1. COS, UMR CNRS 172, Université Paris VI, boîte 45, 4 place Jussieu, 75252 Paris Cedex 05, France, DCMR, URA CNRS 1307, Ecole Polytechnique, 91128 Palaiseau, France, and Department of Chemistry, University of California, Riverside, California 92521-0403