1. School of Chemistry, University of Bristol, Bristol BS8 1TS, U.K.
2. Department of Chemistry, Cardiff University, Cardiff CF1 3TB, U.K.
3. School of Chemical Sciences, University of East Anglia, Norwich NR4 7TJ, U.K.
4. ISIS Facility, Rutherford Appleton Laboratory, Chilton, OXON OX11 0QX, U.K.
5. Department of Chemistry, University of Surrey, Guildford GU2 5XH, U.K.
6. ILL, BP 156-X, F-38042 Grenoble Cedex, France