1. Department of Chemistry and Biochemistry, University of South Carolina, Columbia, South Carolina 29208; Departamento de Fisica y Quimica Teorica, Facultad de Quimica, Universidad Nacional Autonoma de Mexico, Ciudad Universitaria, C. P. 04510, Mexico D. F., Mexico; and Department of Chemistry and Center for Nanoscale Science and Technology, Rice University, MS 222, 6100 Main Street, Houston, Texas 77005