1. Contribution from the Leiden Institute of Chemistry, Gorlaeus Laboratories, Leiden University, P.O. Box 9502, 2300 RA Leiden, The Netherlands, Interuniversity Reactor Institute, Delft University of Technology, Delft, The Netherlands, and Bijvoet Center for Biomolecular Research, Vakgroep Kristal- en Structuurchemie, Utrecht University, Padualaan 8, 3584 CH Utrecht, The Netherlands