[Part 7 of 7]

“The Nature of the Chemical Bond. VII. The Calculation of Resonance Energy in
Conjugated Systems.” The Journal of Chemical Physics, October 1933

The final Paper in Linus Pauling’s earthshaking series on the nature of the chemical bond was the shortest of the seven and made less of a splash than had most of its predecessors. This lesser impact was anticipated and was due primarily to the guiding purpose of the paper: to apply previously developed postulates to compounds that had not been addressed by Pauling in his prior writings. As with the sixth paper in the series, the final publication was co-authored by Caltech colleague Jack Sherman.

In paper seven, Pauling demonstrated how to calculate resonance energy in conjugated systems. A conjugated system is one in which there exists a plane – or alignment – of three or more connecting electrons located in the p orbital. While it was commonly understood by the era’s organic chemists that conjugated systems supplied a compound with more stability than would ordinarily be expected, Pauling’s paper offered the calculations needed to codify this knowledge.

The paper also put forth a collection of rules to help researchers better understand the properties of conjugated systems. For example, Pauling found that “a phenyl group is 20 or 30 percent less effective in conjugation than a double bond, and a naphthyl group is less effective than a phenyl group.” To arrive at these conclusions, Pauling used the equations that he had developed in his previous two papers, applying them this time around to conjugated systems.

Pauling’s seven papers on the nature of the chemical bond came to print over the course of thirty months, from article one in April 1931 to article seven in October 1933. The first three papers laid the groundwork for what was to come by defining chemical bonds in quantum mechanical terms. The fourth paper, published in September 1932, appeared at the midpoint of Pauling’s publishing chronology and also served as a kind of transition paper, connecting the concepts introduced in the first three publications to those in the three more that were forthcoming. (Paper four also contained Pauling’s vital electronegativity scale.) The last three articles were devoted to the concept of resonance and its application to a fuller understanding of the chemical bond.

Taken as a whole, this body of work proved hugely important to the future direction of chemistry. By reconciling and applying the principles of quantum mechanics to the world of chemistry, the articles showed that what had once been mostly a tool for physicists could indeed have great applicability to chemical research. In the process, Pauling and his collaborators also rendered quantum mechanics far more accessible to their colleagues across the field of chemistry. The end result was, to quote Pauling himself, “a way of thinking that might not have been introduced by anyone else, at least not for quite a while.”

This is our forty-eighth and final post for 2020. We’ll look forward to seeing you again in early January!