1. Contribution from the Department of Chemistry, The Pennsylvania State University, University Park, Pennsylvania 16802, IBM Almaden Research Center, 650 Harry Road, San Jose, California 95120, Institut für Physikalische Chemie, Johannes Gutenberg-Universität, Jakob Welder-Weg 11, 55099 Mainz, Germany, and Department of Chemistry, University of Delaware, Newark, Delaware 19716