1. Institute of Physical Chemistry, University of Mainz, Welderweg 15, D—55099 Mainz, Germany
2. Permanent address: Central Research Institute for Chemistry of the Hungarian Academy of Sciences, Pusztaszeri u. 59—67, P.O. Box 17, H—1525 Budapest, Hungary
3. Shell Research and Development Centre, Postbus 3033, NL—1003 AA, Amsterdam, Netherlands