1. Contribution from the Department of Chemistry, University of Calgary, 2500 University Dr. N. W., Calgary, Alberta, Canada T2N 1N4, Bayer AG, Bayer Chemicals AG, BCH-R&D-I1, 51368 Leverkusen, Germany, and Bayer AG, Bayer Industry Services, X-ray Laboratory, Q18, 51368 Leverkusen, Germany.