A Theory of the Color of Dyes

Image credit: Kanwal Jahan.

Image credit: Kanwal Jahan.

Colors convey ideas and emotions in such fundamental ways that being able to capture and use them has been an important component of both cultural and scientific development. The colors of the natural world have fascinated people throughout human history and unending attempts have been made to manipulate and apply color to the items that we use on a daily basis.

Linus Pauling was not immune to humankind’s curiosity for color and as a chemist he was intrigued by dye molecules. Seventy five years ago, in 1939, he attempted to deepen the scientific understanding of how these molecules reflect color.

By the late 1930s, chemists had become comfortable with the concept of electronic resonance – the ability of electrons in a molecule to change orbitals – and were using it to describe a molecule’s capacity to absorb and emit radiation in the reflection of color. Atoms and molecules possess electromagnetic radiation due to the charge of their electrons, and as light hits an atom or a molecule its radiation determines which wavelengths of light are absorbed and which are emitted. When a molecule resonates, the movement of electrons causes a shift in the charges within the molecule which affects its radiation and the distribution of its atoms. All of these processes impact the molecule’s absorption-emission spectra.

By the time that Linus Pauling began working with dyes he had already contributed greatly to the theory of resonance. In 1928, while looking at a series of proposed forms for resonating molecules, he realized that the likelihood that these molecules would resonate directly from one form to another was very low. While many of the resonance forms that had been proposed explained the chemical behavior of molecules, Pauling felt that something was missing in the contemporary understanding of resonance. In his 1928 paper, “The shared-electron chemical bond,” he proposed that the shifts in charge observed in larger molecules required intermediate resonance forms. Pauling then described how these shifts in charge occured from one atom to the next, in the process altering the molecule’s geometry. This idea ran contrary to the notion that electrons shifted directly from one side of the molecule to its opposite.

In 1939 Pauling applied these ideas to the molecules that make up dyes. Dye molecules are often large organic compounds highly affected by resonance. This fact was known to chemists at the time, yet Pauling disagreed with accepted ideas on how these compounds resonate and reflect color. To Pauling, it seemed unlikely that molecules the size and structure of, for example, benzaurin and indigo would resonate in such direct ways as was being proposed by his colleagues.

Although the dramatic changes in charge and structure that had been proposed did account for the colors reflected by dye molecules, Pauling had developed a different understanding of how they came about. Instead of electrons resonating and causing a shift in charge directly from one side of the molecule to the other, Pauling suggested that the shift occurred from atom to atom, giving rise to intermediate forms. Pauling believed that it was necessary to take into account all possible resonance forms in order to fully understand a dye’s emission spectrum.

Some of the multiple resonance forms proposed by Pauling for Döbner's violet. 1939.

Some of the multiple resonance forms proposed by Pauling for Döbner’s violet. 1939.

Pauling’s thinking was published in a 1939 article, “A theory of the color of dyes,” which appeared in the Proceedings of the National Academy of Sciences. The article verifies the notion that color depends on the frequencies of radiation generated by the electrons in a molecule. But it also suggests that in order to understand their molecular structure and explain the colors that these molecules reflect, it is necessary to consider all possible distributions of a molecule’s charges, a combination of which would more accurately describe the observed reflection of color. Scientists now agree that understanding absorption-emission spectra is key in describing molecules because they offer valuable information about a molecule’s components and charges; Pauling’s dye work was a contribution to the development of this understanding.

At the time that Pauling’s theory of dyes paper was published, there were chemists across the country simultaneously trying to understand the color phenomenon. Dr. A. Burawoy’s 1940 article “Light Absorption, Resonance, and Isomerism” (Journal of the Society of Chemical Industry) used Pauling’s 1928 shared electron bond paper in developing his own study of dyes. Not surprisingly, Pauling and Burawoy reached similar conclusions about color.

Crellin Pauling and a friend peer out from a railroad car in an early color image from the Pauling Papers. Image digitized from a Kodachrome slide original.

Crellin Pauling and a friend peer out from a railroad car in an early color image from the Pauling Papers. Image digitized from a Kodachrome slide original.

Other chemists, including L.G.S. Brooker, would contribute to Pauling’s theory of dyes by questioning and expanding upon his work. Brooker was a chemist working for the Eastman Kodak Company in Rochester, New York. The company was naturally interested in producing higher-quality photographic film and, as such, was keen to investigate and understand the chemistry of dyes. Brooker and Pauling exchanged ideas as they studied dyes, and correspondence from December 1937 suggests that the two met in Rochester the following month to discuss their results. When Pauling’s theory of color was published in September 1939, Brooker wrote to issue a disagreement with Pauling’s treatment of carbon molecules. Specifically, Brooker believed that Pauling was overlooking the possible effects of carbon on a molecule’s behavior, though he otherwise agreed with Pauling’s conclusions on radiation and charge migration.

Observations like Brooker’s encouraged Pauling to continue his study of dyes by testing his theory on different molecules, including synthetic dyes like cyanine, which he investigated in 1940. The application of Pauling’s findings on carotenoids, one of the pigments found in tomatoes, was further expanded in a 1941 article published by Laszlo Zechmeister, Pauling and two other Caltech colleagues and titled, “Prolycopene, a naturally occurring stereoisomer of lycopene.” (Proceedings of the National Academy of Science)  Two years later, Zechmeister, Pauling and three others authored “Spectral characteristics and configuration of some stereoisomeric carotenoids including prolycopene and pro-gamma-carotene.” (Journal of the American Chemical Society)  Both publications explored the role of molecular structure in determining the emission spectra of naturally occurring pigments.

The contemporary understanding of how dye molecules reflect color has changed little since Pauling’s 1939 findings. His work, and that of many others scientists, confirms that something as simple as the color of a tomato is the result of a continuing cycle of complex interactions between atoms and their electrons.


The First Two Soviet Trips

Ava Helen and Linus Pauling with Soviet colleagues including A. I. Oparin (front right) and N. M. Sissakian (back right), 1957.

[Part 2 of 3]

Summer 1957 marked the first time that Linus and Ava Helen Pauling visited the Soviet Union. Linus had been invited by A. I. Oparin to deliver a paper at the International Symposium on the Origin of Life on the Earth. At first the Paulings were hesitant to accept due to high costs and questions about their ability to obtain travel visas. But ultimately these issues were resolved and they accepted the invitation, voicing in their correspondence with Oparin their excitement at the prospect of the symposium and the opportunity to visit a new part of the world. And so it was that, in August, they arrived in Moscow to attend the symposium at the Institute of Biochemistry where Pauling presented his paper “The Nature of the Forces of Operation in the Process of the Duplication of Molecules in Living Organisms.”

During their first stay in Russia, Ava Helen kept a private diary to record everything they did and saw – mostly museum visits, festival activities and dance performances. Included were trips to the Bolshoi Theatre to see a ballet, an opera, and an operetta. Other noteworthy excursions included the treasure house of the Kremlin, Cathedral Isaac, the Pushkin Museum and a Russian kindergarten. Of the visit to the kindergarten, Ava Helen noted that the children were presented in such an organized fashion – specifically in their music and gymnastics classes – that she had a hard time buying into what she was seeing and enjoying the visit. Something she did enjoy however, was watching the Youth Festival parade, one which featured spectacular performances and a breathtaking fireworks display.

The Paulings made time to dine with Oparin, their primary contact during their visit, as well as their colleagues the Folkensteins, at the Savoy Hotel in Moscow. The duo also went to an old monastery, since repurposed as the Institute of Chemical Physics, to visit N. N. Semenov’s laboratory. This was just one of a number of laboratory tours, including visits to the nuclear physics lab in Moscow, Oparin’s lab, the Orekhovich Lab, and the Tatyveskis Geo-Chemical Institute Lab.

Pauling in Leningrad, 1957.

Upon returning to the U.S., his visit to Russia completed, Linus Pauling invited new colleagues V. N. Orekhovitch to and Vladimir Knorre to visit him at Caltech. It was not to be however as, in December, Pauling received a letter from the U.S. State Department informing him that Pasadena, San Francisco, and Los Angeles were officially closed to anybody holding a Soviet passport. Outraged by this action, Pauling called State Department official Lawrence Mitchell, urging him to arrange for Orekhovich’s visit to Pasadena. In response, Mitchell informed Pauling that Berkeley, California was open to Soviet visitors, but that the U.S. government could not very well make an exception for Orekhovich, as this would have “little effect in applying pressure on the Russian Government.” Pauling then proceeded to write to the Secretary of State, voicing his opinion on the situation. Pauling claimed that he felt very strongly opposed to this action because, “it gives the Russian scientists who come to the United States a false impression – the impression that we are a police state, where scientists are not free to talk with other scientists, but are ruled by the Department of State.” Orekhovitch eventually made it to the U. S. but was unable to visit Pauling or Caltech.

About a month after Pauling wrote to the Secretary of State, he received a reply from Frederick T. Merrill, Director of the East-West Contacts Staff. In it Merrill reiterated Lawrence Mitchell’s original argument. According to Merrill, it was within the seventeen-point policy of the United States to increase contacts with peoples of Eastern Europe, but this policy had been rejected by the Soviet Union. As such, until negotiations could be revived on the matter of the barriers that had been raised by the USSR to contacts between the two countries, the United States had to restrict Soviet travel as a way of pressuring the USSR into negotiations.

Soviet Academy of Sciences, Certificate of Membership, 1958.

In 1958 Pauling was elected a foreign member of the USSR Academy of Sciences (Akademia Nauk USSR), the second American to receive this honor. Asked for a statement on his selection, Pauling conveyed gratitude to the Academy and commented on the great importance of improving international relations. Since his stance on matters of international relations was well known, colleagues and other figures in Russia wrote to Pauling encouraging him to continue to fight against nuclear testing in the United States.

The Paulings made their second visit to Moscow in November 1961. While there, as an elected member of Akademia Nauk, Linus Pauling gave a speech titled “World Cooperation of Scientists” at a conference hosted by the Academy in celebration of the 250th anniversary of the birth of M.V. Lomonosov. In his speech, Pauling discussed the approaches taken by Lomonosov and other Russian scientists to atomic investigations into the structure of matter. He also commented on the contributions that Soviet scientists had made toward world peace, and reflected on the need to reconsider the Soviet Union’s official decision on Pauling’s chemical theory of resonance.

Pauling expounded on the resonance controversy at a later talk given in Moscow at the Academy’s Institute for Organic Chemistry. His theory of resonance used quantum mechanics and wave functions to model a hypothetical structure of a molecular system as expressed as a sum of wave functions. And his presentation of this theory during the 1961 trip was particulalry important because, ten years earlier, the Institute of Organic Chemistry of the Academy of Sciences, U.S.S.R. had formally rejected the work as “pseudoscientific” and “hostile to the Marxist view.” [For much more on the resonance controversy, see this collection of our posts.]

In response, Pauling had written to Akademia Nauk arguing in support of his theory and asking the organization to reconsider. In 1954 the Soviet group eventually consented to a written debate of the theory between Professor N. D. Sokolov and Pauling – a debate which never actually took place. By 1961, when Pauling gave his lecture on resonance to a Soviet audience, technical facets of the theory remained controversial within the chemistry world and as such provided good fodder for conversation among scientists, irrespective of the political aspects of the debate.

While in Moscow, Pauling likewise gave a talk in which he urged the Soviet Union to end its nuclear testing programs and address its stockpiles of nuclear weapons. He also attended a panel discussion at which he once again called on the Soviet government to halt all nuclear tests.

Diary entry by Ava Helen Pauling, 1961. “6 December. Went to Lenin Library with Angella Gratcheva. It is some experience to ride with her in her car. I only worry about the pedestrians. She does seem a bit crazy.”

Ava Helen attended these events with her husband, but once again found time for adventures of her own. As before she kept a diary during the 1961 trip, most of which is devoted to her husband’s presentations. A substantial portion of the diary is, however, dedicated to documenting the “wild rides” that she experienced with her guide, one Angella Gratcheva. Apparently Gratcheva drove very erratically, and while navigating the Russian roads commonly recited poetry, sang songs and engaged in very animated conversations with Ava Helen. Her driving was so unpredictable that the police stopped them, a “misunderstanding” that the guide cleared up with more animated speech. From scientific controversy to peace activism to crazy driving, it would seem that Russia proved to be an interesting place indeed.

As with much of his international travel, Pauling’s relationship with the Soviet Union and its scientists grew stronger with each visit. The 1957 and 1961 trips set the foundation for Pauling to be viewed as a respected figure in the U.S.S.R., established precedence for future visits to the country and strengthened his position as an advocate for peace in both his home country and its rival nation.

Resonance in Benzene and Beyond

Introductory sentence and diagram from Pauling and Wheland's paper on resonance in benzene and naphthalene, June 1933.

[Part 2 of 2]

Suppose that we ask: is it necessary that a molecule such as CO have a definite valence-bond structure? The answer, which is part of the new idea, is no; instead the CO molecule may have (and does have) a structure which is neither C=O or C≡O, but is somewhere between them, or which rather has some aspects of both. It is customary now to speak of the molecules as resonating between these two structures.

-Linus Pauling, 1936.

As with the structure of carbon monoxide, the principle of resonance also explains what was once a chemical enigma – the true molecular structure of benzene.

Benzene, a double-bond conjugated six member hydrocarbon ring, can be represented by two structures that are equivalent in energy.

A simple model representing oscillation between the two primary structures is, however, insufficient as it does not explain one of the principle chemical properties of the molecule – its inability to saturate.

The application of the theory of resonance permitted the determination of a more accurate model. In the resonance model, the molecular configuration of benzene is such that all possible structures (including ones not shown above) contribute to the true structure – a combination of all structures at once, with each carbon-carbon bond energetically equivalent.  As Pauling wrote in his 1946 Encyclopedia Britannica entry on resonance

It is sometimes found…that a choice cannot be made between two or more structures which are about equally stable, and of which no one accounts in a completely satisfactory way for the properties of the substance. The concept of quantum-mechanical resonance has provided the solution to this problem: namely, the actual normal state of such a molecule does not correspond to any one of the alternative reasonable structures, but rather to a combination of them, their individual contributions being determined by their nature and stability….Just as it is customary to speak of the electrons in an atom in its normal state as moving around the nucleus in roughly the way described by the old quantum theory…so is it customary, and for some purposes useful, to speak of the resonance of a molecule in its normal state between two or more structures.

Based on this theory, the benzene molecule is now often depicted as a hexagon with a circle in the middle representing the true resonating nature of the molecule.

This structure has since been verified by multiple experimental techniques such as electron diffraction, x-ray diffraction, and molecular spectroscopy.

Having recognized resonance as an important missing link in understanding molecular bond structure, Pauling applied his theory to a large collection of empirical results.  (For one, he identified electronic resonance as the principle that allows for the formation of four equivalent bonds to be formed by carbon.)  His analysis consistently explained gaps in the classical models of bond theory and aligned with the quantitative data available.   Pauling’s theory of resonance has since contributed fundamentally to the scientific understanding of molecular shape and stability, and has permitted insight into the true nature of the chemical bond.

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.

Pauling’s Theory of Resonance: A Soviet Controversy

As to the Russian scientists and the scientific controversies, I must say that I have great difficulty in understanding what is happening. The most likely explanation seems to be that some of the Russian scientists are taking advantage of the political situation to advance themselves at the expense of their colleagues. Others are then drawn into the controversy, and required by practical considerations to align themselves with those who say that they are supporting the correct Marxist position. I have read the Russian articles carefully, and I must say that I cannot understand the arguments.
-Linus Pauling to Frank Aydelotte. September 25, 1951.

In the 1947 Russian translation of Pauling’s Nature of the Chemical Bond (Priroda khimicheskoi sviazi) Pauling wrote on the fly leaf in black pen “Moscow, 8 August 1957. Today…ten years after it was published, I have for the first time seen the Russian edition of The Nature of the Chemical Bond – this copy, given to me by Prof. Voevodsy. Linus Pauling.”

Just above this, is Voevodsky’s lightly penciled inscription, reading “To the author, In remembrance of your stay in the URSS, 8 VIII 57”.

The book that Pauling received from Voevodsky – which is held in the Pauling archive today – is a careful translation done by two Soviet scientists, Ia. K. Syrkin, a chemist, and M. E. Diatkina, a mathematician. Syrkin and Diatkina were two of the Soviet Union’s most prominent sympathizers and popularizers of Pauling’s resonance theory of chemical bonds. Their own textbook on the new quantum methods in chemistry, Structure of Molecules and the Chemical Bond, had been published in Moscow in 1946 and, by 1950, already served as a textbook at Moscow State University.

Developed by Pauling in the early 1930s, the theory of resonance was, twenty years later, an accepted component of the scientific lexicon.  As it turned out, however, the theory did not agree with Soviet dogma, at least as conceptualized in the early 1950s.  Biographer Thomas Hager writes

The Lysenko-era Russian researchers, intent on boosting the reputation of Russian achievements in structural chemistry, had for two years been tearing away at Pauling’s ‘reactionary, bourgeois’ chemical ideas, especially his use of idealized resonance structures with no real independent existence.  Resonance theory, it was decided, was antimaterialistic and hence anti-Soviet.  The chemists’ division of the Soviet Academy of Sciences in the summer of 1951 formally resolved that Pauling’s approach was ‘pseudo-scientific’ and ‘idealistic’ and should be rejected.  Pravda trumpeted the decision, which was echoed in Soviet scientific publications with appropriate denunciations of Pauling’s approach to chemistry as ‘contrived, a made-up convenience, an economy of thought that bore no relation to reality.’

Simon Shnol [Russian-language link], a student of Syrkin and Diatkina’s, remembers having attended their lectures on resonance theory in 1950 before its fall from grace. Syrkin’s lectures were full of “brilliant digressions” and “literary analogies,” Shnol recalls, making the complicated topic of quantum mechanics in chemistry seem “accessible and interesting.” Diatkina, somewhat more severe than Syrkin, handled the mathematical aspects of the topic.

Shnol attended the 1951 conference at which the official rejection of resonance theory was formulated, believing that it would merely consist of a conversation about the new theories of the structure of chemical bonds. The conference quickly turned into an auto de fe, and Syrkin and Diatkina were severely criticized. While “the majority felt sympathy for Syrkin and did not want to destroy him,” they nonetheless agreed that his propagation of resonance theory in the Soviet Union had been false and dangerous.

Syrkin and Diatkina, along with several others, issued official and formal recantations of their views, but their careers were effectively stifled. Both Syrkin and Diatkina were made to leave Moscow State University and Syrkin lost his membership in the Academy (though he retained his leadership of the Institute of Fine Chemical Technology until his death in 1974).

Pauling, of course, was not uninformed of the controversy taking place in the Soviet Union. His collection of press clippings is full of mentions and analyses of the resonance theory debate from its inception. I. Moyer Hunsberger acknowledged in his 1954 review of the Soviet resonance theory controversy, “I am indebted to Dr. Linus Pauling for his valuable criticism of this paper. In particular, the contents of footnote 13 were suggested by Dr. Pauling.” Years later historian Loren Graham relied on personal communications with Pauling to construct parts of his article, “A Soviet Marxist View of Structural Chemistry: The Theory of Resonance Controversy,” (1974).

Though he seems to have been unimpressed with Syrkin and Diatkina’s own 1946 text, Pauling was nonetheless concerned by the ideological path that Soviet chemistry was taking. In a characteristically humorous way he alluded to these concerns in a lecture he that gave while visiting the Soviet Union for the first time in 1957.  In his memoir, Simon Shnol recounts this lecture, which was delivered by Pauling at the Institute of Organic Chemistry in the Soviet Academy of Sciences.

It was perfect…with artful gestures, which included his eyebrows, eyes, hands (he reminded me of Louis de Funes), Pauling lectured on the successes of the theory of the structure of chemical bonds….In the lecture room that day the audience separated spatially into three tiers. The most important, high level academic bureaucrats sat in the front rows, the professors and doctors of science sat in the middle rows, and the graduate students filled the back rows. In the course of his lecture Pauling encouraged the students not to repeat the mistakes of those in the front rows (literally: “to not pay attention to them”), a comment that was translated as an encouragement to the students to follow the example of their teachers.

Shnol recalls that the discrepancy was immediately noted by many in the audience. Someone from the back rows then shouted an accurate translation of Pauling’s admonition and the room was filled with laughter and noise.

The Ava Helen and Linus Pauling Papers contain a large collection of correspondence and publications with marginalia in Pauling’s hand that relate specifically to the resonance theory controversy. Pauling’s collections of photographs, press clippings, reprints and official correspondence, combined with recently-added interpretive material, provide a rich archive of this important and in some ways still unexplored scientific controversy.

Learn more about Pauling’s theory of resonance from the website Linus Pauling and the Nature of the Chemical Bond, available at the Linus Pauling Online portal.

The Guggenheim Trip, Part III: Unexpected Colleagues

Walter Heitler, Fritz London, and Ava Helen Pauling in Europe. 1926.

Walter Heitler, Fritz London, and Ava Helen Pauling in Europe. 1926.

The paper of Heitler and London on H2 for the first time seemed to provide a basic understanding, which could be extended to other molecules. Linus Pauling at the California Institute of Technology in Pasadena soon used the valence bond method. . . . As a master salesman and showman, Linus persuaded chemists all over the world to think of typical molecular structures in terms of the valence bond method.” – Robert Mulliken. Life of a Scientist, pp. 60-61. 1989.

After Linus Pauling’s publication of “The Theoretical Prediction of the Physical Properties of Many-Electron Atoms and Ions,” he was ready for an even greater challenge – the problem of the chemical bond was a tantalizing enigma for Pauling, and he wanted more time in Europe to work on it. In the winter of 1926, he applied for an extension of his Guggenheim fellowship and with the help of a particularly complementary cover letter from Arnold Sommerfeld, Pauling was granted six more months of support.

Boosted by this news, he quickly began planning visits to Copenhagen and Zurich, both cities boasting of some of Europe’s finest research facilities. His first stop was Copenhagen, where he hoped to visit Niels Bohr’s institute and discuss ongoing research with the renowned scientist. Unfortunately, he had arrived uninvited and found it almost impossible to obtain a meeting with the physicist. Bohr, with the help of Werner Heisenberg and Erwin Schrödinger, was deeply engaged in research on the fundamentals of quantum mechanics, and was specifically attempting to root out the physical realities of the electron, in the process developing a theory which would eventually be termed the “Copenhagen Interpretation.”

Pauling did, however, did make one valuable discovery in Denmark — that of a young Dutch physicist named Samuel Goudsmit. The two men quickly became friends and began discussing the potential translation of Goudsmit’s doctoral thesis from German to English. Their work did eventually get them noticed by Bohr, who finally granted Pauling and Goudsmit an audience. Unfortunately for the pair, Bohr was neither engaging nor encouraging. Nevertheless, the two continued to work together, their cooperation eventually culminating in a 1930 text, The Structure of Line Spectra, the first book-form publication for either scientist.

In 1926 though, frustrated by his unproductive time in Copenhagen, Pauling departed, stopping briefly at Max Born’s institute in Göttingen before traveling to Zurich where other advances in quantum mechanics promised an interesting stay. Unfortunately, the man Pauling was most interested in, Erwin Schrödinger, proved to be just as unavailable as Bohr. The quantum mechanics revolution was consuming the time and thoughts of Europe’s leading physicists and Pauling, a small-fry American researcher, simply wasn’t important enough to attract the interest of men like Bohr and Schrödinger.

Fritz London

Fritz London

As a result, Pauling chose to converse and work with men of his own status in the scientific community. Fritz London and Walter Heitler, acquaintances of the Paulings, had spent the past several months working on the application of wave mechanics to the study of electron-pair bonding.

Heitler and London’s work was an outgrowth of their interest in the applications and derivations of Heisenberg’s theory of resonance, which suggested that electrons are exchanged between atoms as a result of electronic attraction. Heitler and London determined that this process, under certain conditions, could result in the creation of electron bonds by cancelling out electrostatic repulsion via the energy from electron transfer. Their work on hydrogen bonds likewise agreed with existing theories, including Wolfgang Pauli’s exclusion principle and G.N. Lewis’ shared electron bond. The Heitler-London model was well on its way to contributing to a new truth about the physics of the atom

Walter Heitler

Walter Heitler

Pauling used his time in Zurich to experiment with the Heitler-London work. While he didn’t produce a paper during his stay, the new model made a great impression on him and he returned to Caltech with a renewed sense of purpose. He was preparing to tackle the problem of atomic structure, in all its manifestations, and make history as one of the greatest minds of the twentieth century.

For more information, view our post “Linus Pauling and the Birth of Quantum Mechanics” or visit the website “Linus Pauling and the Nature of the Chemical Bond: A Documentary History.”