High resolution infrared spectrum and global analysis of ν5, ν12, and ν12 + ν6 − ν6 in CH3SiH3

The vibration-torsion-rotation spectrum of CH3SiH3 has been measured from 470 to 725 cm−1 at near-Doppler resolution. The full-width at half - maximum of the lines observed near 600 cm−1 was 0.0011 cm−1. The spectra were obtained using a Bruker IFS 125 HR Fourier transform spectrometer with the broadband source radiation being supplied from the synchrotron emission of the storage ring at the Canadian Light Source. Three vibrational bands were investigated: the lowest lying perpendicular fundamental ν12 centred near 524 cm−1, the lowest lying parallel fundamental ν5 near 703 cm−1, and the torsional hot band ν12 + ν6 − ν6 near 534 cm−1. For ν12 and ν5, the resolution and sensitivity are much improved over those in earlier studies, with many of the torsional multiplets now being resolved even in the cases where the upper levels are unperturbed. The primary motivation for the present work was the hot band, here reported for the first time, where the dependence of the silyl rock in ν12 on the torsional motion is much more pronounced. In addition, for the vibrational ground state (gs), two “forbidden” high torsional overtones v6 = 3 ← 0 and 5 ← 0 have been observed that become allowed through resonant mixing of the upper states with ν12 and ν5, respectively. In each case, two (K, σ) series have been measured where the mixing is largest. Here σ = 0, 1, −1 labels the torsional sub-levels. Using the Fourier transform waveguide spectrometer at E. T. H., the three σ-components of the (J = 1 ← 0) transition in ν12 + ν6 were observed, and a series of direct l-doubling transitions in ν12 + ν6 were measured for σ = 0. In a global fit, all the new data have been analysed along with the frequencies for other transitions obtained in earlier investigations. The analysis takes into account the relevant interactions among the torsional stacks of levels in the gs, ν12, and ν5. These include the previously known (gs, ν12) Coriolis-like and (gs, ν5) Fermi-like interactions along with a higher order (ν12, ν5) Coriolis-like coupling introduced here. This last is responsible for the strong perturbation of the ν5 series with K = 10, 11, and 12, and of the corresponding hot band series. A good fit to 9282 frequencies including 7942 new measurements was obtained both with the Free Rotor model in which the torsion is classified as a rotation, and with the High Barrier model in which the torsion is classified as a vibration. The Hamiltonian is discussed with emphasis on the new terms required for treating ν12 + ν6 − ν6.