The nu(12) band of CH3SiD3

The lowest frequency degenerate fundamental band of CH3SiD3 (nu(12) = 1 <-- 0) centered around 418 cm(-1) was measured in order to investigate the vib ration-torsion-rotation interactions in a symmetric-top molecule with a sin gle torsional degree of freedom. The spectrum was recorded at an instrument al resolution of 0.004 cm(-1) using a Bomem Fourier transform spectrometer. The temperature and pressure of the sample were 180 K and 2 Torr, respecti vely. Because of the Coriolis coupling between the torsional stack with one quantum of the silyl rock excited and the corresponding stack for the grou nd vibrational state, torsional splittings are measured that are substantia lly larger than expected simply from the observed increase in the barrier h eight. Due to the local nature of the Coriolis perturbation, the significan tly enhanced torsional splittings are confined to a few (K, sigma) rotation al series; here sigma = -1, 0, 1 labels the torsional sublevels. The curren t measurements of the nu(12) band and frequencies from previously reported studies in the ground vibrational state were fitted to within experimental uncertainty using an effective Hamiltonian which was used for the analyses of similar spectra in CH3SiD3 and CH3CD3. Spectroscopic parameters characte rizing the states nu(12) = 0 and 1 and their interactions were determined, including several Coriolis-coupling constants. (C) 2000 Academic Press.