1. Department of Electronics, University of Glasgow, Glasgow, G12 8LT, U.K., Institut Laue Langevin, 38042 Grenoble Cedex 9, France, ISIS Facility, Rutherford Appleton Laboratory, Didcot, OX11 0QX, U.K., Department of Chemistry, University of Leicester, Leicester, LE1 7RH, U.K., and Department of Chemistry, Loughborough University, Loughborough LE11 3TU, U.K.