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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]