1. Contribution from the Forschungszentrum Karlsruhe, Institute of Nanotechnology, P.O. Box 3640, D-76021 Karlsruhe, Germany, Institut für Physikalische Chemie, Universität Karlsruhe (TH), D-76128 Karlsruhe, Germany, and Center for Theoretical Chemistry, Department of Chemistry, Århus University, DK-8000 Århus, Denmark