1. Contribution from the Department of Chemistry, University College Dublin, Belfield, Dublin 4, Ireland, School of Chemistry, The University of Birmingham, Edgbaston, Birmingham B15 2TT, The United Kingdom, Department of Chemistry and Biochemistry, University of California at Los Angeles, 405 Hilgard Avenue, Los Angeles, California 90095-1569, and Dipartimento di Chimica “G. Ciamician”, Università di Bologna, via Selmi 2, 40126 Bologna, Italy