1. Department of Chemistry, Imperial College London, SW7 2AZ, U.K., Department of Chemistry, University of Reading, Whiteknights Campus, Reading, RG6 6AD, U.K., Department of Chemistry, Physical Chemistry I, University of Dortmund, Otto-Hahn-Strasse 6, D-44227 Dortmund, Germany, and European Synchrotron Radiation Facility, Boîte Postale 220, F-38043 Grenoble Cedex, France