1. Department of Chemistry and Pharmaceutical Sciences Vrije Universiteit Amsterdam De Boelelaan 1083 1081 HV Amsterdam The Netherlands
2. Department of Chemistry University of Helsinki A. I. Virtasen aukio 1, P.O. Box 55 FIN-00014 Helsinki Finland
3. Department of Chemistry University of Johannesburg Oakland Park 2006 Johannesburg South Africa
4. Present address: Van 't Hoff Institute for Molecular Sciences University of Amsterdam P.O. Box 94157 1090 GD Amsterdam The Netherlands