1. Contribution from the Departamento de Química Orgánica, Facultad de Ciencias, Universidad Autónoma de Madrid, E-28049 Madrid, Spain, Departamento de Química Orgánica, Facultad de Ciencias Químicas, Universidad Complutense de Madrid, E-28040 Madrid, Spain, Department of Chemistry, Clemson University, Clemson, South Carolina 29634, Radiation Laboratory, University of Notre Dame, South Bend, Indiana 46556, and Institute of Physical Chemistry, University of Erlangen, 91058 Erlangen, Germany