1. Contribution from the Physical Chemistry Division, Stockholm University, S-106 91 Stockholm, Sweden, Chemistry Department, Southampton University, S017 1BJ Southampton, U.K., Leiden Institute of Chemistry, Leiden University, NL-2300 RA Leiden, The Netherlands, and Nijmegen Center for Molecular Life Sciences, University of Nijmegen Medical School, NL-6500 HB Nijmegen, The Netherlands