Julia Bursten is the fifth individual to complete research as a Resident Scholar in the Oregon State University Libraries Special Collections. Bursten is a doctoral candidate in the History and Philosophy of Science at the University of Pittsburgh.
[Update: transcribed video of Bursten's Resident Scholar presentation is now available.]
Bursten came to Corvallis to study a specific aspect of Linus Pauling’s valence theory of chemical bonds. In particular, she is interested in the development of Pauling’s ideas on the bent equivalent double bond, or “banana bond” as it is sometimes called.
From his unique vantage point as a structural chemist immersed in contemporary work on both molecular architecture and quantum mechanical behavior, Pauling was well-positioned to make groundbreaking contributions to the scientific understanding of how atoms interact to form molecules. In due course, he proposed that atoms assumed the form of tetrahedra, and that chemical bonds – including the double bond, a shorter and stronger bond that incorporates four bonding electrons rather than the usual two – could be represented in a similar fashion, as tetrahedra that are, roughly speaking, pressed together.
Pauling wrote extensively in support of this theory, though focused much of his attention on single bonds. Indeed, aside from a brief mention in a 1931 paper, Pauling’s only other early reference to the bent equivalent bond was a short passage in the first (1939) edition of The Nature of the Chemical Bond, in which Pauling again reiterated his position in support of the merged tetrahedra.
As with most of Pauling’s structural chemistry work, this picture of the double bond was generally accepted for several years. Gradually though, Pauling’s valence approach began to come under attack by a group of scientists supporting the molecular orbital model of chemical bonds. While confusing to most non-scientists, the differences between the valence bond theory and the molecular orbital approach are ably described as follows by Pauling biographer Thomas Hager.
In Pauling’s approach, derived from the electron-interchange idea of Heitler and London, molecules were aggregates of individual atoms, each linked to its neighbors by bonds formed by electrons localized between two nuclei. The number of bonds equaled the element’s valence, or bonding capacity…. In theory, the total quantum-mechanical state of a molecule could be calculated by adding together the wave functions that were involved in each bond, with appropriate adjustments for the effect of each bond upon its neighbors….
[The] molecular orbital theory [is] an approach predicated on a belief that molecules were not what valence bond advocates thought they were. Molecules to [molecular orbital proponents] were not aggregates of distinct atoms connected by distinct bonds but things unto themselves, with their own odd behavior explicable only in molecular terms…. [The theory posits] that molecules could be more profitably viewed as if their binding electrons were somewhat delocalized and spread across the surface.
According to Bursten, the molecular orbital supporters suggested that both their approach and Pauling’s valency approach yielded the same results in the explanations that they gave for molecular behavior. This noted, the mathematics underlying the molecular orbital techniques were much simpler to apply and, as a result, an improvement on Pauling’s work.
Pauling did not respond to these developments until 1958, when he issued a series of three speeches in support of the valency approach and, specifically, his model of the double bond. These presentations were followed by a detailed technical rebuttal of the molecular orbital school in Pauling’s third (1960) edition of The Nature of the Chemical Bond. As Bursten points out, Pauling had not published this defense in any formal channels before including it in his 1960 text – an approach viewed by many as highly unorthodox.
As it turned out, Pauling’s writings in 1960 marked both his last major defense of the bent equivalent double bond as well as the beginning of the end for Pauling’s valence theory. Over time the molecular orbital approach gained the favor of the scientific community. Indeed, Bursten’s research indicates that Pauling was denied several later grant requests for work on theoretical structural chemistry, precisely because he sought to conduct further research grounded in his valence bond model. As with his and Robert Core’s triple-helix structure for DNA, Pauling’s valence bond approach was swept aside by the research of others.
The OSU Libraries Special Collections Resident Scholar Program is generously supported by the Peter and Judith Freeman Fund. Past recipients have included Dr. Burtron Davis of the University of Kentucky’s Center for Applied Energy Research, Toshihiro Higuchi of Georgetown University, Dr. Mina Carson, professor of history at Oregon State University and Jane Nisselson, a documentary filmmaker based in New York City.
Filed under: Documentary History Websites, Nature of the Chemical Bond, Site and Department News Tagged: | Julia Bursten, Linus Pauling, molecular orbital theory, Resident Scholar Program, Thomas Hager, valence theory