1. School of Chemistry, University of Bristol, Bristol BS8 1TS, U.K., Department of Chemistry, Imperial College, London SW7 2AZ, ISIS-CCLRC, Rutherford Appleton Laboratory, Chilton, Oxon, OX11 0QX, U.K., and Institut Max-von-Laue-Paul-Langevin, BP 156-X, F-38042 Grenoble Cedex, France