1. Contribution from the Department of Chemistry, University of Leuven, Celestijnenlaan 200F, B-3001 Heverlee, Belgium; Lehrstuhl für Organische Chemie II, Ruhr-Universität Bochum, 44780 Bochum, Germany; Department of Chemistry, University of York, York YO10 5DD, U.K.; and Center for Computational Quantum Chemistry, University of Georgia, Athens, Georgia 30602