1. Institut für Physikalische and Theoretische Chemie, Technische Universität Braunschweig, D-38106 Braunschweig, Germany, Fachbereich Analytische Messtechnik and Druck, Physikalisch Technische Bundesanstalt, D-38116 Braunschweig, Germany, Fritz Haber Institute for Molecular Dynamics, Hebrew University, Jerusalem, Israel 91904, and Institute of Organic Chemistry and Biochemistry, Academy of Sciences of the Czech Republic and Center for Biomolecules and Complex Molecular Systems, 16610 Prague 6, Czech Republic