1. Contribution from the Department of Chemistry, McMaster University, 1280 Main Street West, Hamilton, Ontario, Canada L8S 4M1, Department of Organic Chemistry, Weizmann Institute of Science, 76100 Rehovot, Israel, and Department of Chemistry and the Lise Meitner-Minerva Center for Computational Quantum Chemistry, Technion-Israel Institute of Technology, Haifa 32000, Israel