15min:
ROTATIONAL SPECTRA AND AB INITIO POTENTIAL ENERGY SURFACE OF THE H2-OCS VAN DER WAALS COMPLEX.

KELLY HIGGINS, Department of Chemistry, University of Minnesota, Minneapolis, Minnesota 55455; ZHENHONG YU AND WILLIAM KLEMPERER, Department of Chemistry and Chemical Biology, Harvard University, Cambridge, Massachusetts 02138.

The four-dimensional intermolecular potential energy surface for the H2-OCS complex was obtained at the MP4 level. The potential gives a T-shaped global minimum with a distance of 3.2~Å~between H2 and OCS, in which H2 is nearly parallel to OCS. Bound state calculations of para H2-OCS give predicted rotational constants of A = 22652, B = 5994, and C = 4611 MHz, in good agreement with the measured results from high-resolution infrared studies. The calculated binding energy of para H2-OCS is 75 cm-1, almost four times greater than that of He-OCS. Preliminary bound state calculations of ortho H2-OCS predict binding energies of 99 cm-1 for the Sigma state and 64 cm-1 for the Pi state.

a -type rotational transitions of two isotopomers of the ortho H2-OCS complex were observed between 9 to 31 GHz. The spectral constants of ortho H2-OC32S are (B+C)/2 = 5113.372(23), (B-C)/2 = 580.337(40), and DJ = 2.118(2) MHz. The observed 101-000 transition is very close to the expectation of para H2-OCS. This shows that the ground state of the complex is a Sigma bound state with the T-shaped geometry. The H2 nuclear spin dipole-dipole coupling constant dHH is 14.4(1) kHz which indicates a zero-point energy averaged angle of 45\circ of H2 with respect to the molecular a axis. Preliminary spectra of D2-OCS show the existence of both ortho D2-OCS and para D2-OCS complexes.