Knowledge Bank

The lowest frequency parallel fundamental band $\nu 5$ of $CH3CD3$ near $900cm-1$ was measured at low temperature with a resolution of $0.0021cm-1$ 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 ($\nu 12=1$), 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 ($\nu 5=1$) 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 $\nu 5$ band.