Pauling’s Fifth Paper on the Nature of the Chemical Bond

[What follows is Part 5 of 7 in this series. It is also the 800th blog post published by the Pauling Blog.]

The Nature of the Chemical Bond. V. The Quantum-Mechanical Calculation of the Resonance Energy of Benzene and Naphthalene and the Hydrocarbon Free Radicals.” The Journal of Chemical Physics, June 1933.

With his fifth paper in the nature of the chemical bond series, Linus Pauling communicated a new understanding of the structures of benzene and naphthalene. While it had been long accepted that benzene (C6H6) was arranged as a six-carbon ring and naphthalene (C10H8) as two six-carbon rings, the specific organization of electrons and bonds within these structures were not known. Before the publication of Pauling’s fifth paper, several ideas on these matters had been proposed, but all were viewed as flawed in some way or another. But where others had been stymied, Pauling found success, and he did so by fully embracing and utilizing the theory of resonance.

At the time that Pauling began this work, there were five competing structures for benzene, each burdened by its own problems. The one that was the most accepted, despite its inability to connect theory to experimental data, was the Kekulé model. Put forth several decades earlier by the German chemist August Kekulé, this model centered around a six-carbon ring that possessed alternating double bonds. Because the arrangement of these double bonds could differ, Kekulé’s model was actually proposing two potential isomers for benzene. The standard understanding at the time was that these two isomers constantly oscillated between one another.

One major problem with the Kekulé approach was that scientists of his generation had never found evidence of the oscillating structures. Furthermore, the Kekulé structures should have been quite unstable, which was contrary to what researchers were able to observe in the laboratory. As such, even though it was compelling in the abstract, the Kekulé model was known to be imperfect.

In his paper, Pauling pointed out the flaws in Kekulé’s work as well as four other concepts published by other researchers. In doing so, he suggested that a common hindrance to all of the approaches was a reliance upon the laws of classical organic chemistry, and a concomitant lack of application of the new quantum mechanics. It was Pauling’s belief that the structure of benzene could be explained using quantum mechanics, as could the structures of all aromatic compounds.

In a handful of previous papers, Pauling had used the theory of resonance to explain a variety of chemical phenomena, but in thinking about benzene and naphthalene he committed more fully to its principles. According to Pauling, all observable data that had been collected for benzene, particularly its bond energies, suggested that benzene was much stronger than any models had yet to predict. But none of the previous models had entertained the possibility of a resonate structure, by which he meant an aggregate structure that was essentially a blend of all possible structures. A structure of this sort, Pauling argued, would conform to a lower, more stable energy state, and would accurately map with the observed data.

For Pauling, therefore, the structure of benzene was not the result of rapid isomerization as put forth by Kekulé, but rather a blend of states. “In a sense,” he wrote, “it may be said that all structures based on a plane hexagonal arrangement of the atoms – Kekulé, Dewar, Claus, etc. – play a part” but “it is the resonance among these structures which imparts to the molecules its peculiar aromatic properties.”

To support his theory, Pauling considered all five possible structures of benzene – which he called “canonical forms” – calculating the energy of each structure as well as the combined resonance energy. Having done so, Pauling then noted that it was the resonance energy that most closely matched the observed data.

In addition to its utility, the elegance of Pauling’s approach compared favorably with similar work being published by a contemporary, the German chemist Erich Hückel. Situating this thinking within Molecular Orbital theory, Hückel was able to arrive at a similar conclusion for benzene, but his calculations were quite cumbersome and could not be applied to larger aromatic compounds. By contrast, Pauling was now firmly rooted in Valence Bond theory and his formulae could be applied to all aromatics, not just benzene. In particular, by simplifying some of the calculations that Hückel had made, Pauling was able to overcome some of the mathematical hurdles posed by the free radicals in benzene and other aromatics.

To demonstrate the broad applicability of his ideas, Pauling applied his theoretical framework to naphthalene, which consists of two six-carbon rings and had forty-two canonical structures — a great many more than benzene’s five. Despite this significant difference, Pauling was successful in applying the same basic math to determine that the structure was also in resonance.

Indeed, Pauling was certain that his calculations were relevant to all aromatic compounds, noting specifically that “this treatment could be applied to anthracene [a three-ringed carbon molecule] and phenanthrene [a four-ringed carbon molecule], with 429 linearly independent structures, and to still larger condensed systems, though not without considerable labor.” Were one willing to expend this labor, the calculations would show that the “resonance energy and the number of benzene rings in the molecule would be substantiated” and the structure correctly predicted.

G.W. Wheland

The fifth paper was unique in part because it was the first in the series to be co-authored. The article also marked a switch in publishing forum: whereas the first four had appeared in The Journal of the American Chemical Society, this paper (and the two more still to come) was published in volume 1 of The Journal of Chemical Physics.

Pauling’s co-author for the paper was George W. Wheland, a recent doctoral graduate from Harvard who worked with Pauling from 1932-1936 with the support of a National Research Fellowship. This collaboration proved noteworthy both for the quality of the work that was produced and also because Wheland later became a vocal supporter, advocate and contributor to resonance theory.

Pauling Supports His Resonance Theory

Portrait of Linus Pauling, 1930s

[Exploring the theory of resonance and the Soviet resonance controversy. Part 2 of 7.]

Linus Pauling was able to develop his resonance theory because of his belief that quantum mechanical principles could be applied to molecular architecture. And even though the theory did not always predict chemical structures and bond strengths with supreme accuracy, Pauling worked hard to defend his ideas. This firm belief in resonance permeated many aspects of Pauling’s work for years to come, and the imperative to defend it arose in more than one instance as well.

Resonance theory informed the two schools of thought then-prevailing for chemists interested in quantum mechanics and the chemical bond. These two modes of thinking were the Valence Bond (VB) theory and the Molecular Orbital (MO) theory. And while both theories used the concept of resonance to explain molecular structure, neither was able to predict all structures all of the time.

One might assume that this variability would have troubled Pauling, but if so, it did not cause him to falter in his conviction that resonance was the best tool available for accurately describing states of matter on the molecular level. Perhaps because of this, Pauling ultimately preferred the VB theory, for which hybridization of orbitals is the mechanism by which molecular shape is explained. That said, Pauling was also aware of the utility of MO theory and, in at least one instance – a study of cyameluric acid – he was forced to use MO theory to explain the structure, having concluded that his application of VB theory was inaccurate.

But incidents like this did not weigh heavily on Pauling, who remained very confident in the soundness of his ideas. Notably, in his 1939 book, The Nature of the Chemical Bond, Pauling explained away the incongruities between VB and MO theory and upheld resonance by stressing that, “the convenience and value of the concept of resonance in discussing the problems of chemistry are so great as to make the disadvantage of the element of arbitrariness of little significance.”

While he took pains to press the usefulness of resonance theory, Pauling sometimes found it more difficult to explain it vagaries. On a conceptual level, one of the theory’s biggest hindrances to acceptance was its statement that the “correct” structure of a molecule is a hybrid of all possible isomers of the molecule. This effectively meant that, in a resonating structure, all possible isomers exist simultaneously, an idea that proved confounding for many. Prior to resonance, the conventional wisdom was based on August Kekulé’s description of rapidly changing isomers. Pauling was sure that, in order for resonance theory to work, the idea of isomers shifting from one to another had to be abandoned.

In making the case for resonating structures, Pauling wrote that

a substance showing resonance between two or more valence-bond structures does not contain molecules with the configurations and properties usually associated with these structures.

Of the various Kekulé structures, Pauling suggested that, “taken together, [they] provide a rough description of the wave function of the molecule in its normal state.” The implication, of course, was that resonance theory offered a more precise set of tools for understanding molecular architecture, once one was ready to clear a few conceptual hurdles.

Christopher K. Ingold

In time the initial sense of confusion was overcome, and the utility and brilliance of Pauling’s resonance theory made it widely accepted. While some saw resonance as heralding a promising new direction for the application of chemistry, others found the theory appealing simply because it seemed to solve so many problems. Early praise came in 1933, when the esteemed British chemist Christopher K. Ingold declared the theory as having effectively resolved lingering questions about the stability of aromatic compounds.

Ingold’s colleague Nevil V. Sidgwick also became a big supporter of resonance theory. Sidgwick was a well-respected scientist, and his endorsement of resonance helped to cement its status as a leading model for understanding chemical structures. A few years later, in his 1944 book, The Theory of Resonance and its Applications to Organic Chemistry, University of Chicago professor George Wheland confirmed resonance as being

the most important addition to chemical structural theory that has been made since the concept of the shared-electron bond was introduced by G.N. Lewis.

Pauling too believed in the utility of resonance throughout his life. He argued in particular that the theory was especially well-suited to “aromatic molecules, molecules containing conjugated systems of double bonds, hydrocarbon free radicals, and other molecules to which no satisfactory single structure in terms of single bonds, double bonds, and triple bonds can be assigned.” He also believed that “resonance provides an explanation of the properties of many inorganic molecules. For example, the carbon monoxide molecule.” Pauling likewise suggested that resonance theory “permitted the discovery” of the alpha helix and associated models comprising “the most important secondary structures of polypeptide chains in proteins.”

In time, resonance theory would become universally used and applied, but not before Pauling was forced to wrestle with an unusual conflict coming out of the Soviet Union. This controversy, which lasted for nearly five years, will be introduced in our next post.

Becoming An Asset for the Guggenheim Foundation

Simon Guggenheim

[Ed Note: In much the same fashion as last year’s examination of Linus Pauling’s administrative work at Caltech, the Pauling Blog is once again going long form, this time with a detailed look at Pauling’s connections with the Guggenheim Foundation. This is the first post in a series that will occupy much of our schedule through the late Spring.]

Simon and Olga Guggenheim established the John Simon Guggenheim Memorial Foundation in 1925 to honor their son, who had died in 1922 just before he was to enter college. The Guggenheims’ intentions for this new foundation were “to improve the quality of education and the practice of the arts and professions in the United States, to foster research, and to provide for the cause of better international understanding.”

The Foundation aimed to do so by offering “promising scholars, both men and women, opportunities under the freest possible conditions to carry on advanced study and research in any field of knowledge, or opportunities for the development of an unusual talent in any of the fine arts, including music.” That charge supported fifteen fellows in the organization’s first year and forty-three in its second. Within its first decade, the Foundation had extended its ideals by supporting applicants from Latin America and Canada, in addition to the United States.

Shortly after setting up the Foundation, Simon Guggenheim stated that

It has been my observation…that just about the time a young man…is prepared to do valuable research, he is compelled to spend his whole time in teaching. Salaries are small; so he is compelled to do this in order to live, and often he loses the impulse for creative work in his subject, which should be preserved in order to make his teaching of the utmost value, and also for the sake of the value of the researches in carrying on of civilization.

Linus Pauling was one such beneficiary and his 1926 Guggenheim Fellowship proved to be a crucial step forward for both his career and, indeed, the arc of twentieth century chemistry. The experience likewise initiated a relationship with the Foundation, lasting several decades, that placed Pauling in a position to mold his own field and, to a lesser extent, American culture more generally.

In 1975, author John H. Davies, who was researching a biography of the Guggenheim family, asked Pauling to reflect on what his Guggenheim Fellowship had meant to his life. In response, Pauling described how he had been learning about quantum mechanics in the mid-1920s as a graduate student at the California Institute of Technology, but that the opportunity to meet both leading and emerging physicists in Europe gave him more confidence in his own ideas on the application of quantum theory to chemical problems. Much of his theoretical work that followed was based on those nineteen months in Europe, and the freedom granted by the Guggenheim Fellowship, especially when compared to the more restrictive grants offered by the Rockefeller Foundation, was just what he needed at the time.

Pauling also told Davies that in addition to Frank Aydelotte, who chaired the Foundation’s Committee of Selection from its first meeting in 1925 until 1950, Henry Allen Moe, the Foundation’s Secretary, was the most influential figure in shaping what the Foundation would ultimately become. Notably, from the start Moe made it his goal to know each and every Fellow. And indeed, when Linus and Ava Helen Pauling first arrived in New York before heading off to Europe in 1926, Moe invited them to stop by and see him, giving the young couple advice on how to navigate and best utilize their time overseas. Remarkably, during a forty-year tenure at the Foundation that saw the appointment of thousands of Fellows, Moe was largely able to achieve his goal to at least meet each selected individual. Moe and Pauling, however, developed an especially unique relationship over the years as the two became close colleagues and friends.

Maurice Huggins

After Pauling completed his tenure as a Fellow, he quickly emerged as an important asset for Moe and the Foundation. When the search for new Fellows began in the fall of 1928, Moe wrote to Pauling asking for recommendations and ensuring him that they would remain confidential. This guarantee of confidentiality also extended to the many letters of reference that Pauling supplied for scores of applicants. Over time, Pauling’s judgments became central to the Foundation’s selection process, but early on, Pauling’s opinions did not always line up with final decisions on who received funding.

In 1930 Moe requested Pauling’s opinion of Maurice Loyal Huggins, then a Stanford professor and formerly a lab-mate of Pauling’s when the two were in graduate school at Caltech. Pauling’s reply was exacting in it assessment of both strengths and weaknesses, a characteristic that would remain consistent throughout Pauling’s professional life.

According to Pauling, Huggins was one of the top six chemists then looking at crystal structures in the United States. Pauling also told Moe that ever since Huggins had moved to Stanford, he had found it difficult to obtain funding. This, in turn, had affected his research, leading him to follow unfruitful lines focusing on organic crystal structures. But these issues did not dissuade Pauling from recommending Huggins. Instead, they seemed to highlight the strength of his proposed research on surface structures, which Pauling judged to be more appropriate to his talents. Pauling’s recommendation, however, was not enough for Huggins to be awarded a Fellowship.

Pauling also used his position as a Fellow to recommend those around him. Early on, in 1931, he put forth his assistant Boris Podolsky for a Fellowship to visit Albert Einstein in Berlin and Vladimir Fock in Leningrad, in both instances to discuss ideas on combining quantum theory and relativity. As was the case with Huggins, Pauling’s belief in Podolsky’s work was not enough to convince the Committee of Selection that he was worthy of a Fellowship.

William Zachariasen

In other instances, Pauling’s negative assessments were not enough to prevent someone from receiving support. On one such occasion, in 1934, Moe wrote to Pauling asking if University of Chicago physicist William H. Zachariasen was a “first-rate scholar with a first-rate project.” Similar to Huggins, Pauling described Zachariasen as being among the top six crystal structure researchers in the United States and also the head of an important project on the structure of borates. On the same token, Pauling also pointed out that Zachariasen had already spent time with Viktor M. Goldschmidt in Gottingen and Lawrence Bragg in Manchester, and was likewise equipped with a good lab in Chicago. As such, Pauling concluded that Zachariasen, while indeed being “first-rate,” would not gain much from a return trip to Europe and recommended that he not be awarded a Fellowship.

In so doing, Pauling also took the opportunity to share his thoughts on the true value and impact of a Guggenheim Fellowship, an opinion that seems clearly to have been based on his own experiences as a Fellow.

It is my conviction that the Guggenheim Foundation can contribute more to the development of American scientists by awarding fellowships to able and promising young men (only two or three years beyond the Ph.D.) who can learn a new technique in a foreign laboratory or from a foreign professor and bring it back to America, or who can obtain training in one branch of knowledge which in combination with earlier training may lead to the development of a new field of research, rather than by awarding fellowships to mature first-rate scientists to enable them to prosecute definite research, except when the research is of great importance and requires the facilities of a foreign country for its prosecution.

Pauling’s conclusions with respect to Zachariasen and the mission of the Guggenheim Fellowships did not, in this instance, hold sway as Zachariasen was awarded the Fellowship.

But not all of Pauling’s instincts on potential fellows were at odds with those of the Foundation. By 1936, George W. Wheland and Pauling had worked together at Caltech for three years, enough time for Pauling to forecast his becoming “one of the leading university research men and teachers of the country.” By Pauling’s reckoning, Wheland was the only person in the world to be thoroughly trained in organic chemistry and to have mastered the principles of quantum mechanics. The only person close to Wheland’s caliber was Erich Hückel in Stuttgart, but he did not possess the same depth of training in organic chemistry.

Prompted by Pauling’s encouragement, Wheland submitted an application for a 1936-1937 Fellowship. In it, he outlined plans to visit Christopher K. Ingold at University College London, and Robert Robinson and Nevil Sidgwick at Oxford. In his letter of support, Pauling pointed out to Moe that Wheland was the first applicant to apply from Caltech’s Chemistry Division since Pauling had received his own award some ten years prior, perhaps implying in the process that the Division was due. This time the Committee of Selection agreed and, one year later, Caltech and Pauling enjoyed another success when nominee Lawrence O. Brockway also received funding.

As Pauling’s judgements came more and more in line with those of the Foundation, Moe and others began to take notice. That attention would earn Pauling a place on the Guggenheim Advisory Board in 1939 and, not long after, on its Committee of Selection.

The Story of “The Nature of the Chemical Bond”: Coordinating Research & Funding

[Ed. Note: This year marks the 75th anniversary of Linus Pauling’s publication of his landmark text, The Nature of the Chemical Bond.  For the next six weeks we will take a detailed look at the creation, release and impact of a book that changed the scientific world.]

Linus Pauling’s The Nature of the Chemical Bond, first published in 1939, was the product of over two decades of diligence, sacrifice, and collaboration among a broad range of actors that included Pauling’s family, research assistants, professional colleagues and a variety of institutions. Pauling’s prefatory remarks to the book – “For a long time I have been planning to write a book on the structure of molecules and crystals and the nature of the chemical bond” – give an indication of the extent to which this was a long-term objective for Pauling, despite his being only 38 years old.

Looking back at his process, Pauling’s application for a grant from the Carnegie Institute in February 1932 provides a more detailed affirmation of his ambitions. In it, Pauling relayed how his undergraduate research in crystal structures at Oregon Agricultural College between 1917 and 1922 had laid the foundation for his current work by bringing him into contact with contemporary questions in structural chemistry. As a graduate student at Caltech, Pauling began to search for answers to those questions in the newly developing field of quantum mechanics.

In pursuit of those answers, Pauling and his wife Ava Helen, with the support of a Guggenheim Fellowship, left their one-year-old son, Linus Jr., with Ava Helen’s mother in Portland and traveled to Europe in 1926 to study quantum mechanics at its source. There, Pauling deepened his understanding and immersed himself even more by beginning to apply the new physics directly to chemical bonding.

J. Holmes Sturdivant

Upon returning to Caltech in 1927, Pauling began to seek funding so he could continue what he had begun. Let down by the National Research Fund, Pauling supported his work with funding from Caltech and the National Research Council, money which allowed him to hire a full time assistant, J. Holmes Sturdivant, who focused on x-ray crystallography and continued to work with Pauling for many years. Pauling also brought aboard Boris Podolsky for nine months to assist him with the more detailed technical components of connecting quantum mechanics to chemical bonding.

In 1932 Pauling expressed a hope that, with help from the Carnegie Institute, he could expand his work by funding more assistants and purchasing equipment like an “electric calculating machine,” a “specialized ionization spectrometer,” and a microphotometer. The Carnegie Institute was not interested. Luckily for Pauling, the Rockefeller Foundation came through with a general grant of $20,000 per year over two years, to be split between the physics and chemistry departments at Caltech. This allowed Pauling to keep Sturdivant on staff while adding George Wheland, Jack Sherman, and E. Bright Wilson, Jr. to his research team.

This scramble to secure funding and bring new people into the lab came amidst the publication of Pauling’s first four “Nature of the Chemical Bond” articles for the Journal of the American Chemical Society, proof positive that Pauling’s work was bearing fruit. Once the funding was secured and Sherman and Wheland began producing results, Pauling wrote – with Sherman and Wheland as co-authors – three more “Nature of the Chemical Bond” articles the following year, published in the newly established Journal of Chemical Physics. Wheland also worked with Pauling on a monograph discussing the application of quantum mechanics to organic molecules. Wheland finished his part of the book by 1937, but Pauling never got around to his portion: his desire to write a book length treatment of chemical bonds began, more and more, to take center stage.

Warren Weaver

In order to keep the funding coming in through the lean years of the Great Depression, Pauling was compelled to follow the lead of his patrons, the Rockefeller Foundation. Warren Weaver, Director of Natural Sciences for the foundation, told Pauling in December 1933 that the organization was “operating under severe restrictions” and that funding would go to projects “concentrated upon certain fields of fundamental quantitative biology.” That Pauling’s work had “developed to the point where it promises applications to the study of chlorophyll, haemoglobin and other substances of basic biological importance” was key to his potential receipt of continued dollars.

The commitment of Caltech’s chemistry department to continue pursuing the line of research suggested by Weaver helped Pauling to secure funding for the following year. A three-year commitment came after that, providing the Caltech group with a reliable source of support into 1938. Pauling thanked Weaver in February of that year for his direction, writing,

I am of course aware of the fact that our plans for organic chemistry not only have been developed with the aid of your continued advice but also are based on your initial suggestion and encouragement; and I can forsee that I shall be indebted to you also for the opportunity of carrying out on my own scientific work in the future to as great an extent as I have been during the past six years.

Secure funding allowed Pauling to maintain a research group consisting of graduate students and post-doctoral fellows. In his preface to The Nature of the Chemical Bond, Pauling expressed his gratitude to several of these individuals, including Sherman and Sturdivant. Another, Sidney Weinbaum, earned his doctorate under Pauling and continued on afterwards, helping Pauling with quantum mechanical calculations and molecular structures.

Fred Stitt worked as research fellow with Pauling and assisted him in teaching his graduate course on the applications of quantum mechanics to chemistry – an exercise, no doubt, that helped to shape Pauling’s own thoughts on the subject, crystallizing them in preparation for the book.

Charles Coryell and Linus Pauling, 1935.

Charles Coryell and Linus Pauling, 1935.

Charles Coryell worked as a research fellow at the Caltech lab with Pauling on the topic of magnetic susceptibilities, which were central to investigating chemical bonds.  (Coryell also later helped Pauling to construct a magnet for the Caltech labs, based on one already in place at Cornell.)

Edwin H. Buchman, according to a 1985 oral history interview, was self-supporting due to royalties from his synthesis of vitamin B1. Buchman told Pauling in May 1937 that he would assist Pauling “on any problem in which an organic chemist could be useful and for which extra space could be had.”

Once assembled, Pauling’s team helped him to refine his understanding of chemical structures and bonding as the time approached when he could produce a book-length treatment on the subject.