1. Department of Chemistry, University of Wales, Bangor, Gwynedd LL57 2UW, U.K. Peboc Division of Eastman Chemical (UK) Ltd., Industrial Estate, Llangefni, Anglesey, Gwynedd LL77 7YQ, U.K., and Department of Chemistry, University of Wales, Cardiff, P.O. Box 912, Park Place, Cardiff CF1 3TB, U.K.