The powerful concept of resonance was now entering its full flower. The sixth in Pauling's chemical bond series, written in
1933 with postdoctoral fellow Jack Sherman, extended the concept to more chemical puzzles involving variations from classical
single, double, and triple bonds. This work, too, was groundbreaking.
In "The Nature of the Chemical Bond VI. The calculation from the thermochemical data of the energy of resonance of molecules
among several electronic structures" Pauling showed that molecules were not restricted to whole-integer links (the classic
single, double and triple bonds) but could take on intermediate forms. Here again he melded the bond lengths and strengths
from his ever-growing library of molecular structures with his ideas about the stabilizing influence of resonance, and came
up with novel explanations for a whole slew of problems.
This was particularly important for structural chemistry. Atoms connected by single bonds were known to be able to rotate
like wheels on an axle, for instance, while those linked by double and triple bonds were held rigidly and double bonds — with
what he called "partial double-bond character" — were also restricted from rotating, an important factor in focusing the approach
to potential structures and predicting new ones. In his sixth paper Pauling explained the restriction on rotation in quantum-mechanical
terms, then applied his idea of resonance between single and double bonds to explain the bond lengths and rotational properties
of a number of intermediate cases.
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"Scientists Publish New Journal Jan. 1." December 23, 1932.
"The nature of the chemical bond. VI. The calculation from thermochemical data of the energy of resonance of molecules among
several electronic structures." April 13, 1933.
"I published a paper with Jack Sherman on the calculation of some of these overlap integrals with a simplification.... It's
in The Nature of the Chemical Bond, the results are -- with a simplification of some sort; it's like taking Slater functions, I don't know what it was, but
actually evaluating the overlap integrals. Our conclusion was that the bond strength function giving angular dependence alone
is really pretty good -- not perfect but pretty good."
March 27, 1964