1. Departament de Química Física, Universitat de Barcelona, Martí i Franquès 1, 08028, Barcelona, Department of Chemistry, King's College London, Strand, London, WC2R 2LS U.K., and Dipartimento di Chimica “G. Ciamician” dell' Universitá di Bologna, Via Selmi 2, 40126 Bologna, Italy