Erich Hückel, Pioneer of Organic Quantum Chemistry: Reflections on Theory and Experiment

In the period between 1930 and 1937, Erich Hückel, a theoretical physicist, made profound contributions to organic chemistry in his quantum mechanical descriptions of unsaturated and conjugated compounds. A brief account of his academic career is followed by simplified expositions, from the point of view of an organic chemist, of the highly approximate theoretical methods he used. Of special significance in the case of cyclic molecules of the class C n H n is the concept of filled shells when the number of π electrons is (4 N + 2) ( N = 0,1,2,…). Examinations of key applications of Hückel's ideas reveal how they eventually motivated the exploration of new fields in organic synthesis and organic reaction mechanism. Another significant (but until recently, virtually ignored) contribution by Hückel was the recognition that atomic connectivity is a strong determinant of spin multiplicity in non‐Kekulé molecules. This idea provides, in principle, a basis for predicting violations of Hund's rule, as recent computational and experimental developments confirm. The interaction of experiment with different styles of quantum theory, and the impact of this relationship on the development of chemistry, are briefly considered. Speculations are given on why Hückel's work exerted so little influence on organic chemistry for decades before its importance finally began to be recognized.