1. Department of Chemistry, University of Warwick, Coventry CV4 7AL, U.K.
2. Department of Physics, University of Warwick, Coventry CV4 7AL, U.K.
3. Institut Laue-Langevin, 38042 Grenoble Cedex 9, France
4. ISIS Neutron and Muon Facility, Rutherford Appleton Laboratory, Didcot OX11 0QX, U.K.