1. Contribution from The School of Chemistry, University of Birmingham, Edgbaston, Birmingham B15 2TT, U.K., Dipartimento di Chimica “G. Ciamician” dell'Università di Bologna, via Selmi 2, I-40126 Bologna, Italy, Department of Chemistry, Odense University, Campusvej 55, DK-5230 Odense M., Denmark, Department of Chemistry and Biochemistry, University of California, Los Angeles, California 90095, and Department of Chemistry, Imperial College, South Kensington, London SW7 2AY, U.K.