1. Contribution from the Department of Physical Chemistry, Chalmers University of Technology, S-412 96 Göteborg, Sweden, Laboratoire des IMRCP, Université Paul Sabatier, 118 route de Narbonne, Toulouse Cedex 31062, France, and Department of Chemistry, Bedson Building, University of Newcastle, Newcastle upon Tyne NE1 7RU, U.K.