1. Departement für Chemie und Biochemie, Universität Bern, Freiestrasse 3, CH-3012 Bern, Switzerland, Department of Chemistry, University of Florida, Gainesville, Florida 32611, ISIS Facility, CCLRC Rutherford Appleton Laboratory, Chilton, Didcot, Oxfordshire OX11 0QX, UK, and Institut Laue-Langevin, 6 Rue Jules Horowitz, BP 156-38042, Grenoble Cedex 9, France