1. Dipartimento di Chimica, Università di Perugia, 06123 Perugia, Italy, Istituto di Metodologie Inorganiche e dei Plasmi - C.N.R., 70126 Bari, Italy, Laboratory of Molecular Structure and Quantum Mechanics, Department of Chemistry, Moscow State University, 119992 Moscow, Russia, and Department of Chemistry, University of British Columbia, V6T 1Z1, Vancouver, Canada