1. Adolphe Merkle Institute, University of Fribourg, Route de l’Ancienne Papeterie, P.O. Box 209, CH-1723 Marly 1, Switzerland, Departamento de Química Fundamental, Instituto de Química, Universidade de São Paulo, Av. Prof. Lineu Prestes 748, CEP 05508-000 São Paulo, SP, Brazil, and Institut Max von Laue-Paul Langevin, F-38042 Grenoble Cedex 9, France