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.