1. Contribution from the MIT/Harvard Center for Magnetic Resonance, Francis Bitter Magnet Laboratory and Department of Chemistry, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, Massachusetts 02139, Department of Chemistry, Queen's University, Kingston, Ontario K7L 3N6, Canada, and Department of Chemistry and Biochemistry and the Molecular Biology Institute, University of California, Los Angeles, 405 Hilgard Avenue, Los Angeles, California 90095-1569