1. Hoard, J. L., in Structural Chemistry and Molecular Biology (edit. by Rich, A., and Davidson, N.), (Freeman, San Francisco, 1968).
2. Countryman, R., Collins, D. M., and Hoard, J. L., J. Am. chem. Soc., 91, 5166–5167 (1969).
3. Perutz, M. F., Nature, 228, 726–734 (1970).
4. Coulson, C. A., Valence, second ed., 39 (Oxford University Press, London, 1961).
5. Perkins, J. F., J. chem. Phys., 39, 687–693 (1963).