1. Contribution from Daresbury Laboratory, Warrington, Cheshire WA4 4AD, United Kingdom, School of Chemical Sciences, University of Birmingham, Edgbaston, Birmingham B15 2TT, United Kingdom, Department of Chemistry, University of Alberta, Edmonton, Alberta T6G 2G2, Canada, Neutron Program for Materials Research, NRC, Chalk River Laboratories, Chalk River, Ontario, K0J 1J0, Canada, and Organic Chemistry Laboratory, Swiss Federal Institute of Technology, ETH-Zentrum, CH-8092 Zürich, Switzerland.