1. Centro C.N.R. “M.I.S.O.”, Dipartimento Chimica, Università di Bari, v. Amendola 173, I-70126 Bari, Italy, the Chemistry Department, Brown University, Providence, Rhode Island 02912, and the Dipartimento Scienze Ambientali, University of Venice, Dorsoduro 2137, I-30123 Venice, Italy csmirc17@area.ba.cnr.it