene in which the highest negative charge is on the carbon atom para to the methyl group. Proton migration from the ipso to para position requires the lowest (X = C) and highest (X = Si) barriers to be overcome compared with the other barriers to proton migration over the benzene ring. These barriers and relative stabilities of the isomers correlate well with the charge distribution on benzene carbon atoms in toluene and phenylsilane.     

Conformational analysis of 2-methyl-5-nitro-1,3,2-dioxaborinane

Quantum-chemical methods HF/6-31G(d), HF/6-31+G(d), MP2/6-31G(d)//HF/6-31G(d), and MP2/6-31+G(d)//HF/6-31+G(d) were used to investigate the conformational isomerization of 2-methyl-5-nitro-1,3,2-dioxaborinane. It has been shown that a potential energy surface of this compound includes two minima: an axial form of semi-chair and equatorial sofa together with a transition state belonging to the conformation of 2,5-twist-form. A comparison between experimental NMR ¹H and theoretical vicinal coupling constants was used to determine the quantitative conformational composition of cyclic boric acid ester and a value of ΔG ⁰ for nitro group at the ring carbon atom C⁵ in CCl₄ and C₆D₅NO₂ solutions.