MICROWAVE-SPECTRUM, STRUCTURE, BARRIER TO INTERNAL-ROTATION, DIPOLE-MOMENT, AND DEUTERIUM QUADUPOLE COUPLING-CONSTANTS OF THE ETHYLENE SULFUR-DIOXIDE COMPLEX

The microwave spectra of the complex between ethylene and sulfur dioxide and nine of its isotopic species have been observed in a Fourier transform microwave spectrometer. The spectra exhibit a and c dipole selection rules; transitions of the normal species and several of the isotopically substituted species occur as tunneling doublets. The complex has a stacked structure with Cs symmetry; the C2H4 and SO2 moieties both straddle the mirror plane with the C2 axis of SO 2 crossed at 90° to the carbon-carbon bond axis (i.e., only the S atom lies in the symmetry plane). The distance between the centers of mass (Rcm) of C2H4 and SO2 is 3.504(1) Å and the deviation of their planes from perpendicular to Rcm is 21 (2)° and 12(2)°, respectively. The tunneling splittings arise from a rotation of the ethylene subunit in its molecular plane. The barrier to internal rotation is 30(2) cm-1. The dipole moment of the complex is 1.650(3)D. The deuterium nuclear quadrupole coupling constants for C 2H3D·SO2 are χaa = -0.119(1) MHz, χbb = 0.010(1) MHz, and χcc = 0.109(1) MHz. The binding energy is estimated to be 490 cm-1 from the pseudo-diatomic approximation. A distributed multipole electrostatic model is explored to rationalize the structure and binding energies. © 1990 American Institute of Physics.