Knowledge Bank

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 {\AA} between $H2$ and OCS, in which $H2$ is nearly parallel to OCS. Bound state calculations of $paraH2-OCS$ give predicted rotational constants of $A=22652,B=5994$, and $C=4611$ MHz, in good agreement with the measured results from high-resolution infrared $studies.a$ The calculated binding energy of $paraH2-OCS$ is $75cm-1$, almost four times greater than that of $He-OCS.b$ Preliminary bound state calculations of $orthoH2-OCS$ predict binding energies of $99cm-1$ for the $\Sigma$ state and $64cm-1$ for the $\Pi$ state. a-type rotational transitions of two isotopomers of the $orthoH2-OCS$ complex were observed between 9 to 31 GHz. The spectral constants of $orthoH2-OC32S$ 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 $paraH2-OCS$. 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 $dHHc$ 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 $orthoD2-OCS$ and $paraD2-OCS$ complexes.