GIANT TORSIONAL SPLITTINGS IN THE PARALLEL FUNDAMENTAL BAND $(\nu_{5}=1\leftarrow 0)$ OF $CH_{3}SiH_{3}$

$^{a}$ Present address: Department of Physical Sciences, University of Oulu, P.O. Box 3000, Oulu, Finland, FIN 90401Author Institution: Department of Physics and Astronomy, University of British Columbia; University of Lethbridge, Lethbridge, Alberta; Department of Physics and Astronomy, University of British ColumbiaThe lowest frequency parallel fundamental band $\nu_{5}$ of $CH_{3}SiH_{3}$ near $700 cm^{-1}$ has been measured at a resolution of $0.004 cm^{-1}$ with Fourier transform spectroscopy. The torsional fine structure in the spectrum is increased from $\sim 0.005$ to $\sim 1 cm^{-1}$ by the anharmonic interactions between the torsional stack of levels $(\nu_{6}=0,1,2..)$ in the ground vibrational state and the corresponding stack with $(\nu_{5}=1)$. For given $(\nu_{5}, \nu_{6}, J, k)$, the torsional sub-levels are labelled by the index $\sigma=0,+1, -1$. For several $(k,\sigma)$, resonant or near-resonant interactions occur between the vibration-torsion levels $(\nu_{5}=0, \nu_{6}=5)$ and $(\nu_{5}=1, \nu_{6}=0)$. For $(k=1, \sigma=-1)$, the mixing is so severe that perturbation-allowed transitions $(\nu_{6}=5\leftarrow 0)$ have been observed. In spite of the fact that $\nu_{6}$ changes by 5 units, the mixing causes the probability density as a function of the torsional angle to have a local maximum for $(\nu_{5}=1, \nu_{6}=0)$ in the centre of the classically forbidden region inside the torsional barrier. This severe mixing with such large $|\Delta \nu_{6}|$ has serious implications for vibration-rotation relaxation in molecules with low frequency, highly anharmonic vibrational modes. A detailed fit to over 3400 transition frequencies has been made to within experimental accuracy using a vibration-torsion-rotation Hamiltonian with 45 parameters. The treatment includes a third stack of torsional levels in the lowest-lying degenerate mode $\nu_{12}$ near $525 cm^{-1}$. The Hamiltonian is severely constrained by a wide variety of infrared, microwave and molecular beam transitions. The analysis shows that the second term in the Fourier expansion of the hindering potential characterized by the parameter $\nu_{6}$ is dominantly due to these Fermi-type interactions. The changes in the effective molecular parameters caused by removing the non-resonant anharmonic interactions by contact transformation will be discussed