1. Department of Chemistry, Virginia Polytechnic Institute and State University, Blacksburg, VA 24061‐0212 (USA)
2. Present address: E.I. duPont de Nemours and Company, Wilmington, DE 19880‐0402 (USA)
3. Department of Chemistry, University of Delaware, Newark, DE 19716 (USA)
4. Present address: X‐ray Crystallography Facility, University of Oregon, Eugene, Oregon 97403 (USA)
5. Present address: Department of Chemistry and Biochemistry, University of California, San Diego, La Jolla, CA 92093 (USA)