1. Contribution from the Department of Physics, University of Stockholm, Box 6730, S-113 85 Stockholm, Sweden, the Department of Biochemistry, Centre of Chemistry and Chemical Technology, P.O. Box 124, S-221 00 Lund, Sweden, the Department of Chemistry, Michigan State University, East Lansing, Michigan 48824, and Organic Chemistry, Department of Chemistry, Royal Institute of Technology, S-100 44 Stockholm, Sweden