1. Leiden Institute of Chemistry, Gorlaeus Laboratories, Leiden University, Einsteinweg 55, P.O. Box 9502 2300 RA, Leiden, The Netherlands, Department of Chemistry and Biochemistry, University of Colorado, Boulder, Colorado 80309-0215, and Departement de Chimie, CNRS UMR 8640 PASTEUR, Ecole Normale Supérieure, 24 rue Lhomond, Paris 75231, France