1. Contribution from the Institute of Life Science and Chemistry, Roskilde University Center, DK-4000 Roskilde, Denmark, Steacie Institute of Molecular Sciences, National Research Council Canada, Ottawa, Canada, KIA 0R6, Department of Chemistry, Nuclear Chemistry, Royal Institute of Technology, SE-100 44, Stockholm, Sweden, and Department of Chemistry, University of Aarhus, Langelandsgade 140, DK-8000 Århus C, Denmark