1. Van't Hoff Institute for Molecular Sciences, University of Amsterdam, Science Park 904, 1098 XH, Amsterdam (The Netherlands) fax: +31 20 525 5604 http://www.science.uva.nl/research/imc/HomKat/
2. Current address: IFP Energies nouvelles Rond point de l'échangeur de Solaize, 69360 Solaize (France)