1. Department of Chemistry, University of Toronto, Toronto, Ontario, Canada M5S 3H6, Department of Chemistry, Queen's University, Kingston, Ontario, Canada K7L 3N6, Institut für Organische Chemie, Universität Würzburg, D-97074 Würzburg, Germany, and Max-Planck-Institut für Festkörperforschung, D-70569 Stuttgart, Germany