1. Department of Chemistry, Washington University, St. Louis, Missouri 63130-4889, Department of Chemistry, North Carolina State University, Raleigh, North Carolina 27695-8204, Departments of Chemistry and of Molecular and Cell Biology, University of Connecticut, Storrs, Connecticut 06268, Department of Chemistry, University of Notre Dame, Notre Dame, Indiana 46556, and Department of Chemistry, University of California Riverside, Riverside, California 92521-0403