1. Contribution from the The Department of Chemistry, Columbia University, New York, New York 10027, Laboratory of Chemical and Biological Dynamics, K. U. Leuven, Celestijnenlaan 200D, B-3001 Heverlee, Belgium, FB Chemie, Universität Kaiserslautern, 67653 Kaiserslautern, Germany, and Bell Laboratories, Lucent Technologies, Murray Hill, New Jersey 07974