1. Contribution from the School of Chemistry, University of Bristol, Cantock's Close, Bristol, UK BS8 1TS, Instituto de Química-Física “Rocasolano”, CSIC, E-28006 Madrid, Spain, Center for Computational Chemistry, University of Georgia, Athens, Georgia 30602, and Institut für Organische Chemie der Universität Erlangen-Nürnberg, Henkestrasse 42, D-91054 Erlangen, Germany, and Institut de Ciència de Materials de Barcelona, CSIC, Campus de Bellaterra, Cerdanyola, E-08193 Barcelona, Spain