1. Davy-Faraday Research Laboratory, The Royal Institution of Great Britain, 21 Albemarle Street, London, W1X 4BS, U.K., Laboratoire des Sciences Moleculaires, Institut de Chimie de la Matière Condensée de Bordeaux, 33608 Pessac Cedex, France, Department of Chemistry, University of Durham, South Road, Durham, DH1 3LE, U.K., Chemistry Department, University of Southampton, Highfield, Southampton, SO17 1BJ, U.K., and Institute of Molecular Sciences, Myodaichi, Okazaki, 444 Japan