1. Center for Chemistry and the Environment, Leiden Institute of Chemistry, Gorlaeus Laboratories, Leiden University, P.O. Box 9502, 2300 RA Leiden, The Netherlands, Universitat de València, 46100 València, Spain, Grinnell College, Grinnell, Iowa 50112, Université de Strasbourg, 67000 Strasbourg, France, and Chemistry Department, Environmental Chemistry, Umeå University, SE- 901 87 Umeå, Sweden