The most public demonstration of the power of Pauling's resonance ideas came when
he used them to solve one of the oldest problems in organic chemistry.
Benzene was a mystery. It was known to be composed of six carbon atoms and six hydrogen
atoms, but the structure of the benzene molecule had eluded definitive analysis. In
the winter of 1932-33, Pauling and a graduate student, George Wheland, set out to
solve the structure according to Pauling’s concept of resonance.
By spring they finished a paper, "The Nature of the Chemical Bond V. The quantum-mechanical
calculation of the resonance energy of benzene and naphthalene and the hydrocarbon
free radicals," in which benzene was described as resonating between five extreme,
or "canonical," structures. "The properties of the molecule," Pauling wrote, "would
then be expected to be a sort of average of the properties of the individual molecules."
Pauling and Wheland's approach seemed to work: The values calculated from their resonating
structure fit what was known about the molecule's structure, reactivity, and stability.
They expanded their approach to other aromatic molecules like naphthalene (using no
fewer than forty-two canonical structures) and to hydrocarbon free radicals.