1. Contribution from the Department of Chemistry, University of Bath, BA2 7AY, U.K., Institut Laue-Langevin, 6, rue Jules Horowitz, BP 156 - 38042 Grenoble, Cedex 9, France, School of Chemistry and EaStChem, The University of Edinburgh, The King's Buildings, Edinburgh, EH9 3JJ, U.K., Department of Chemistry, University of Victoria, Victoria, BC V8W3V6, Canada, and CCLRC Daresbury Laboratory, Daresbury, Warrington, WA4 4AD, U.K.