New Insights into Metals and More

Linus and Peter Pauling at Warwick Castle, England. 1948.

[The Paulings in England: Part 3 of 5]

In his lab, a five minute walk from his office at Balliol College (where he was once caught boiling an egg on his electric space heater), Linus Pauling’s research took a turn from the contents of his lectures – intermolecular forces and biological specificity – and he found himself devoting his research time to metal theory. Pauling had planned to revise the index for his newly published freshman text, General Chemistry, during his Eastman Professorship, but couldn’t seem to get metals off his mind.  As he wrote in a letter to his Caltech colleague J. Holmes Sturdivant, “I thought that I would be doing work in connection with my freshman text while in England, but it has turned out that I have devoted all of my time, and presumably shall continue to do so, to work on the theory of metals and intermetallic compounds.”

He was aided in his lab by three other researchers – David Shoemaker, Hans Kuhn, and a young man from Holland, Dr. F. C. Romeyn. Pauling’s circumstances were proving to be highly productive, and in a March letter to Robert Corey, Pauling wrote of the impact that the change of setting was having in stimulating his thoughts:

I have been having wonderful success in my development of a theory of metals. I think that it has really been very much worthwhile for me to get away for this period of time, under circumstances favorable to my thinking over questions and trying to find their solution. The problem of metals has been on my mind for a number of years, and I haven’t been able to leave it alone, so it is a good thing that I have now managed to get it solved.

This new theory of metals was an extension of Pauling’s valence-bond approach to determining the structure of molecules, as initially developed in the late 1920s. Pauling was first exposed to quantum mechanics as an undergraduate at Oregon State University (then known as Oregon Agricultural College) and retained that interest as he transitioned to graduate studies and faculty employment at the California Institute of Technology.

In 1926 Pauling traveled on a Guggenheim Fellowship to study the developing field of quantum mechanics with physicists in Europe, and especially Germany. He brought these new ideas back to Caltech in the form of quantum chemistry, which he used to compute the electronic structures of molecules. This intuitive valence-bond approach was quickly judged a success and had been popular since the 1930s as a simple model for studying the electron dispersal in the bonds between molecules.

But all the while another chemist, Robert Mulliken (recipient of the 1966 Nobel Prize for Chemistry) had been steadily fostering a rival approach: the molecular orbital theory. While the Pauling family enjoyed springtime in Paris at the beginning of April, Pauling and Mulliken met head to head at a conference on Isotopic Exchange and Molecular Structure. There an entire day was devoted to the comparison of the two theories before a group of quantum chemists. Pauling had written earlier that molecular orbitals were confusing to students, but he learned at this meeting that one always has to stay one’s toes: with more mathematics under their belts, advanced chemistry students were increasingly hungry for the more quantitative approach that Mulliken’s theory offered.

Sometimes ideas come upon the great thinker at surprising times, and Pauling experienced just such a eureka moment during one of his twice-weekly Oxford lectures in February.  As he wrote to Holmes Sturdivant,

I have just had a great stroke of luck. While giving my lecture on Tuesday I suddenly realized that a calculation about resonance energy of metals that I had just made and was reporting contained the key to the strange valence numbers and numbers of atomic orbitals and unused orbitals that have turned up in my theory of valency of metals.

Notes on intermetallic compounds by Linus Pauling, March 1948.

Pauling worked out his ideas on electron theory and the structure of metals and intermetallic compounds through pages and pages of careful handwritten calculations. In looking at each manuscript now, Pauling presents a hypothesis about some aspect of metal theory and then proceeds to calculate, revise, and recalculate until the theory and the experimental x-ray diffraction data line up. For instance, on one day in March, Pauling was exploring intermetallic compounds from several different angles.  He writes “I shall now treat intermetallic compounds, with my new ideas – resonance of bonds when an extra orbital is available, importance of n=1/2, 1/4 etc., concentration of bonding electrons into strong bonds (Zn-Zn, etc as compared with Na-Na) , transfer of electrons with increase in valence.” Hybrid orbitals, bond lengths, and the overall stability of structures were other items on Pauling’s research agenda.

Of course, not every idea is a winner and a few theories led Pauling down the wrong path; in one manuscript Pauling set out to, as he wrote, “consider sp hybridization – how can we set up a secular equation to give the results given by my bond-strength postulate?”  In the end Pauling found that “the ratio does not come out as desired. It is evident that my assumption that the energies can be taken proportional to ‘bond strengths’ is not right.”  Missteps such as these didn’t deter Pauling from pressing on with his research, for as he often said, “The way to have good ideas is to have lots of ideas, and throw away the bad ones.”

Chemistry boasts its own special language, or nomenclature, and chemists like Pauling are to thank for the terms that make chemical jargon unique. As research advances, sometimes an entire new word is needed to describe an innovative concept. While tackling the nuances of metal theory at Oxford, Pauling wrote to Sturdivant about this very problem.

By the way, I think that we should do something toward improving the nomenclature. For example, coordination number is an awkward and unwieldy expression – we need one short, precise word for this concept. Perhaps ligancy could be used. It would fit in well with ligand and the verb to ligate. We also need some general words to express the bonds between one atom and the surrounding atoms – we now use the word bond to refer both to the electron pair bond that is resonating around among alternative positions and to the fraction of an electron pair bond that is a portion to a particular position. I have also felt troubled about using the word position in this way – to mean the region between two atoms. If we do introduce any change in nomenclature, it must be very well thought out, and must not involve too great a strain on the memory, or too great a departure from the past.

New fields also call for innovations in instrument development and research programs. Pauling was in constant communication with his colleagues back home about new tools that might be constructed to aid the researchers. He admired the Cavendish’s vast x-ray crystallography laboratory and also gained new insights from reading British journals devoted to scientific instrumentation. He would frequently send word back as to how Caltech workers could improve on a complex apparatus such as the specialized cameras for x-ray diffraction of metallic crystals.

Pauling was likewise intrigued by the English system of graduate education, wherein graduate students would take class work completely during the first year and then spend practically 100% of their time on research during the other two years. Pauling was always looking to improve upon existing programs, but as appealing as the English system was, he acknowledged that in implementing it one would run the risk of not knowing whether a student was an apt researcher for their entire first year!

Julia Bursten, Resident Scholar

Peter Freeman, Julia Bursten and Judith Freeman.

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.