1. Contribution from CEA Saclay, DSM/DRECAM/SPAM, Bât. 522, 91191 Gif sur Yvette, France, Department of Chemistry and Biochemistry, University of Colorado, Boulder, Colorado 80309-0215, and Département de Chimie, CNRS UMR 8640, École Normale Supérieure, 24 rue Lhomond, 75231 Paris, France