1. Contribution from the Institute for Theoretical Chemistry and Molecular Structural Biology, University of Vienna, UZAII, Althanstrasse 14, A-1090 Vienna, Austria, Department of Chemistry, University of Basel, Klingelbergstrasse 80, CH-4056 Basel, Switzerland, and Istituto di Fotochimica e Radiazioni d' Alta Energia, CNR Bologna, Via P. Gobetti 101, 40129 Bologna, Italy