1. Contribution from the Laboratoire de Chimie Physique, Groupe de Chimie Théorique, Université de Paris-Sud, 91405 Orsay Cédex, France, the Department of Chemistry and State Key Laboratory for Physical Chemistry of Solid Surfaces, Xiamen University, Xiamen, Fujian 361005, P.R. of China, and the Department of Organic Chemistry and the Lise-Minerva Center for Computational Chemistry, The Hebrew University, Jerusalem 91904, Israel