1. Department of Chemistry, King's College London, Strand WC2R 2LS, U.K. GlaxoSmithKline, Old Powder Mills, Nr. Leigh, Tonbridge, Kent TN11 9AN, U.K. Department of Chemistry, Imperial College London, Exhibition Road, South Kensington SW7 2AZ, U.K., and EPSRC X-ray Crystallography Service, Southampton University, Highfield SO17 1BJ, U.K.