A Resonating-Valence-Bond Theory of Metals and Intermetallic Compounds, 1949

Linus Pauling, 1949

As we have written elsewhere, Linus Pauling developed and championed the theory of resonance early on in his career. But for almost 20 years, the ways that metals might conform to the theory remained elusive. Even though he had a hunch that they too adhered to the tenets of resonance, he was not able to prove it definitively at the outset. Not until 1949, with the publication of “A Resonating-Valence-Bond Theory of Metals and Intermetallic Compounds” was he able to demonstrate that metals do resonate. 

Pauling’s interest in metals dated at least as far back as his undergraduate years at Oregon Agriculture College and continued to flourish during his graduate training at the California Institute of Technology. Later in his career, when asked to reflect on his contributions to the field of chemistry, he often spoke of his early work with metals as being important. This was especially so with his work on metals and resonance.

“A Resonating-Valence-Bond Theory of Metals and Intermetallic Compounds,” which was published in the Proceedings of the Royal Society London, serves as an addendum of sorts to Pauling’s previous papers on the nature of the chemical bond. Even though Pauling’s theory of resonance had been well-received for several years, confusion still existed amongst chemists (including Pauling) about how the theory might apply to metals; particularly the iron-group transition metals. Pauling believed that, like other elements, resonance must be used to explain the way that these metals bond, but the specifics proved difficult to pin down.

Pauling had always aligned himself with the notion that the properties of an element were connected to the configuration of its valence electrons. For example, Pauling knew that the arrangement of valence electrons gave Carbon its stable tetrahedral properties and salts their ionic properties. Metals, however, were harder to define due to the fact that they “showed great ranges of values of their properties, such as melting and boiling point, hardness and strength, and magnetic properties.”

Pauling initially focused on magnetism, and from this work, it was determined that metals had a high ligancy of either 8 or 12. Ligancy, or the number of compounds that each metal could bond to, (often referred to as Coordination Number) is similar but still distinct from valency, and for the metals that Pauling was studying, a ligancy of 8 or 12 was higher than their valence. This discrepancy seemed to indicate that resonance could not be applied to explain how metals bonded. But Pauling still believed that resonance was the answer, and set about trying to confirm this belief.

In his quest to understand if or how metals resonated, Pauling first needed to clarify whether or not previous thinking about metals was correct. Prior to the publication of his 1949 paper, it was believed that the d orbitals did not participate in bonding with the iron-group transition metals, such as Manganese, Iron, Cobalt, Nickel, and Copper. Using these assumptions, Pauling modeled the predicted properties of such metals and found that they would have low melting points and weak bonds. The lab work indicated otherwise however, leading Pauling to conclude that the current understanding was incorrect, and that something else had to be going on with the bonds in order to account for their physical properties.

For several years, Pauling had been unable to devise a competing theory that would explain the unique properties of metals, but he felt certain that the answer would be revealed through the application of quantum mechanics. By specifically using the wave theory of quantum mechanics, Pauling calculated that metals used 8.28 orbitals instead of the expected 9. While he was sure that the math was correct, he struggled to understand what was accounting for the missing 0.72 orbitals, and for almost nine years he worked to reconcile the discrepancy. Ultimately though, he realized that 0.72 orbitals was, in fact, the answer he was looking for; they were what gave metals their unique properties.

In its essence, Pauling breakthrough was that the extra 0.72 orbitals was not a mathematical anomaly, but instead an extra orbital, one that he called a “metallic orbital.” The metallic orbital was, according to Pauling, “required” to give metals their “characteristic properties, especially that of electronic conductivity of electricity.” In Pauling’s view, substances lacking the metallic orbital that still maintained some metallic properties, such as electric conductivity, should be classified as metalloids.

Pauling’s conceptualization of the metallic orbital allowed him to subsequently reframe his understanding of resonance. When Pauling developed the theory, it was more specifically known as synchronized resonance; a state in which all bonds resonate in the same way and valence is undisturbed by the process. Pauling realized that, in order for metals to resonate, a different kind of resonance – unsynchronized resonance – would be needed. In unsynchronized resonance, instead of all bonds resonating simultaneously, a single bond could resonate on its own. In the case of metals this happened in the metallic orbital, and it was found that unsynchronized resonating metals conferred unusual stability and high ligancy.

Once Pauling made these connections, he was ready to publish his paper, which, first and foremost, sought to prove the existence of a metallic orbital. To do this, Pauling had to show that the existing understanding of valency was incorrect, and then to demonstrate that resonance – specifically unsynchronized resonance – could account for both the predicted and the observed properties of metals.

Using Lithium as his model element, Pauling explained that the current understanding posited a valence structure that consisted of 2s orbitals, and that this was the commonplace belief because, “the [proposed] molecular orbitals correspond to electron energies.” By using the wave function however, Pauling found that the assumed orbital structure did not correspond to observed electron energies. Specifically, he calculated that if Lithium had 2s orbitals, the predicted heat of formation would be much different than was the observed heat of formation; a calculated difference of 32.4 kcal/g. In short, the current understanding was incorrect.

Pauling’s next step was to use resonance to explain Lithium’s metallic properties, and to devise an arrangement of valence electrons that would match predicted energies with observed energies. Using the idea of unsynchronized resonance, Pauling suggested that if, instead of 2s orbitals, Lithium’s valence electrons were actually in resonance, this new arrangement could be “responsible for the difference in energy.” In other words, without resonance the electrons existed in a fixed energy state. But if the electrons were in resonance, their energy state would instead be a hybrid of all possible energy states. This circumstance, Pauling postulated, would confer lower energies to the molecule and bring predicted and observed energies into alignment. In the case of Lithium specifically, Pauling found that the valence electrons were in 2s orbitals, but also 2p orbitals. Pauling worked through different metals throughout the remainder of the paper to further support his thinking.

The theoretical work that Pauling put into this paper completed the framework for resonance. And interestingly, even though the ideas that he presented were accepted and widely used for decades, some of the math in the paper was not fully validated at the time. In fact, it wasn’t until 1984, when Pauling revisited the 0.72 orbitals, that the math was completed. By then Pauling had acknowledged that the calculations he used to get to 0.72 orbitals had been crude, and based on in part on educated guesses. But by the 1980s, many of the unknowns were known, which allowed Pauling to revisit the math with more precision and compare it to his work from the late 1940s. Using clean, contemporary data, Pauling confirmed that the new calculation was “in exact agreement with the observed value of 0.72.”

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

[Part 7 of 7]

“The Nature of the Chemical Bond. VII. The Calculation of Resonance Energy in
Conjugated Systems.” The Journal of Chemical Physics, October 1933

The final paper in Linus Pauling’s earthshaking series on the nature of the chemical bond was the shortest of the seven and made less of a splash than had most of its predecessors. This lesser impact was anticipated and was due primarily to the guiding purpose of the paper: to apply previously developed postulates to compounds that had not been addressed by Pauling in his prior writings. As with the sixth paper in the series, the final publication was co-authored by Caltech colleague Jack Sherman.

In paper seven, Pauling demonstrated how to calculate resonance energy in conjugated systems. A conjugated system is one in which there exists a plane – or alignment – of three or more connecting electrons located in the p orbital. While it was commonly understood by the era’s organic chemists that conjugated systems supplied a compound with more stability than would ordinarily be expected, Pauling’s paper offered the calculations needed to codify this knowledge.

The paper also put forth a collection of rules to help researchers better understand the properties of conjugated systems. For example, Pauling found that “a phenyl group is 20 or 30 percent less effective in conjugation than a double bond, and a naphthyl group is less effective than a phenyl group.” To arrive at these conclusions, Pauling used the equations that he had developed in his previous two papers, applying them this time around to conjugated systems.

Jack Sherman and Linus Pauling, 1935.

Pauling’s seven papers on the nature of the chemical bond came to print over the course of thirty months, from article one in April 1931 to article seven in October 1933. The first three papers laid the groundwork for what was to come by defining chemical bonds in quantum mechanical terms. The fourth paper, published in September 1932, appeared at the midpoint of Pauling’s publishing chronology and also served as a kind of transition paper, connecting the concepts introduced in the first three publications to those in the three more that were forthcoming. (Paper four also contained Pauling’s vital electronegativity scale.) The last three articles were devoted to the concept of resonance and its application to a fuller understanding of the chemical bond.

Taken as a whole, this body of work proved hugely important to the future direction of chemistry. By reconciling and applying the principles of quantum mechanics to the world of chemistry, the articles showed that what had once been mostly a tool for physicists could indeed have great applicability to chemical research. In the process, Pauling and his collaborators also rendered quantum mechanics far more accessible to their colleagues across the field of chemistry. The end result was, to quote Pauling himself, “a way of thinking that might not have been introduced by anyone else, at least not for quite a while.”

This is our forty-eighth and final post for 2020. We’ll look forward to seeing you again in early January!

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’s Third Paper on the Nature of the Chemical Bond

[Part 3 of 7]

“The Nature of the Chemical Bond. III. The Transition from One Extreme Bond Type to Another.” Journal of the American Chemical Society, March 1932.

In his third paper exploring the nature of the chemical bond, Linus Pauling dug into the unsolved question of how molecules transition from one kind of bond type to another. While it had been determined that molecules do switch from one kind of bond to another – from an ionic bond to an electron-pair bond, for example – the specifics of how that transition happens remained elusive.

Prior to the third paper, two prevailing ideas were being debated by chemists. One concept, as Pauling wrote, was that “all intermediate bond types between the pure iconic bond and the pure electron-pair bond” exist in some kind of infinite transitionary state. A contrary viewpoint put forth instead that molecules “transition from one extreme bond type to another” in an abrupt manner. Pauling suggested that the answer lie somewhere in between.

In order to determine how molecules transition, Pauling first needed to establish the bond structures of given molecules in their initial states. He did so by defining the bonding characteristics of molecules, a task that takes up the majority of the paper. But amidst this discussion, Pauling arrived at several key conclusions.

To begin, Pauling described many cases where a relationship existed between atomic arrangement – as determined by x-ray crystallographic analysis – and bond energies. When, for instance, a strongly electropositive and strongly electronegative molecule bonded, it was reasonable to assume that the bond was ionic. This presumed, Pauling then used electron energy curves to show that an example group, the alkali halide molecules, were strongly ionic, and that they might generally be thought to form ionic bonds.

As Pauling pointed out however, these presumptions were faulty. In fact, studies of the bonding in hydrochloric acid (HCl) and hydrobromic acid (HBr) indicated that both molecules were essentially covalent in make-up, whereas hydrofluoric acid (HF) was ionic. So even though it might reasonably have been assumed that the initial states for HCl, HBr and HF would be similar in their bonding, the experimental data indicated otherwise. These findings led Pauling toward the conclusion that there is no single universal answer to the question of how molecules transition, because there is no steadfast rule determining the types of bonds that hold molecules together before they transition.  

Having arrived at the conclusion that one could not lean upon a guaranteed universal bond type, Pauling then turned to his burgeoning theory of resonance to develop more precise thinking about transition mechanisms. Pauling specifically argued that when bonds transition from one type to another, rather than shifting either abruptly or in a continuous state – as the two competing models then prevailing put forth – they instead shift to an intermediate resonant state before switching to a new bond type.

Pauling was in essence suggesting that, in between classically-defined “completed” bond states, there also existed an intermediate bonding state that could best be understood through the theory of resonance. Moreover, Pauling argued that idealized bonds, such as pure covalent bonds or pure ionic bonds, did not technically exist. Rather, bonds might more accurately be described as constantly transitioning through resonant states, some of which more closely approximated a classic bond type.

Pauling understood that the concept he was putting forth was quite theoretical and that, in practical terms, it was hard to work with molecules if they existed in a constant state of transition. As such, Pauling allowed that, for purposes of discussion, it was acceptable to think of molecules as residing in discrete bonding states. He likewise acknowledged the convenience of using more traditional names (ionic, covalent, etc.) when referring to bonds, even if they never fully existed.

Pauling then concluded that, even though bonds were constantly transitioning, for certain bond types – such as “when the normal states for the two extremes have the same number of unpaired electrons” – it could be assumed that they had transitioned in a continuous state. But, of course, continuous state transition was definitely not always the case and could not be universally applied.

In conducting the work that led to his third paper, Pauling had sought to define a universal rule that would govern the transition between bond types. By the time that he delivered his manuscript though, he had recognized that not only was a universal law unattainable, but that what he did find had its limitations. In particular, Pauling suggested of his approach that “It is not possible at the present time to carry out similar calculations for more complicated molecules,” though “certain less specific conclusions can, however, be drawn.”

Regardless, Pauling’s third paper broke new ground on a topic of keen importance to structural chemists. By applying the theory of resonance, Pauling helped chemists to understand that there was a spectrum of polarity, and that bonds were not always strictly of one kind or the other. Importantly, in this same paper Pauling did not fall prey to dogmatism, and allowed that bonds residing near the ends of one spectrum or another might fairly be said to represent so-called “classic” bond types.

The Soviet Resonance Controversy: Outside Views

[Part 7 of 7]

Even though the resonance controversy was largely one contested between Soviet scientists and Linus Pauling, many outside members of the scientific public, as well as the media, weighed in on the dispute. By and large, the reaction was quite supportive of Pauling, with most commentators coming to his defense.

One scientist who definitely fit this bill was George W. Wheland, a colleague of Pauling’s who worked with him to develop resonance theory. In a letter to the editor of Chemical and Engineering News dated August 4, 1952, Wheland denounced the Soviet attack on resonance as being driven by a “patriotic, political, and ideological invective, which from the Western viewpoint, has no scientific content.”

Moyer Hunsberger, a chemist from the University of Chicago and Fordham University, felt similarly, and in 1952 he delivered a series of lectures meant to thwart “the Russian chauvinistic ideological approach” to scientific inquiry. In them, Hunsberger labeled the Soviet position and its steadfast support of A.M. Butlerov an “extremely obvious exaggeration of [his] contributions to organic chemistry.” Likewise, “the intensity and crudeness” of the Soviet condemnation of Pauling’s ideas “appear[ed] to be without parallel in the annals of chemistry.”

Other scientists added their voices in concert. Notably, Harvard University president James Bryant Conant argued that,

If the Russians continue to attempt to force science to follow along a path determined by politics, Russia is sure to grow weaker. If Russians are not allowed to use the resonance theory or are deprived of scientific freedom in any other direction, Russian science will fall behind western science and Russian technology will suffer. I think that Syrkin and Diatkina and other Russian chemists who have been criticized for using the theory are among the most able in Russia today. Their book, The Chemical Bond and the Structure of Molecules is an excellent work. It is based almost entirely on resonance theory – and I think there is no substitution for that.

But scientists weren’t the only ones who came to Pauling’s defense. Many media outlets also published articles about the controversy and, as with the academics, they were strongly in support of Pauling and his resonance theory.

Perhaps the first mention of the controversy in the stateside media appeared in a July 15, 1951 New York Times article titled, “Soviets Dispute a Chemical Theory.” In the article, the author impugns the Soviets for not acknowledging the centrality of resonance theory and even asserts that the Soviet atomic bomb projects were necessarily informed by the work. As such, it was clearly hypocritical for the theory to be described as “bourgeois mysticism” when it had been so important to advancing goals fundamental to the national interest.

Shortly thereafter, on September 1, 1951, the California Institute of Technology News Bureau released a bulletin about the controversy that sought to summarize the Soviet attack and defend the importance of Pauling’s ideas. “The theory is now an accepted part of chemistry,” the News Bureau wrote, “at least in the Western world. It is taught in general and organic chemistry textbooks and has contributed to the simplification of the science.”

These early analyses in hand, other newspapers began to report on the controversy, with some even drawing comparisons between the Soviets’ stance and actions taken by the U.S. government a year prior. In particular, a Seattle Times piece from September 2, 1951 compared the resonance controversy to accusations made by the House Un-American Activities Commission a year earlier, wherein Pauling was branded a communist. By recognizing and naming the similarities between the two controversies, the author of the Seattle Times piece put forth both an exoneration of Pauling’s political activities in the U.S. as well as the perception of his science in the USSR.

That same day, the Washington Star published its own article containing a very similar sentiment, and the steady flow of support continued from there. As the year moved forward, the Los Angeles Times, Newsweek, Time, and the Pasadena Independent were among those publishing popular articles that upheld resonance theory and denounced the Soviet position.

In 1952, the New York Times published a second article investigating the topic. The piece sought to explain resonance theory in basic terms that “every schoolboy” could understand while also responding to lingering objections from Soviet scientists.

According to the article, scientists in the U.S.S.R. had recently reaffirmed their denouncement of resonance theory, calling its ideas “mere illusions, senseless structures.” These critics continued that

Pauling and his like are said to cherish perverted concepts which are typical examples of bourgeois thinking. The man to follow is A.M. Butlerov, who has a more materialistic concept of chemical structures.

This established, the Times article then put forth a clarification. “It may be that resonance structures are illusory,” the author wrote, but “That is not the point. A theory is not a statement of absolute truth. It is an invention, a tool. […] So far, the theory of resonance explains what cannot be explained by older theories of valence.” The piece concludes of resonance theory, “Benighted Western chemists will continue to apply it.”

But not all outside observers were entirely behind Pauling, and one figure in particular deserves mention. In 1977, two years after his passing, a biographical memoir was written of Robert Robinson, the former president of the British Royal Society. In this piece, it was revealed that Robinson did not at all agree with Pauling’s ideas on resonance.

Robinson and Pauling had known one another for over two decades and were clearly friends. When Robinson received the Nobel Prize in chemistry in 1947, Pauling wrote to convey his “heartiest congratulation,” and at nearly the same time Robinson helped Pauling to secure his Eastman fellowship at Oxford University. When Robinson’s wife died in 1954, Pauling reached out to express his “deep affection.” Some years later, Robinson’s daughter gave Pauling a portrait of her father from 1955, which Pauling later put on his desk as a reminder of the times they had shared together “sitting on stools at the little eating house after the Royal Society Meeting” and their “boat trip down the St. John’s River.”

But despite this close personal connection, Robinson differed on the issue of resonance theory. In the biographical memoir, Robinson was quoted as having referred to Pauling’s resonance work as “very misleading” and an “unfortunate contribution” to science. Pauling was upset by this revelation, so much so that he wrote a response in which he argued that Robinson’s beliefs were “based entirely on misunderstanding or incompleteness of knowledge of the nature and early history of the theory of resonance.”

The late Robinson’s critique stood as an outlier though and, upon final reflection, it appeared that Pauling’s experience with the Soviets did not leave a lasting mark. In a Royal Society paper about resonance theory that he wrote in 1977, Pauling addressed the controversy just once, and in it he did not even specifically mention the Soviets. Rather, Pauling’s text indicated that resonance theory had been “rather strongly attacked…because of the failure of critics to understand it.” And with that, he seems to have summarized his feelings about the Soviet resonance controversy in as succinct a manner as possible.

The Soviet Resonance Controversy: Dying Embers

“Glory and Pride of Russian Science,” Pravda, November 22, 1961. Note Pauling’s annotation at far right.

[Part 6 of 7]

After the death of Joseph Stalin in 1953, scientists in the Soviet Union began to decrease the intensity of their attacks on Linus Pauling and his theory of resonance. This process played out for at least another ten years, during which time the combatants’ points of emphasis gradually shifted.

Instead of arguing against the theory because it opposed Soviet ideology, Pauling’s critics now focused on his failure to acknowledge the work of Russian scientists whom they argued were influential to his breakthrough. This difference marked a monumental deviation from the scientists’ earlier platform for opposition. Before, the Soviets put forth a blanket rejection of resonance as an idea, and thus abandoned its use in developing their scientific work. However, post-Stalin, the Soviets tacitly began to recognize the utility of resonance while still arguing for credit that they felt was owed to luminaries of their past.

Pauling also began to soften his rhetoric in defending his theory, perhaps in part because of the complex nature of his relationship with the Soviet Union. The years of the resonance controversy coincided with stateside accusations that Pauling was a communist and a Soviet sympathizer. During this period, Pauling had also been very public in urging the Soviets to slow down the pace of their atomic weapons development and testing. In the midst of these heady issues, the battle to defend resonance was perhaps just not quite so pressing, and it was in this context that the final phase of the resonance theory controversy took place.

Notes used by Pauling in his 1961 Moscow lecture.

The last chapter of the dispute began when Pauling gave a speech on resonance in Moscow, a talk that would have been unimaginable during Stalin’s lifetime. Delivered on December 3, 1961, Pauling made his remarks to the chemistry division of the Soviet Academy of Sciences.

At his hosts’ urging, Pauling began by praising a Russian chemist, Mikhail Vasilyevich Lomonosov (1711-1765), for first theorizing that the properties of chemicals are a result of “their structure as aggregates of atoms.” Pauling’s offering of acclaim was not without calculation however, in that he also argued that that Lomonsov was a free thinker, and that he had used his “imaginative effort” and “high originality” to develop his ideas.

Perhaps most importantly, Pauling stressed that Lomonosov’s ideas were based on conceptual findings, not ideological doctrine. In this, Pauling was clearly trying to emphasize the ways in which Russian science had differed from the more contemporary Soviet approach, with old masters like Lomonosov developing their thinking on the basis of experimental data. By using Lomonosov as an anchor for this argument, Pauling made clear throughout his speech his continuing objection to the Soviet ideological attack on his theory.

Question submitted to Pauling by an audience member following his 1961 Moscow lecture.

The controversy had been brewing for well over a decade by the time Pauling gave his speech, so it would stand to reason that many in attendance still doubted the validity of resonance theory. And indeed, handwritten questions offered to Pauling at the conclusion of his lecture ranged from outright denial of resonance theory to asking whether or not molecular orbital theory or other explanations could be used instead to understand chemical structures and their properties.

Generally speaking though, these audience-generated questions were not ideologically based, perhaps because Pauling had deliberately chosen at the outset to “ignore the criticism [of his theory] on ideological grounds” and instead devoted most of his speech to systematically arguing in support of the science. The case that he made was clear and straightforward, but also imbued with undertones of sadness. Specifically, Pauling mourned the loss of scientific contributions that could have been made had the Soviet establishment bought in to resonance from the beginning. This sentiment comes across most acutely in the talk when Pauling laments that there is not a single “textbook of chemistry published by any Soviet scientist during the past ten years in which the theory of resonance is presented in a reasonably satisfactory way,” an omission that would guarantee that future Soviet students would be “seriously hampered.”

Pauling’s Moscow lecture signaled a clear shift in the arc of the controversy. Though still critical of Soviet science and its lack of fundamental integration of resonance theory, Pauling was at least willing to visit the Soviet Union and concede a small piece of intellectual ground. In the wake of these actions, the controversy moved into its final, least aggressive form.

Publications authored in the months following Pauling’s visit increasingly came to show that the Soviets no longer rejected resonance, but did want to be included in the story of its discovery. One notable example was a May 1962 article by G.M. Bykov titled “The Origin of the Theory of Chemical Structure.” The piece was written in English, a clear indication of its intended audience, and argued that all chemical structure theory should be based on the work of Russian scientists. It is “[t]rue,” Bykov writes, that Russian scientists such as A.M. Butlerov “did not develop and did not always support” other ideas about chemical structures. But for Bykov and his colleagues, Butlerov was “the founder of the theory of chemical structures” and “the establishment of a correct conception of chemical atoms” would necessarily be based on his ideas.

Excerpt from “In the Memory of a Theory.”

Responses of this sort allowed the Soviet academy to protect national pride while simultaneously updating their practices to align with the rest of the world. The shift in rhetoric also helped to end the extreme animosity that had been directed towards Pauling and, by the 1970s, the Soviets had essentially retracted any ideological objection to resonance theory. In fact, in 1972 the popular Soviet science magazine, Znanie Sila (“Knowledge is Power”) published a retrospective titled “In the memory of a theory” that used interviews with M.E. Diatkina to recount the resonance story and its importance in the world of chemical structures.

By the 1970s, Pauling and the Soviets had effectively ended their feud with both sides having offered a bit of a compromise. Even though Pauling never fully acknowledged Russian science as directly influencing his theory, he did concede for the records that Russians had developed some early ideas regarding the structure of chemicals. For their part, the Soviets lessened the intensity and direction of their objections and gradually adopted resonance theory into their modeling.

Despite this thawing of relations, other scientists and the public at large continued to weigh in on the controversy, as they had done since its genesis. The story of these outside perspectives, and Pauling’s responses to them, will be the subject of our final post in this series.

The Soviet Resonance Controversy: Pauling Fights Back

[Part 5 of 7]

Beginning in the late 1940s, Linus Pauling’s theory of resonance came under attack in the Soviet Union, first by scientists and then by the Soviet state itself. The primary points of contention for Pauling’s Soviet critics hinged on the seeming abstractness of his theory as well as his failure to acknowledge Russian and Soviet scientists deemed important to the development of his ideas. The more they considered the theory, the more their contempt grew, to the point where Soviet chemists were eventually called upon to lead an “army of many thousands strong” against Pauling. These sentiments did not reach Pauling until 1951, a full two years after the debate within the Soviet Union began. Though surprised by the tone of the Soviets complaints, Pauling did not stand down and instead launched a counter-offensive of his own in support of his scientific work.

Pauling became aware of the brewing controversy in August 1951, when a translated article from the Soviet scientific literature was sent to his office. In response, Pauling wrote a charged letter to the Soviet Academy of Sciences condemning the piece and upholding the importance of resonance theory as key to growth in the field of structural chemistry. In the letter, Pauling made clear his belief that the critics’ rhetoric was falsely placed and, indeed, that “the attempted suppression of a part of science is based upon a faulty conception of the nature of science.”

Pauling took particular objection to the ideological arguments against his theory, finding them baseless since they did not intersect at all with the scientific principles that informed his theory. He also warned of the dangers of refusing to incorporate resonance into an entire nation’s scientific vernacular, judging that “Any chemist in the modern world who attempts to carry on his work without making use of the theory of resonance […] is seriously handicapped.”

At the conclusion of the letter, Pauling took pains to praise the two Soviet scientists, Ia. K. Syrkin and M.E. Diatkina, who had translated his work, had spread word of his theory among their colleagues, and who had ultimately been forced to recant their support. In Pauling’s view, the duo were “among the most able chemists in the USSR.”

Pauling’s reply was met with scorn in the Soviet Union, and as a direct result of the letter, the Soviet Academy organized a special meeting to specifically discuss and condemn resonance theory. Held at the end of 1951, the meeting was attended by over 400 people and featured public denunciations of Pauling’s work by no fewer than forty-four prominent Soviet scientists.

One especially notable speaker was Ia. K. Syrkin, the erstwhile translator of Pauling’s Nature of the Chemical Bond. In his presentation, Syrkin introduced his remarks by suggesting that Pauling’s

substitution of the real molecule by a set of resonance structures for the sake of convenience led to an arbitrary and speculative element, and brought about what looks like an explanation, instead of an actual critical analysis and discussion of the mechanism of chemical reactions.

While it is unclear if this notion represented Syrkin’s true feelings, it is reasonable to presume that his words echoed the sentiments of many in attendance. As he moved forward through his talk, Syrkin put forth a standard criticism of resonance objecting to the notion that a molecule might not have a discrete structure. This argument, of course, was grounded fully in Soviet ideology, and specifically the belief that everything must be real and known.

Joseph Stalin lying in state

As it turned out, Pauling’s letter and the Soviet Academy meeting that followed marked the crescendo of the resonance theory controversy. Following the conclusion of the 1951 conference the debate continued to simmer, with small attacks volleyed by both sides for another two years. But in 1953, curiously, the controversy came to an end, specifically because of the death of Soviet Premier Joseph Stalin.

The end if Stalin’s reign ushered in a new era for the Soviets, one where it was no longer imperative to uphold orthodoxy for fear of severe punishment, including death. And the fact that the Soviet scientific community did not pursue its condemnation of resonance theory with any vigor following Stalin’s demise suggests that the debate was never actually rooted in beliefs about problems with the science of resonance. More likely, resonance was just one of many convenient targets for Stalin and other Soviet figureheads in their war against ideas that were ideologically incongruent with Soviet doctrine.

Whatever the case, after 1953 the resonance controversy was effectively concluded, though a few actions remained to be taken by both parties involved. These activities, as well as the western scientific world’s take on the dispute, will be the focus of our final two posts in this series.

The Soviet Resonance Controversy: Escalating the Attack on “Bourgeois Science”

O.A. Reutov

[Part 4 of 7]

In late 1940s and early 1950s, the culture of Soviet science was dominated by Lysenkoist thinking, with little room permitted for other points of view. Indeed, any work that did not fall strictly in line with Lysenkoist ideology was condemned, and those scientists who were brave enough to uphold contrary beliefs were routinely imprisoned or even killed. This period, which coincided with the end of Joseph Stalin’s reign as premier, was clearly not an environment that encouraged innovation; rather, conformity ruled the day.

By extension, the most lauded Soviet scientists of the era were those who remained ideologically attuned to accepted beliefs, and it was in this environment that pitched opposition to Linus Pauling’s resonance theory emerged. The controversy then, was not so much one based on scientific opposition, but was instead steeped in perceived ideological differences.

As noted in our previous post, Ia. K. Syrkin and M.E. Diatkina’s books – including their translation of The Nature of the Chemical Bond – did much to increase awareness of resonance theory within the Soviet scientific community, and in due course the theory was attacked. In 1949 V.M. Tatevskii and M.I. Shakhparanov became the first authors to criticize the theory in the Soviet literature, and seven more articles followed from there. Much of this work received the formal backing of the Soviet state, and all were approved for publication by the Scientific Council of the Institute of Organic Chemistry of the Academy of Sciences.

Reviewing these articles today, it would appear that more than four hundred Soviet scientists were working in some way to disprove resonance theory within the Soviet Union. The sentiment that emerged from this work held that Pauling’s resonance theory was scientifically unsound because 1) it was not ideologically congruent with Soviet ideology, and 2) it did not recognize the work of previous Soviet or Russian scientists.

The framework established by the seven articles set the stage for the next chapter in the controversy: the approval of an official stance on resonance by the Soviet government.

As part of the tradition of Lysenkoist thinking, Soviet scientists periodically held conferences where theoretical doctrine was debated and a specific piece of ideology was declared the “winner.” These winning ideas would, in turn, become the Soviet government’s supported theory, at which point no other competing theory would be allowed.

In 1951 it was resonance theory’s turn to be debated and, to the surprise of few, it was ultimately decided that Pauling’s ideas were not to be considered as part of the Soviet doctrine surrounding the structure of molecules. Furthermore, according to a U.S. State Department document, a decree was issued that “all remnants of the mistaken conceptions of resonance” were to be eradicated.  

The official Soviet position on resonance theory was disseminated in an article by O.A. Reutov in the Journal of General Chemistry of the USSR and titled, “Some Problems of the Theory of Organic Chemistry.” In it, Reutov systematically attacked Pauling’s ideas as “bourgeois science” and “alien reactionary ideas” that did not take into account Soviet foundational work on molecules.

A.M. Butlerov

According to Reutov, Russian scientist Aleksandr Mikhailovich Butlerov had developed the “true” chemical structure of the molecule, and he argued that Pauling evinced an “insufficient appreciation as well as an incomplete understanding and a perversion of the Butlerov theory of structure.” Pauling’s omissions were so egregious as to force Reutov to conclude that the theory of resonance had been formulated, at least in part, for “the belittlement of the importance of Russian science.”

Reutov’s rhetoric only escalated from there. “[I]t is urgently necessary,” he wrote, to criticize resonance on the “basis of Marxist-Leninist methodology,” a passage that all but admits that the controversy had little to do with scientific thinking. And this needed criticism would reliably be carried out by Soviet organic chemists who, to use Reutov’s words, “form an army of many thousands strong.” By the end of the article, it was clear: the Soviet scientific community was ready to attack western ideas on the chemical bond, and Pauling was their primary target.

Because Reutov’s article was originally published in Russian and chiefly circulated within the Eastern Bloc, Pauling did not find out about it for several months. In August 1951 he finally received a copy from the Consultants Bureau, a translation center specializing in scientific journal articles and based in New York. Pauling was understandably unhappy with the content and tone of the article, and expressed hope that “some action can be taken to stop this attack,” which to him was a “very vigorous one.”

The Soviet Resonance Controversy: Beginnings

Trofim Lysenko

[Part 3 of 7]

Linus Pauling’s resonance theory, while initially a bit difficult to grasp, gradually became an essential tool for predicting and understanding chemical structures. Widely lauded for the links that it provided between classical chemistry and the new quantum mechanics, the theory was also valued for its practical applications in molecular modeling.

But not all found comfort and solace in resonance. In particular, many Soviets viewed Pauling’s theory as an affront to their ideology, regardless of its chemical “correctness.” For these critics, an abstract hybrid model of the molecule – one lacking a foundation in a quantifiable reality – was too troublesome to ignore.

Pauling developed his theory of resonance in a series of papers authored in the early 1930s, and eventually codified the work in his 1939 book, The Nature of the Chemical Bond. Soviet scientists were surely aware of resonance theory throughout this time period, but it appears that ideological interest in Pauling’s ideas did not fully manifest until after World War II. This was likely because, during the war, the Soviet government diverted scientific work away from theoretical investigations in favor of practical applications that would benefit the war effort. Shortly after Word War II ended however, Soviet scientists Ia. K. Syrkin and M.E. Diatkina translated The Nature of the Chemical Bond into Russian. The tandem then wrote their own book outlining ideas on structural chemistry; a book that was based on resonance theory.

Though they had grounded their text in well-established chemical theory, Syrkin and Diatkina’s work was almost immediately criticized within the Soviet Union. In 1949, Soviet scientists V.M. Tatevskii and M.I. Shakhparanov published an article in the journal Voprosyi Filosofii (Questions in Philosophy) titled, “About a Machistic Theory in Chemistry and its Propagandists.” In it, the authors contended that Syrkin and Diatkina were obliged to carefully critique Pauling’s resonance theory, rather than bestow upon it “laudatory” praise. The paper then points out that Syrkin and Diatkina did not include

even one mention of the works of Russian or Soviet scientists…it was expected that the translator and editor, fighting for the honor of the Soviet science, would have seen to it to fill in this gap.

“Still worse,” the authors continued, “the annotations which Syrkin gives at the end of the book are represented primarily as indication of the works of American and English chemists.” This given, one could only conclude that Syrkin and Diatkina were merely “propagandists for the known-to-be erroneous and vicious theory of the American chemist, Pauling.”

Tatevskii and Shakhparanov’s views were not anomalous. In fact, the Soviet Academy of Sciences itself viewed Syrkin and Diatkina as being in “serious error” for “championing the theory of resonance or failing to expose its supposed fallacy.” The rhetoric only escalated from there, and in 1951 Syrkin and Diatkina were forced to recant their belief in resonance theory. In his recantation, Syrkin was famously quoted as saying that he had “overestimated second rate works of foreign scientists.”

On the surface, the criticism of Syrkin and Diatkina seems to have emerged primarily from their support of a non-Soviet scientist. And while this was certainly a factor, the attack was also a product of a larger ideological push within Soviet science that traced its origins to the 1930s and a man named Trofim Lysenko.

An agronomist, Lysenko came to symbolize the ideal Soviet man; a notion that received the full support of Premier Joseph Stalin. Lysenko had been born into relative poverty but eventually gained entry into the Kiev Agricultural Institute, where he studied wheat production. It was there that he began to develop the ideas for which he would become well known.

Lysenko believed that if he could freeze winter wheat, he would be able to force early germination of the crop, a process known as vernalization. He also believed that this early germination would in turn become a heritable characteristic. A breakthrough of this sort was highly desirable in the Soviet Union, in part because the vast nation had experienced multiple food shortages due to unseasonably cold winters and poorly structured collectivist agricultural practices. In Lysenko, Stalin saw the promise of more easily feeding the Soviet population, and the scientist quickly grew in stature as a “man of the people.”

But Lysenko’s ideas were attractive on ideological grounds as well, chiefly because his ideas on vernalization took a Lamarckian approach to evolution. Simply put, Lysenko believed that evolution was something that could be controlled by man; that the people had the means and ability to enact and propel genetic change.

This idea ran counter to those prevailing in other parts of the world and specifically the United States, where a more Darwinian notion of evolution had been adopted. In the Darwinian model, evolutionary change occurs by random chance and individuals do not possess the ability to create lasting heritable impact through their own actions. For the Soviets, the Darwinian model was seen as bourgeois, a word that was widely held in contempt. In contrast, Lysenko’s view of evolution was one that put the state in control, and it was absolute.

Given this context, it becomes easier to see how resonance could be viewed as being counter to Soviet ideology. And within the USSR, an alternative theory was accordingly developed by a Soviet chemist named Gennadi Chelintsev, who aspired to become the chemistry and biology equivalent of Lysenko.

Chelintsev applied Soviet ideology to chemistry in his book, Essays on the Theory of Organic Chemistry. Importantly, Chelintsev used the book to argue that every molecular compound had only one discrete structure. His ideas were appealing to many Soviet scientists because, as with Lysenko, they were primarily based in the mechanistic world view that was so deeply enmeshed in Soviet ideologies.  

By translating The Nature of the Chemical Bond, Syrkin and Diatkina had brought Pauling’s ideas on resonance to the widespread attention of Soviet chemists, a process that allowed many to become aware of a scientific concept that was contrary Soviet ideology. In reaction, Chelintsev and others found in Pauling’s theory a perfect tool for villainizing non-Soviet science. And while Chelintsev’s book did not target Pauling outright, it did bring attention to a conflict that would continue to grow in the years to come.

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.