Rotational spectra of five isotopomers of the methylacetylenesdot;SO2(MAsdot;SO2) van der Waals complex have been observed with a Fourier transform microwave spectrometer. Each species showed two sets of rotational transitions, one associated with theA(m=0) and the other with theE(m=plusmn;1) methyl group internal rotation states. The rotational transitions of the isotopomers with Sthinsp;16O2and the doubly substituted Sthinsp;18O2also showed inversion splitting ranging from tens of kHz to a few MHz. This splitting was absent in the Sthinsp;16Othinsp;18O isotopomers. The spectra of these species have been assigned and fit, yielding rotational constants, which allowed a complete determination of the structure of the complex. The SO2was found to sit above the carbonndash;carbon triple bond, with one of the Sndash;O bonds roughly parallel to the symmetry axis of methylacetylene. The centershyphen;ofhyphen;mass distance between the two monomers was determined to be 3.382(10) Aring;. The center frequencies of the inversion doublets (or quartets) were used in a fit of both theAand theEtransitions; the barrier hindering the internal rotation of the methyl group was determined to be 62.8(5) cmminus;1. Based on the dependence of the inversion splitting on the transition dipole direction and isotopic substitution, the inversion motion was identified as an lsquo;lsquo;in planersquo;rsquo; wagging of the SO2relative to methylacetylene. A pure inversion splitting of 3.11 MHz (free from rotation) was extracted from theAhyphen;state spectrum of the normal species, from which an inversion barrier height of about 63 cmminus;1was estimated.
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