Rotational spectrum,structure and internal motions of the ethylene-OCS weakly bound dimer

The rotational spectra of five isotopomers of the ethylene-OCS dimer have been observed by Fourier transform microwave spectroscopy and its structure was determined. The dipole moment components and rotational constants for this complex are consistent with a stacked geometry in which the OCS lies above the ethylene molecular plane, approximately parallel to the C=C bond. Two internal motions of the monomer subunits split each rotational transition into four components. The larger tunneling splittings have been analysed to give a twofold barrier for the internal motion of the ethylene subunit about its c inertial axis of 16(3) cm^?1. The results are compared with calculations with a semi-empirical model employing electrostatic, dispersion and repulsion interactions.