1. Department of Chemistry, University of Arizona, Tucson, Arizona 85721, Institute for Low Temperature Physics and Engineermg, National Academy of Sciences of Ukraine, 47 Lenin Avenue, Kharkov 310164 Ukraine, Department of Chemistry, Faculty of Science, lJniversity of Lisbon, R. Ernesto de Vasconcelos, ed. C1 1700 Lisboa, Portugal, and Department of Chemistry, University of Coimbra, P-3049 Coimbra, Portugal