EXPLORATIONS ON THE POTENTIAL SURFACES OF AH-ETA-BENZENE COMPLEXES

Gradient search techniques have been employed in conjunction with ab initio molecular orbital theory to locate minima in the potential-energy surfaces of complexes between benzene and the hydrides of several first- and second-row elements. The small molecules considered in this study are NH3, H2O, HF, H2S, and HCl. Most of the geometry optimizations were carried out by using a small split-valence basis set with polarization functions added on the second-row atoms. In order to reduce the basis-set superposition error and include effects of electron correlation, second-order Møller-Plesset (MP2) theory was used with a larger basis set to determine the energy in a single-point calculation at each minimum. Local minima with the heavy atom of the small molecule lying near the plane of the benzene ring were found only for the first-row hydrides; both H2S and HCl moved away from the plane into a π hydrogen-bonding configuration. Polarization effects and electron correlation were found to be extremely important in determining favored configurations of all systems. On the basis of the MP2 results, the π hydrogen-bonding configurations of HF and H2O are favored over structures featuring an approach to the edge of the ring by nearly 2 kcal mol-1. Although NH3 prefers to remain near the benzene plane, the π hydrogen-bonding configuration is disfavored by less than 1 kcal mol-1. © 1990 American Chemical Society.