A general harmonic force field for methyl silane

The number of accumulated spectroscopic data sensitive to the harmonic force constants of methyl silane has been augmented recently through high resolution infrared gas-phase studies of ¹²CH ₃SiH ₃, ¹³CH ₃SiH ₃ and ¹²CH ₃SiD ₃ species (J.L. Duncan, A.M. Ferguson and D.C. McKean, J. Mol. Spectrosc, 168 (1994) 522). This permits the determination of an empirical harmonic force field, in which 24 A, and E species force constants out of the total of 36 are independently defined in terms of 72 data across five isotopic species. The empirical constants are compared with those from a scaled ab initio force field using a basis set of double zeta plus polarization quality, where very good agreement is found throughout. Ultimately, a best estimate general harmonic force field is presented in which the 12 undermined and numerically small interaction force constants are constrained to their ab initio predicted scaled values.