1. Contribution from The School of Chemistry, The University of Birmingham, Edgbaston, Birmingham B15 2TT, UK, Department of Chemistry, Imperial College, South Kensington, London SW7 2AY, UK, Merck Sharp & Dohme Research Laboratories, Neuroscience Research Centre, Terlings Park, Harlow CM20 2QR, UK, Dipartimento di Chimica “G. Ciamician” dell'Università, via Selmi 2, I-40126 Bologna, Italy, and Istituto FRAE-CNR, via Gobetti 101, I-40129 Bologna, Italy