Modeling chemisorption of benzene and its hydrogenation on platinum surfaces. 1. Complexes of benzene with Pt and Pt-2

The potential energy surfaces (PESs) of the interaction of benzene with Pt and Pt-2 are thoroughly studied at the density functional B3LYP computation al level using the Hay-Wadt relativistic core potential. The PES of the ben zene-Pt complexation reveals two stable structures, the traditional di-pi c omplex and a a one which appears to be more stable than the former one by 9 .5 kcal/mol. The optimized geometry and vibrational modes of the a complex are reported in the present work for the first time although it has been pr edicted experimentally by Gland and Somorjai nearly 30 years ago. We show t hat the di-pi double left right arrow sigma transition is governed by the t ransition structure with the three-center two-electron bond C H Pt. A slidi ng motion of the Pt atom over the benzene ring to the di-pi complex involve s a transition structure whose activation barrier amounts 4.7 kcal/mol. The PES of the interaction of bare benzene with Pt-2 consists of three stable structures referred to as di-pi, pi-sigma, and di-sigma types. The latter o ne constitutes a global minimum. The formation of all reported structures i s rationalized in terms of molecular orbital patterns.