The rotational spectra of eight isotopomers of the propene * SOz complex have been observed with a Fourier transform microwave spectrometer. The rotational constants of the normal species are A=4269334 MHz, B= 1577.2661 MHz, and C= 1469.6335 MHz. The structure of the complex was derived from least-squares fitting of the 24 moments of inertia. It has a stacked, near-parallel planes configuration. The distance between the centers of mass of the two monomers is 3.26(5) A. The sulfur atom is

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... mately above the propene double bond. The C2 axis of SOz nearly eclipses the carbon-carbon single bond with the oxygen atoms towards the methyl group. The dipole moment of the complex was determined by Stark effect measurements to be ,u = 1.34( 3) D. The binding energy is estimated to be 2.9 kcal/mol from the pseudodiatomic model. Both electrostatic and ab initio calculations have been carried out to rationalize the structure and properties of the complex. The effect of methyl group substitution on the structures and properties of the ethylene * SO" propene * SO" and toluene . SOZ complexes is discussed.