1. Contribution from the Department of Chemistry, University of British Columbia, 2036 Main Mall, Vancouver, B.C., Canada V6T 1Z1, Istituto di Chimica Farmaceutica, Università di Milano, 42 Viale Abruzzi, I-20131 Milano, Italy, Institut Max von Laue-Paul Langevin, 38042 Grenoble Cedex 9, France, and Department of Chemistry, Brookhaven National Laboratory, P.O. Box 5000, Upton, New York, 11973-5000