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 b een observed by Fourier transform microwave spectroscopy and its structure was determined. The dipole moment components and rotational constants for t his complex are consistent with a stacked geometry in which the OCS lies ab ove the ethylene molecular plane, approximately parallel to the C=C bond. T wo internal motions of the monomer subunits split each rotational transitio n into four components. The larger tunnelling splittings have been analysed to give a twofold barrier for the internal mot ion of the ethylene subunit about its c inertial axis of 16(3) cm(-1). The results are compared with c alculations with a semi-empirical model employing electrostatic, dispersion and repulsion interactions.