Search

The optimized molecular structures, vibrational wavenumbers and corresponding vibrational assignments of the syn and anti conformers of 2-, 3- and 4-pyridine carboxaldehydes and their sulfur and selenium analogues were calculated using the density functional theory method employing the B3LYP functional with the 6-311++G(d,p) basis set and second order Møller-Plesset Perturbation Theory (MP2) method with the cc-pVDZ basis set. The calculations were adapted to the C symmetries of all ground state structures. The transition state arising from syn-anti isomerization was also modelled. All structures were fully optimized, and the geometries, rotational constants, dipole moments, charges, thermodynamic properties and energies are reported. Energies of the optimized structures were used to obtain the energy difference between the syn and anti conformers and the rotational barrier. The calculated vibrational wavenumbers were appropriately assigned to the various fundamental modes of vibrations. The Integral Equation Formalism Polarization Continuum Model was used to calculate the optimized geometry and the vibrational wavenumbers for all of the compounds in nine solvents: heptane, chloroform, tetrahydrofuran, dichloroethane, acetone, ethanol, methanol, dimethylsulfoxide and water. In addition, the LUMO, HOMO and energy band gap are also reported. The results indicate that the anti conformer is preferred with a rotational barrier of at least 19 kJ·mol. Graphical Abstract: [ABSTRACT FROM AUTHOR]

Abstract: Quantum mechanical methods have been employed to investigate the structural and spectroscopic properties of the novel trisilylsilylcyanide and trigermylgermylcyanide isolated molecules, in C3v symmetry, in the gas phase. The levels of theory used are DFT and MP2, the functional employed for the DFT method is B3LYP and the basis set used for all atoms is 6-311G(d,p). En route, the structures of these compounds have been analysed using the natural bond orbital approach. The latter confirms the presence of all single bonds and the only triple bond, namely, the Cn both of these molecules. Substituting silicon and germanium for carbon in the parent molecule, trimethylmethylcyanide, leave the bond length Cinvariant. Both the ab initio methods used perform satisfactorily when the predicted parameters from the two methods are compared. All the normal modes of vibrations were assigned to one of the 11 types of motion namely X–C stretch, X–X stretch, C, X–H stretch, X–X–C bend, X–X–X bend, X–C–X–H bend, XH3 rock, XH3 def and XH3 twist (X=Si or Ge). An interesting outcome of this work is that the HOMO–LUMO energy gap decreases from trimethylmethylcyanide to trigermylgermylcyanide. Since these compounds have not yet been synthesised, we have successfully compared the predicted parameters with related molecules for which results have been published in the literature. [Copyright &y& Elsevier]

Abstract: Theoretical methods have been employed to investigate the structural, spectroscopic and energetic properties of difluoro(germylthio)phosphine in the C s symmetry in the gas phase. The levels of theory used are MP2 and DFT/B3LYP and the basis set used for all atoms is 6-311G(d,p). The computed structural parameters, namely, bond lengths and bond angles and spectroscopic parameters, namely, infrared frequencies, Raman activities and NMR chemical shifts have been compared with experimental data reported in literature. The structure of this compound has been analysed using natural bond orbital approach. Both the ab initio methods perform satisfactorily when the predicted and experimental parameters are compared. We have extended this study, using the same methods and basis set, to the selenium analogue, namely, difluoro(germylseleno)phosphine for which literature is elusive. The MP2/6-311G(d,p) geometrical parameters for difluoro(germylseleno)phosphine are: r(P–Se)=2.270, r(Ge-Se)=2.386, r(P–F)=1.604, r(Ge–H)=1.530Å, ∠(Ge–Se–P)=91.4, ∠(F–P–F)=96.7, ∠(Se–P–F)=99.9 and ∠(Se–Ge–H)=109.5°. Apart from these, we have also studied the energetic parameters for their synthesis. The results indicate that formation of the sulphur-containing compound is favoured thermodynamically. [Copyright &y& Elsevier]