1. Dipartimento di Chimica “G. Ciamician”, Università di Bologna, via Selmi 2, Bologna, I-40126, Italy, Department of Chemistry, Odense University, Campusvej 55, Odense M, DK-5230, Denmark, and Department of Chemistry and Biochemistry, University of California, Los Angeles, 405 Hilgard Avenue, Los Angeles, California 90095-1569