Microwave spectrum and structure of the acetylene-OCS dimer

The rotational spectra of four isotopomers of the HCCH-OCS van der Waals di mer have been observed with a Fourier transform microwave spectrometer. The rotational constants for the normal isotopomer were determined to be A = 5 839.1071(20) MHz, B = 2055.4388(13) MHz, and C = 1514.9785(12) MHz. The rot ational constants are consistent with a nearly parallel arrangement of the monomer subunits with a calculated R-CM distance of 3.6062(13) Angstrom. A semiempirical model employing electrostatic, dispersion, and repulsion inte ractions was used to study the structure and binding of the system and repr oduced the geometry of the dimer successfully. Comparisons with the model r esults for the closely related HCCH-CO2 and HCCH-N2O systems will also be p resented.