Torsionsfeinstruktur im Rotationsspektrum des Dimethylsulfids und internes Hinderungspotential

Abstract

The microwave spectra of the three naturally occurring isotopic species of dimethyl sulfide have been recorded between 12 and 63 kmc/s. The lines of the common isotope and most of those of the two rarer isotopes are found to be split into triplets or quartets by the interaction of over-all and two-top internal rotation. The perturbation treatment of the top-frame coupling terms of this two-top internal rotation is extended to fourth order by a repeated application of a VAN-VLECK-transformation. The multiplet widths (torsional ground state) of 54 rotational transitions (J up to 12) of the common isotope are fitted with high precision (mean width 1.5 mc/s, rms error 12 kc/s) to the extended splitting formula; the multiplet widths of the ten resolved rotational transitions of the S34-substituted isotope fit equally well. For high J the fourth order contribution may account for as much as a third of the total width. Parameters to be determined in the fitting process are the barrier potential V3 of internal rotation and the angle ϑ between the CH3-symmetry axes and the z principal axis. Subsequently the moment of inertia Ia of the CH3-groups is included, since it has proved to be the main source of error in the barrier determination in similar cases. The Iα for best fit is significantly lower than the value computed from molecular structure, resulting in a marked disagreement in the dependent barrier values. Only part of the discrepancy may be explained by a slightly corrected methyl group structure (yielding Iα=3.1615 amu A2) as required by the assumption that the internal top axis as gien by the angle ϑ is normal with respect to the plane of the top's three Η-atoms. The methyl group thereby becomes slightly asymmetric. Correcting for kinetic energy top-top coupling by a 2 per cent correction of the perturbation coefficient (PIERCE-HAYASHI) one obtains using Iα=3.204 amu A2 Ia=3.0985 amu A2 (PIERCE-HAYASHI. from structure) (best fit for KRAITCHMAN'S method yields the following structural parameters: CS = 1.802 Ā, angle CSC 98° 52' (partial determination). This implies that the two methyl groups are tilted outward with respect to the CS bond by an angle of 2° 10'.