1. The Chemistry Department, University of Reading, Whiteknights P.O. Box 224, Reading RG6 6AD, United Kingdom
2. The Department of Chemistry of the University of Newcastle, Newcastle upon Tyne, NE11 7RU, United Kingdom, and CLRC, Daresbury Laboratory, Warrington WA4 4AD, United Kingdom
3. The Royal Institution of Great Britain, 21 Albemarle Street, London W1X 4BS, United Kingdom, and CLRC, Daresbury Laboratory, Warrington WA4 4AD, United Kingdom