1. Contribution from the Physical and Theoretical Chemistry Laboratory, Oxford University, South Parks Road, Oxford OX1 3QZ, UK, Freie Universität Berlin, Institut für Physikalische und Theoretische Chemie, Takustrasse 3, D-14195, Berlin, Germany, Department of Chemistry, King's College London, Strand, London WC2R 2LS, UK, and Department of Chemistry and Biochemistry, University of California, Los Angeles, California 90095