Knowledge Bank

The microwave spectrum of the propylene-$SO2$ complex has been observed with a pulsed beam, Fourier transform microwave spectrometer. The rotational constants were determined as $A=4269.564(2)$ MHz, $B=1577.2661(3)$ MHz, and $C=1469.6335(3)$ Mhz. In addition to the normal species, the spectra of five other isotopic species $C3H6,\cdot S18O2,C18,C3H6\cdot S18OO,C3H6\cdot SO18O,C3H6\cdot 34SO2$, and $CH3CDCH2\cdot SO2$ were assigned. The structure of this complex has been determined from the moments of inertia of all the isotopic species in a least-squares fitting procedure. The complex has a stacked near parallel planes configuration with the centers of mass of the two monomers separated by 3.444(7) {\AA}. Splittings of about 10-20 kHz were observed in many transitions, which can be understood as arising from the internal rotation of the methyl group in propylene. Electrostatic energy modeling to better understand the structure will also be discussed.