1. Contribution from the Department of Chemistry, Johns Hopkins University, 3400 North Charles Street, Baltimore, Maryland 21218, Chemistry Program, Roosevelt University, Chicago, Illinois 60605, Dipartimento di Chimica e UdR INSTM di Firenze, Polo Scientifico, Via della Lastruccia 3, 50019 Sesto Fiorentino, Italy, and Department of Chemistry and Biochemistry, University of California, San Diego, 9500 Gilman Drive, La Jolla, California 92093