1. Department of Chemistry and CSGI, University of Florence, via G. Capponi 9, 50121 Florence, Italy, Department of Organic and Industrial Chemistry, University of Parma, Viale delle Scienze 78, 43100 Parma, Italy, and Laboratoire Léon Brillouin, CEA-CNRS, CEN-Saclay, 91191 Gif-sur-Yvette, France