Microwave and sub-mmwave study of CH 3SiH 3 including the perturbation-allowed torsion–vibration difference band ( ν12=0, ν6=3)↔( ν12=1, ν6=0)

Abstract

The vibration–torsion–rotation Hamiltonian in CH 3SiH 3 has been investigated using Fourier transform microwave methods and tunable sideband far-infrared spectroscopy. Four different studies have been carried out. First, the Q-branch of the torsion–vibration difference band ( ν 12=0, ν 6=3)↔( ν 12=1, ν 6=0) has been measured between 17.8 and 26.6 GHz. When three quanta of the torsional mode ν 6 are excited in the ground vibrational state (gs) for ( σ=−1) torsional sublevels with K=6, these transitions become allowed through resonant Coriolis-like coupling to the lowest lying degenerate mode ν 12 with no quanta of ν 6 excited. Second, direct l-doubling transitions in the state ( v 12=1, v 6=0) have been observed between 8.3 and 17.5 GHz for both torsional sublevels σ=0 and σ=±1. In the limit that the intervibrational interactions vanish, the σ-splitting between lines of the same J would be difficult to resolve, but frequency differences of more than 1 GHz due to these interactions have been determined. Third, the ( J=1←0) spectrum just below 22 GHz has been re-measured with higher resolution for 0⩽ v 6⩽4 in the gs and for ( v 6=0) in ν 12. Finally, the ( J=45←44) spectrum near 1 THz has been obtained for 0⩽ v 6⩽2 in the gs. A global data set of 3423 frequencies has been formed by merging the present 123 measurements with the data set used recently in the simultaneous analysis of the ν 12 and ν 5 bands by Schroderus et al. [J. Chem Phys. 115 (2001) 1392]. By refining the (gs/ ν 12/ ν 5) Hamiltonian developed in this earlier work in which the torsional motion is grouped with the vibrational degrees of freedom, a good fit to within experimental error has been obtained by varying 45 parameters. A fit of comparable quality has also been obtained using a similar analysis in which the torsional motion is grouped with the rotational degrees of freedom. The values of the molecular constants determined in the two models are compared.