1. Leiden Institute of Chemistry, Gorlaeus Laboratories, Leiden University, PO Box 9502, 2300 RA Leiden, The Netherlands, Shell Research and Technology Center Amsterdam, Shell International Chemicals B.V., PO Box 38000, 1030 BN Amsterdam, The Netherlands, and Department of Crystal and Structural Chemistry, Bijvoet Center for Biomolecular Research, Utrecht University, Utrecht, The Netherlands