1. Institut für Physikalische Chemie, Universität Basel, Klingelbergstrasse 80, CH-4056 Basel, Switzerland, Institut für Chemie, Universität Hohenheim, Garbenstr. 30, D-70599 Stuttgart, Germany, Dipartimento di Chimica “G. Ciamician” dell'Università di Bologna, Via Selmi 2, 40126 Bologna, Italy, and the Department of Chemistry, University of Ottawa, 10 Marie Curie, Ottawa, Ontario K1N 6N5, Canada