Torsional Splitting in the ν5 Fundamental Infrared Band of CH3CD3 and 13CH3CD3

Abstract

The ν5 fundamental (C–C stretching) of CH3CD3 shows a resolved torsional structure, caused by perturbations due mainly to the linear dependence of the torsional potential barrier on the normal coordinate Q5. We were able to analyze this structure and to assign vibration–rotation transition wavenumbers for all five torsional components, classified according to the symmetry species of the G18(3) extended molecular group. The torsional splitting pattern is qualitatively similar to that of a nondegenerate vibrational state with an even number of excited torsional quanta v6. Explorative calculations show that the main perturber system should consist of the torsional components of the vibrational ground state correlating with v6=4 in the high barrier limit. The strength of the perturbation on the Er0 torsional components of ν5 increases rapidly with r, the E40 component being the most affected. The observed transition wavenumbers can be reasonably fitted by a simplified model containing independent effective vibration–rotation parameters for the five different torsional components of ν5, for both CH3CD3 and 13CH3CD3. The trend of the determined values of the effective vibrational wavenumbers and rotational parameters over the torsional components supports the proposed vibration–torsion interaction mechanism, responsible for the observed torsional splittings. A strong anomaly observed in the rotational intensity distribution of ν5 is discussed.