1. Unilever Research and Development Laboratory, Port Sunlight, Quarry Road East, Bebington, Wirral, ISIS, STFC, Rutherford Appleton Laboratory, Chilton, Didcot, OXON, Physical and Theoretical Chemistry Laboratory, Oxford University, South Parks Road, Oxford, Institute Laue-Langevin, 6 Jules Horowitz, F-38042 Grenoble, Cedex 09, France, and National Institute of Science & Technology, Gaithersberg, Maryland