1. Contribution from the Chemistry Department, Physical and Theoretical Chemistry Laboratory, Oxford University, South Parks Road, Oxford OX1 3QZ United Kingdom, Sanofi-Aventis, Centre de Recherche de Paris, 13 quai Jules Guesde, BP14, 94403 Vitry-sur-Seine, France, Chemistry Department, Chemistry Research Laboratory, Oxford University, 12 Mansfield Road, Oxford OX1 3TA United Kingdom, and FOM Institute for Plasmaphysics Rijnhuizen, Edisonbaan 14, NL-3439 MN, Nieuwegein, The Netherlands