1. ISIS, Rutherford Appleton Laboratory, Chilton, Didcot, OXON, OX11 0QX, U.K., Unilever Research and Development Laboratory, Port Sunlight, Quarry Road East, Bebington, Wirral, CH63 3JW, U.K., and Physical and Theoretical Chemistry Laboratory, Oxford University, South Parks Road, Oxford, OX1 2JB, U.K.