1. Laboratoire de Chimie Théorique, Université Pierre et Marie Curie, Paris VI, Case 137, 4 Place Jussieu, F-75252 Paris Cedex 05, France, Departamento de Química, Física y Analítica, Universidad de Oviedo, E-33006 Oviedo, Spain, and Laboratory for Heterogeneous Catalysis, Catalan Institution of Research and Advanced Studies (ICREA) and Institute of Chemical Research of Catalonia (ICIQ), Avenue Països Catalans s/n, E-43007 Tarragona, Spain