1. School of Chemistry, University of Birmingham, Edgbaston, Birmingham B15 2TT, U.K., ISIS Facility, Rutherford Appleton Laboratory, Chilton, Didcot OX11 0QX, U.K., Department of Chemistry, University College London, 20 Gordon Street, London WC1H 0AJ, U.K., and School of Chemistry, University of St. Andrews, St. Andrews, Fife KY16 9ST, U.K.