1. Department of Chemistry, University of Leuven, Celestijnenlaan 200F, B-3001 Leuven, Belgium, Institute of Atomic and Molecular Sciences, Academia Sinica, P.O. Box 23−166, Taipei 10764, Taiwan, and Laboratory of Organic Chemistry, University of Mons-Hainaut, Avenue Maistriau 19, B-7000 Mons, Belgium