High-Resolution Far-Infrared Torsional Spectrum of CH3SiD3 Using a Synchrotron Source

The far-infrared torsional spectrum of CH₃SiD₃ has been measured in a continuing effort to quantify the coupling between the small amplitude vibrations and the large amplitude internal rotation of the methyl group in symmetric tops. It is hoped that this will help in understanding the role of torsional motion in intramolecular vibrational relaxation. The spectrum was recorded with the Bruker IFS120 HR interferometer that is coupled to the MAX-I synchrotron radiation source in Lund, Sweden. High-resolution (0.002 cm⁻¹) spectra of the very weak torsional overtone 2ν₆ and hot band 3ν₆−ν₆ were recorded between 230 and 350 cm⁻¹. A total of 1413 frequencies in these two bands were assigned. In a separate experiment, a high-resolution (0.00125 cm⁻¹) spectrum of the lowest-lying degenerate fundamental band ν₁₂ was measured between 360 and 480 cm⁻¹, and 3263 frequencies belonging to this band were assigned. This spectrum was recorded with the Bruker IFS120 HR interferometer located at the University of Oulu, Finland. The frequencies from the aforementioned three bands and the data from the recent molecular beam measurements reported by Ozier and Meerts (J. Chem. Phys.109, 4823 (1998)) were analyzed using a model which considered three interacting torsional stacks: one for the ground vibrational state and two for v₁₂=1. A fit to within the experimental error was obtained by varying 36 molecular parameters. Several interstack coupling constants have been determined. A comparison of the leading parameters between CH₃SiD₃ and CH₃SiH₃ is presented.