1. Unilever Research and Development Laboratory, Port Sunlight, Quarry Road East, Bebington, Wirral CH63 3JW, U.K., ISIS, STFC, Rutherford Appleton Laboratory, Chilton, Didcot, OXON, U.K., Physical and Theoretical Chemistry Laboratory, Oxford University, South Parks Road, Oxford, U.K., Institut Laue-Langevin, 6 Jules Horowitz, F-38042 Grenoble, Cedex 09, France, and Nationial Institute of Standards and Technology, Center for Neutron Research, 100 Bureau Drive, Gaithersburg, Maryland 20899