1. Contribution from the Department of Chemistry, University of Aarhus, Langelandsgade 140, 8000 Aarhus C, Denmark, Instituto de Investigaciones Químicas, C.S.I.C., Isla de la Cartuja, Sevilla, Spain, Unidade Resonancia Magnética, RIAIDT, CACTUS, Santiago de Compostela, 15706 Spain, and Centro de Investigaciones Biológicas, C.S.I.C., Juan de la Cierva 3, 28006 Madrid, Spain