1. School of Chemistry, University of Manchester, Oxford Road, Manchester M13 9PL, U.K., Molecular Materials Research Center, Beckman Institute, MC 139-74, California Institute of Technology, 1200 East California Boulevard, Pasadena, California 91125, and Departamento de Química Orgánica, ICMA, Universidad de Zaragoza-CSIC, E-50009 Zaragoza, Spain