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 I_a 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 I_α 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 I_α=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 I_α=3.204 amu A² I_a=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'.