Torsional splitting in the nu(5) fundamental infrared band of CH3CD3 and (CH3CD3)-C-13

The nu (5) fundamental (C-C stretching) of CH3CD3 shows a resolved torsiona l structure, caused by perturbations due mainly to the linear dependence of the torsional potential barrier on the normal coordinate Q(5). We were abl e to analyze this structure and to assign vibration-rotation transition wav enumbers for all five torsional components, classified according to the sym metry species of the G(18)((3)) extended molecular group. The torsional spl itting pattern is qualitatively similar to that of a nondegenerate vibratio nal state with an even number of excited torsional quanta nu (6). Explorati ve calculations show that the main perturber system should consist of the t orsional components of the vibrational ground state correlating with nu (6) = 3 in the high barrier limit. The strength of the perturbation on the E-r 0 torsional components of nu (5) increases rapidly with r, the E-40 compone nt being the most affected. The observed transition wavenumbers can be reas onably fitted by a simplified model containing independent effective vibrat ion-rotation parameters for the five different torsional components of nu ( 5), for both CH3CD3 and (CH3CD3)-C-13.The trend of the determined values of the effective vibrational wavenumbers and rotational parameters over the t orsional components supports the proposed vibration-torsion interaction mec hanism, responsible for the observed torsional splittings. A strong anomaly observed in the rotational intensity distribution of nu (5) is discussed. (C) 2001 Academic Press.