1. General account of substituent effects: C. K. Ingold, Structure and Mechanism in Organic Chemistry, 2nd edn. ( Bell, London, 1969 )
2. L. P. Hammett, Physical Organic Chemistry, Chapter 11 (McGraw-Hill, New York, 2nd edn., 1970 )
3. H. B. Watson, Modern Theories of Organic Chemistry, p. 241 (Oxford U.P., 2nd edn., 1941 )
4. R. W. Taft in Steric Effects in Organic Chemistry, Chapter 13, M. S. Newman, ed. (Wiley, New York, 1956 )
5. V. Meyer and J. J. Sudborough, Chem. Ber., 27, 1580 (1894)