1. R. Robinson Outline of an Elektrochemical Theory of the Course of Organic Reactions 1932
2. L. Pauling The Nature of the Chemical Bond 1941
3. G. W. Wheland The Theory of Resonance (New York 1945); B. Eistert, Chemismus und Konstitution 1948
4. L. P. Hammett Physical Organic Chemistry 1940 314