1. Department of Chemistry, University of Sheffield, Sheffield, S3 7HF, U.K.
2. Department of Chemistry, University of Southampton, Highfield, Southampton SO17 1BJ, U.K.
3. Department of Chemistry, University of Wales, P.O. Box 912, Park Place, Cardiff CF1 3TB, U.K.
4. Rhône-Poulenc Rorer Ltd., Rainham Road South, Dagenham, Essex RM10 7XS, U.K.