1. Department of Molecular Theory and Spectroscopy, Max-Planck-Institut für Kohlenforschung, Kaiser-Wilhelm-Platz 1, D-45470 Mülheim an der Ruhr, Germany
2. Institute of General and Inorganic Chemistry, Bulgarian Academy of Science, Akad-Georgi Bontchev Str. Bl.11, 1113-Sofia, Bulgaria