THE MICROWAVE-SPECTRUM, STRUCTURE, AND LARGE-AMPLITUDE MOTIONS OF THEMETHYLACETYLENE-CENTER-DOT-SO2 COMPLEX

Rotational spectra of five isotopomers of the methylacetylene.SO2 (MA. SO2) van der Waals complex have been observed with a Fourier transform microwave spectrometer. Each species showed two sets of rotational tr ansitions, one associated with the A (m=0) and the other with the E (m =+/-1) methyl group internal rotation states. The rotational transitio ns of the isotopomers with S O-16(2) and the doubly substituted S O-18 (2) also showed inversion splitting ranging from tens of kHz to a few MHz. This splitting was absent in the S O-16 O-18 isotopomers. The spe ctra of these species have been assigned and fit, yielding rotational constants, which allowed a complete determination of the structure of the complex. The SO2 was found to sit above the carbon-carbon triple b ond, with one of the S-O bonds roughly parallel to the symmetry axis o f methylacetylene. The centers-of-mass distance between the two monome rs was determined to be 3.382(10) Angstrom. The center frequencies of the inversion doublets (or quartets) were used in a fit of both the A and the E transitions; the barrier hindering the internal rotation of the methyl group was determined to be 62.8(5) cm(-1). Based on the dep endence of the inversion splitting on the transition dipole direction and isotopic substitution, the inversion motion was identified as an ' 'in plane'' wagging of the SO2 relative to methylacetylene. A pure inv ersion splitting of 3.11 MHz (free from rotation) was extracted from t he A-state spectrum of the normal species, from which an inversion bar rier height of about 63 cm(-1) was estimated.