1. Department of Chemistry, University of the Free State, P. O. Box 339, Bloemfontein 9300, South Africa
2. Current address: Sasol Technology R & D, P. O. Box 1, Sasolburg 1947, South Africa
3. Department of Chemistry, Carnegie Mellon University, 4400 Fifth Av., Pittsburgh, PA 15213, USA
4. Department of Chemistry, M. V. Lomonosov Moscow State University, 119899 Moscow, Russia