1. Contribution from the Department of Chemistry, Washington University, Saint Louis, Missouri 63130, Department of Physics, State University of New York at Binghamton, Binghamton, New York 13902, Lehrstuhl für Organische Chemie und Biochemie, Technische Universität München, D-85747 Garching, Germany, and Department of Medicinal Chemistry and Molecular Pharmacology, School of Pharmacy, Purdue University, West Lafayette, Indiana 47907