AB-INITIO STUDY OF THE H2CO-AR COMPLEX

The potential energy surface of the H2CO-Ar complex is calculated at the second-order Moller-Plesset perturbation theory and analyzed using the perturbation theory of intermolecular forces. The equilibrium geometry (D(e) = 171 cm-1) involves a T-shape structure with the Ar atom nearly perpendicular to the C-O bond of H2CO and in the molecular plane of H2CO. The equilibrium configuration results from a minimum in the exchange repulsion. It is conceivable that H2CO undegoes a hindered internal rotation in the complex. A barrier to such a motion is estimated at 32 cm-1. The potential energy surface is very complex. The anisotropy of the surface involving the in-plane motion of Ar is very high and resembles that of the Ar-H2O complex. The anisotropy involving motion of Ar perpendicular to the molecular plane of H2CO is much weaker, and it is strikingly similar to that of the Ar-CO complex.