1. Contribution from the Los Alamos National Laboratory, Los Alamos, New Mexico 87544, Department of Biological and Chemical Sciences, University of the West Indies, Cave Hill, Barbados, West Indies, Department of Chemistry, DePaul University, 25 East Jackson Boulevard, Chicago, Illinois 60604, Department of Chemistry, Manhattan College, Riverdale, New York City, New York 10471, and Department of Chemistry, Princeton University, Princeton, New Jersey 08544