1. Chemistry Research Laboratory, University of Oxford, Mansfield Road, Oxford OX1 3TA, U.K., Laboratoire de Structure et Dynamique des Systèmes Moléculaires et Solides (UMR 5636 CNRS-UM2), Institut Charles Gerhardt, cc 14, Université Montpellier 2, 34095 Montpellier Cedex 5, France, and School of Chemistry, University of Nottingham, University Park, Nottingham NG7 2RD, U.K.