Infra-red spectra of (CD3GeH3)-C-13 and DFT studies of force constants and electrical properties in methylsilane and methylgermane

New infra-red data are reported for some fundamental bands of (CD3GeH3)-C-13 in the gas phase. Triple-zeta DFT calculations of geometry and force field have been made for CH4, SiH4, GeH4, CH3SiN3, and CH3GeH3. The DFT force fields for methylsilane and methylgermane are scaled (SDFT) on all available data, including harmonised frequencies and frequency shifts, Coriolis constants and centrifugal distortion coefficients. Comparison of scale factors reveals considerable variation. Empirical force fields (ESDFT) are determined, a number of off-diagonal constants being constrained to SDFT values. Problems remaining in the observed data are identified. Atomic polar tensors from the DFT calculations are used to obtain King effective atomic charges Chi and bond dipole derivatives d mu/dr with respect to terminal bond stretching. Comparison between silanes and germanes reveals common trends in bending and stretching dipole moments together with the inductive effect of a methyl group. (C) 1997 Elsevier Science B.V.