Ab initio and density functional studies of the structure, vibrational spectra and force field of trimethylsilane

Ab initio and density functional Q-M treatments have been carried out on trimethylsilane, SiHMe3. The harmonic force fields so obtained are scaled to frequencies selected from four isotopic species, SiH(CH3)(3), SiD(CH3)(3), SiH(CD3)(3) and SiD(CD3)(3), some new infrared data for the d(0) species being employed. The region 250-200 cm(-1) is reinterpreted in terms of the skeletal bending modes nu(8)(a(1)) and nu(23)(e), with nu(8) > nu(23). Microwave information regarding the positions of the methyl torsions apparently conflicts with evidence both from the Q-M calculations and from infrared combination bands, which favour values less than 160 cm(-1). All the scaled force fields give a poor fit to frequencies in the E symmetry species and nine force constants there are further refined. The spread of scale factors narrows when a larger basis set or density functional theory is employed, but widens slightly when electron correlation is introduced through an MP2 calculation. Valence interaction constants are reported for bond stretching motions and their significance assessed. Coriolis and centrifugal distortion constants are listed for the preferred B3LYP/6-311G** based force field. Electrical properties of the atoms and bonds are discussed. (C) 1999 Elsevier Science B.V. All rights reserved.