1. Contribution from the Department of Chemistry and Biochemistry, University of Bern, Freiestrasse 3, CH-3000, Bern 9, Switzerland, Institut Laue-Langevin, BP156, 38043, Grenoble Cedex 9, France, High Magnetic Field Laboratory, CNRS/MPI, B.P. 166, 38042 Grenoble, France, Institut de Chimie, Université de Neuchâtel, Avenue de Bellevaux 51, C.P. 2, CH-2007 Neuchâtel, Switzerland, Laboratory for Neutron Scattering, PSI, CH-5232 Villigen, Switzerland, and Department of Physical Chemistry, Saarland University,...