1. Department of Chemistry, Clemson University, Clemson, South Carolina 29634-0973, Department of Chemistry, Austin College, Sherman, Texas 75090-4440, and Centre de Recerca en Química Teòrica, Departaments de Quimica Física and Quimica Organica, Universitat de Barcelona, Marti i Franques 1, 08028-Barcelona, Catalonia, Spain