1. Contribution from the Institute of Organic Chemistry and Biochemistry and the Institute of Physical Chemistry, University of Freiburg, Albertstrasse 21, D-79104 Freiburg, Germany, Institute of Physical Chemistry, ETH Zürich, ETH Zentrum, CH-8000 Zürich, Switzerland, and Institute of Physical Chemistry, University of Basel, Klingelbergstrasse 80, CH-4056 Basel, Switzerland