[1] NIST, Gaithersburg, MD 20899 USA
[2] Palo Alto Res Ctr, Palo Alto, CA 94304 USA
[3] Stanford Synchrotron Radiat Lab, Menlo Pk, CA 94025 USA
[4] Merck Chem, Southampton S016 7QD, Hants, England
[5] Univ London Imperial Coll Sci Technol & Med, Dept Chem, London SW7 2AZ, England