1. Fakultät für Chemie, Universität Bielefeld, Postfach 100131, 33501 Bielefeld, Germany
2. Department of Chemistry, Georgetown University, Box 571227, Washington, D.C. 20057, USA
3. Dipartimento di Chimica, Universitá di Firenze, Via G. Capponi 9, 50121 Firenze, Italy