1. Department of Organic Chemistry University of Geneva Quai Ernest Ansermet 30 1211 Geneva 4 Switzerland
2. Laboratoire de Cristallographie University of Geneva Quai Ernest Ansermet 24 1211 Geneva 4 Switzerland
3. Dipartimento di Chimica e Chimica Industriale University of Pisa Via G. Moruzzi 13 Pisa Italy