1. Contribution from the Department of Chemistry, The Ohio State University, 120 West 18th Avenue, Columbus, Ohio 43210-1173, Centre de Recherches sur la Physique des Hautes Températures, 1D Av. de la Recherche Scientifique, 45071 Orléans CEDEX 2, France, and Department of Geological and Environmental Sciences, Stanford University, Stanford, California 94305