1. Department of Chemistry, University of Exeter, Stocker Road, Exeter EX4 4QD, U.K., Institut für Theoretische Physik, Technische Universität Dresden, Mommsenstrasse 13, D-01069 Dresden, Germany, Department of Chemistry, University of Nottingham, University Park, Nottingham NG7 2RD, U.K., and Dipartimento di Chimica, ‘G. Ciamician', Universitá di Bologna, via F. Selmi 2, 40126 Bologna, Italy