1. Contribution from the Department of Chemistry, University of Florida, Gainesville, Florida 32611-7200, Laboratoire Louis Néel-CNRS, BP 166, 25 Avenue des Martyrs, 38042 Grenoble, Cedex 9, France, Department of Chemistry and Molecular Structure Center, Indiana University, Bloomington, Indiana 47405-7102, and Department of Chemistry, University of California at San Diego, La Jolla, California 92093-0358