1. Contribution from the School of Chemistry, The University of Edinburgh, West Mains Road, Edinburgh, EH9 3JJ, UK, Laboratoire Louis Néel-CNRS, 38042 Grenoble, Cedex 9, France, Department of Chemistry, University of Florida, Gainesville, Florida 32611-7200, and Department of Chemistry, University of Patras, 26504 Patras, Greece