15min:

J. R. COOPER, Department of Physics and Astronomy, University of Calgary, Calgary, Alberta T2N 1N4, Canada; A. R. W. MCKELLAR, Steacie Institute for Molecular Sciences, National Research Council of Canada, Ottawa, Ontario K1A 0R6, Canada; I. OZIER, Department of Physics and Astronomy, University of British Columbia, Vancouver, B.C. V6T 1Z1, Canada; N. MOAZZEN-AHMADI, Department of Physics and Astronomy, University of Calgary, Calgary, Alberta T2N 1N4, Canada.

The lowest frequency parallel fundamental band ₅ of CH₃CD₃ near 900 cm^-1 was measured at low temperature with a resolution of 0.0021 cm^-1 using Fourier transform spectroscopy. The band is weak, and an absorption path of 60m was used. Large torsional splittings due to inter-vibrational coupling have been observed. Building on previous studies of the torsional levels in the ground vibrational state and and in the methyl rocking state ( ₁₂=1), a three-band analysis including this most recent data has been completed. The combined data set of more than 2,200 frequencies was fitted to within experimental accuracy using a 43-term model Hamiltonian. The results were found to bear a striking resemblance to those of an earlier, analogous study of CH₃SiH₃. In both cases, Fermi coupling between the ( ₅=1) state and the ground state was found to be the dominant interaction responsible for the observed torsional splittings. Inclusion of this coupling results in a simplification of the ground-state Hamiltonian, so that only eight additional terms were required with the introduction of the ₅ band.