1. Contribution from the School of Chemistry, University of Bristol, Cantock's Close, Bristol BS8 1TS, U.K., Department of Chemistry, University of Surrey, Guildford, Surrey GU2 5XH, U.K., Dipartimento di Chimica Fisica ed Elettrochimica, Università di Milano, Via Golgi 19, 20133 Milano, Italy, and Department of Chemistry, University of Liverpool, P.O. Box 147, Liverpool L69 3BX, U.K.