1. Contribution from the Laboratoire de Chimie Théorique, Unité de Recherche Associée au CNRS No. 510, Université Henri Poincaré-Nancy I, BP. 239, 54506 Vandœuvre-lès-Nancy Cedex, France, Exobiology Branch, NASAAmes Research Center, MS 239-4, Moffett Field, California 94035-1000, and Department of Pharmaceutical Chemistry, University of California, San Francisco, San Francisco, California 94143-0446