1. Laboratory of Physical Chemistry, Department of Chemistry, University of Helsinki, P.O. Box 55 (A. I. Virtasen aukio 1), FIN-00014 Helsinki, Finland, Institute of Organic Chemistry and Biochemistry, Academy of Sciences of the Czech Republic, Flemingovo n. 2., CZE-16610 Praha 6, Czech Republic, and NMR Research Group, Department of Physical Sciences, University of Oulu, P. O. Box 3000, FIN-90014 Oulu, Finland