1. Contributions from the Department of Chemistry, U.M.I.S.T., P.O. Box 88, Manchester, M60 1QD, U.K., Department of Chemistry, University of Edinburgh, West Mains Road, Edinburgh, EH9 3JJ, U.K., School of Chemistry, The University, Cantock's Close, Bristol, BS8 1TS, U.K., and Department of Biological and Molecular Sciences, University of Stirling, Stirling, FK9 4LA, U.K.