Knowledge Bank

The interaction between molecular hydrogen and carbonyl sulfide was studied through {\it ab initio} calculations and microwave spectroscopy of {\it p}H$2$-OCS, {\it o}H$2$-OCS, {\it p}D$2$-OCS, {\it o}D$2$-OCS, and HD-OCS. The intermolecular potential surface (IPS) encompasses all four intermolecular degrees of freedom and is an extensive refinement of the IPS previously presented.} The IPS was calculated at the MP4/aug-cc-pVTZ + bond functions level of theory at a total of 1836 unique geometries. The global minimum is -210.3 \wn and places the hydrogen on the side of the OCS in a near parallel arrangement. The interaction is dominated by dispersion with little contribution from the electrostatic dipole-quadrupole term. Four dimensional bound state calculations using this IPS yield binding energies of -76.7 \wn for {\it p}H$2$-OCS and -90.3 \wn for {\it o}H$2$-OCS relative to {\it j} = 0 or 1 H$2$, respectively. Surprisingly, the ground state for all species, including {\it o}H$2$-OCS and {\it p}D$2$-OCS with hydrogen angular momentum {\it j} = 1, have total angular momentum {\it J} = 0. \vspace{1em} Eleven to fifteen {\it a}- and {\it b}-type pure rotational transitions were measured for each of the five species using a Fourier transform microwave spectrometer. Careful control of the gas mixture was required to observe {\it p}H$2$-OCS in the presence of the more strongly bound {\it o}H$2$-OCS species. The observed transition frequencies of each species can be fit using a standard asymmetric rotor Hamiltonian with the exception of {\it o}H$2$-OCS, for which the effects of internal rotation require a more complicated treatment. Comparison will be made between the {\it ab initio} calculated and the observed transition frequencies and fitted spectroscopic constants.