Developing the Theory of Resonance

Linus Pauling, 1930.

[Part 1 of 2]

“I think my work on the chemical bond probably has been most important in changing the activities of chemists all over the world – changing their ways of thinking and affecting the progress of the science.”

Linus Pauling, 1977.

In early 1932, Linus Pauling spent several months visiting the University of California, Berkeley and the Massachusetts Institute of Technology to present two different lecture series on the theory of resonance and its implications for molecular structure and function. This topic, a product of Pauling’s adventures in Europe as a Guggenheim fellow, would profoundly impact the ways in which twentieth-century chemists ultimately understood the chemical bond and predicted molecular structures.

Throughout his long career, Pauling sought to improve his understanding of molecular structure in order to better predict chemical function. As Pauling saw it, molecular structure dictates function and should thus be considered accordingly.  As he wrote in 1946

…I am confident that, as our knowledge of the structure not only of simple molecules but also of proteins and other complex constituents of organisms increases, we shall in time achieve an insight into physiological phenomena which will serve as an effective guide in biological and medical research, and will contribute to the solution of such great practical problems as those presented by cancer and cardiovascular disease.

Indeed, much of Pauling’s work sought to develop the tools necessary to enable chemists to bridge the gap between structure and function.  Pauling spoke somewhat literally of this quest in 1936, in a speech where he compared the tools of the chemist to those of an architect.

The structural chemist of the past and present has been an architect working with materials of whose nature he is largely ignorant – an architect who does not know what an I beam is, but only that it can be used in his construction, and who must proceed to design structure after structure, to find ultimately that certain designs lead to satisfactory results – to a building with rooms adapted to the use of certain visitors, to bridges strong enough to hold their load, and so on. The structural chemist of the future will be able to plan his structures and forecast their properties in the same definite way that the architect and engineer plan the macroscopic, even gargantuan, structures of modern civilization.

In the years just prior to, and at the start of, Pauling’s career, great strides had been made in  molecular structure determinations.

G.N. Lewis, ca. 1930s.

In 1916, for example, Gilbert Newton Lewis developed a theory of molecular diagrams based on valence electrons, now referred to as Lewis-dot structures. The subsequent application of spectroscopic methods to molecular chemistry allowed for more direct quantitative studies of atomic and molecular structure. Later, advancements in quantum mechanics increased chemists’ and physicists’ understanding of the detailed interactions that occur between nuclei and electrons that ultimately determine atomic and molecular structure.  Meanwhile, various valence bond theories had been developed but were not applicable to all matter uniformly.

While these and other contributions were significant, many questions still remained as not all quantitative data aligned with current theories. To provide an explanation for the many apparent holes in understanding, Pauling developed his theory of resonance – an idea which became the central concept of Pauling’s valence theory.

Resonance, or electron exchange, is a property integral to the formation and maintenance of chemical bonds, as it accounts for the formation of hybrid structures that cannot be explained by the classical models of molecular structure alone.

Pauling used his theory of resonance to explain why many molecules can be drawn in various forms according to Lewis’s scheme even though no single structure could be differentiated as the “correct” structure based on energy theories and quantum mechanics.

According to Pauling’s theory, these structures could not be differentiated quantitatively because the electrons exchanged between atoms caused the molecule to resonate between multiple structures. Thus the structure of a molecule is not made up of one single structure, but in some cases, such as in carbon monoxide (CO), the true nature of the molecule resonates between multiple structures. Pauling therein predicted that the CO molecule fluctuates rapidly between multiple conformations thus creating a more stable structure, known as a “resonance hybrid.”

By Pauling’s way of thinking, the theory of resonance explained many of the obvious inconsistencies in the understanding of specific molecules at that time, and he argued that the theory should be applied when predicting new molecular structures and functions.  In our next post, we’ll talk more about the impact of the theory of resonance by examining its application to the study of the enigmatic structure of benzene.

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