1. Contribution from the Physical and Theoretical Chemistry Laboratory, University of Oxford, South Parks Road, OX13QZ Oxford, United Kingdom, Sanofi-Aventis, Centre de Recherche de Paris, 13 quai Jules Guesde, BP14, 94403 Vitry-sur-Seine, France, and Chemistry Department, Chemistry Research Laboratory, 12 Mansfield Road, Oxford OX1 3TA United Kingdom