1. Departamento de Química Orgánica (C-I), Universidad Autónoma de Madrid, Cantoblanco, E-28049 Madrid, Spain, Institute for Physical Chemistry, Friedrich-Alexander-Universität Erlangen-Nürnberg, Egerlandstrasse 3, D-91058 Erlangen, Germany, and Department of Chemistry, Clemson University, Clemson, South Carolina 29634