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

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Linus and Ava Helen Pauling in Munich, with Walter Heitler (left) and Fritz London (right), 1927.

[An examination of Linus Pauling’s first paper on the nature of the chemical bond, published in April 1931. Part 2 of 2.]

In 1928 the German physicists Walter Heitler and Fritz London published a paper that appeared to have beaten Linus Pauling to the punch in its application of quantum mechanics to the theory of chemical bonding. As with Pauling, the duo was interested in Erwin Schrödinger’s wave function, and in their paper they applied it to the simplest bond: that formed by two hydrogen atoms. In so doing, Heitler and London did indeed become the first scientists to publish an application of this type.

The German colleagues also incorporated Werner Heisenberg’s ideas on exchange energy. Heisenberg had theorized that the electrons of two given atoms would find themselves attracted to the positively charged nuclei of their atomic pairs. As such, a chemical bond, according to this theory, consisted simply of two electrons jumping back and forth between two atoms, belonging simultaneously to both and to neither.

Heitler and London extended this idea, proposing that chemical bonds sourced their lengths and strengths from the amount of repulsion extant between two positively charged nuclei. The balance between the electrons’ attraction to these nuclei, coupled with the quantifiable repulsion existing between the two nuclei, ultimately served to create the bond.


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Pauling on the precipice of greatness, 1928.

Invigorated by the promise of competition, Pauling set to work applying Heitler and London’s theory to more complex molecules. In part to motivate himself, but also to ensure that he received recognition for his research, Pauling announced in the Proceedings of the National Academy of Sciences that he believed he could solve the tetrahedral binding of carbon using the ideas put forth by quantum mechanics.

This declaration piqued significant interest throughout the scientific community, striking a nerve for chemists and physicists alike, both groups of whom had been puzzling over this specific structure in different ways. On one hand, physicists believed that carbon should have a valence of two because, of its six electrons, four were located in two different subshells. Both sets of two would then be expected to pair off with each other, leaving only two electrons logically available for bonding. On the other hand, chemists found in the laboratory that carbon typically offered four electrons for bonding in nature. In essence, both theory and experiment indicated that neither party was completely right, but so too could neither point of view be completely wrong. Pauling believed that quantum mechanics could illuminate the paradox.

In addition to his theoretical study, Pauling’s extensive graduate training in x-ray crystallography strengthened both his interest in and his flair for atomic structure. By 1928, after a busy year of research, he had established five principles for determining the structure of complex covalent and ionic crystals, later dubbed “Pauling’s Rules.” He used these rules to predict models for particular molecular structures, and then worked backward from the theoretical model to develop a more concrete picture based on x-ray data. When a colleague remarked that this technique resembled the Greek stochastic method – an approach based largely on applied guess work – Pauling offered a correction, stating that

Agreement on a limited number of points cannot be accepted as verification of the hypothesis. In order for the stochastic method to be significant, the principles used in formulating the hypothesis must be restrictive enough to make the hypothesis itself essentially unique.

In other words, Pauling was relying on his knowledge of chemical principles to develop meticulous and educated hypotheses that he could go back and prove. And as he would hasten to add, he placed very little stock in luck or guesswork.


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As work moved forward, Pauling added his rules to three others that had been established by G.N. Lewis – and then expanded and formalized by Heitler and London – concerning the electron pair bond. These rules set parameters for the circumstances in which an electron would be theoretically available to form a chemical bond.

Though equipped with a solid toolkit of his and others’ making, it ultimately took Pauling almost three years to solve the carbon tetrahedron, with his big breakthrough coming in December of 1930. Inspired by the work of MIT physicist John C. Slater, Pauling found a way to reduce the complexity of the radial wave function, a component of bond orbital theory the application of which had been giving him some trouble. With this solution in hand, the math required for solving further steps of the carbon puzzle became significantly more manageable.

Pauling’s subsequent equations led him to develop a model for the structure that consisted of four equal orbitals oriented at the angles of a tetrahedron. Using these equations, Pauling further discerned that the strength of the bonds within the structure increased in accordance with greater degrees of orbital overlap between two atoms. The overlap, Pauling found, produced more exchange energy and this in turn created a stronger bond.

Pauling sent his paper to the Journal of the American Chemical Society (JACS) in February 1931. In it, Pauling detailed three rules governing eigenfunctions that complemented G.N. Lewis’ rules about electron pairs. Pauling used this collection of guidelines to explain relative bond strength, finding that the strongest bonds occurred on the lowest energy level and where orbitals overlap. He also developed a complete theory of magnetic moments and ended the paper stressing the important role that quantum mechanics had played in his formulation of the rules and theories expressed in the work.

The paper, titled “The Nature of the Chemical Bond: Application of Results Obtained from the Quantum Mechanics and from a Theory of Paramagnetic Susceptibility to the Structure of Molecules,” was accepted and published in record time. The subject matter was so new and the ideas so fresh that Arthur Lamb, the editor of JACS at the time, had trouble finding a group qualified enough to review it. Even so, he scheduled the article for the April issue and in so doing published Pauling’s paper a mere six weeks after he had received it.


In 1926, whether he knew it or not, Linus Pauling embarked down a path toward the transformation of chemistry and the way that it would be studied for generations to come. The ideas that he began developing during this time gradually became the standard model for those studying chemistry while simultaneously launching Pauling to dizzying heights. His April 1931 paper, the first in a series of seven, also became the basis for his 1939 book, The Nature of the Chemical Bond, which was almost immediately recognized as a classic of twentieth-century scientific writing.

Largely on the strength of the April JACS article, Pauling also received the 1931 Langmuir Prize from the American Chemical Society, and used the money that came with the prestigious award to further his research. Now that he was interested in molecular structure, he saw it’s promise everywhere within a rapidly expanding research program. In fact, the chemical bond work of the late 1920s and early 1930s laid the foundation for his subsequent program of hemoglobin research, which in turn led to his sickle cell anemia discovery almost twenty years later. In hindsight, it is easy to see how Pauling could have looked back on the achievements of early 1931 as being “the best work I’ve ever done.”

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“The Best Work I’ve Ever Done”

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Linus Pauling, 1931.

[An examination of Linus Pauling’s first paper on the nature of the chemical bond, published in April 1931. This is part 1 of 2.]

“It seems to me that I have introduced into my work on the chemical bond a way of thinking that might not have been introduced by anyone else, at least not for quite a while. I suppose that the complex of ideas that I originated in the period of around 1928 to 1933 – and 1931 was probably my most important paper – has had the greatest impact on chemistry.”

-Linus Pauling, 1977

One of the major film documentaries chronicling Linus Pauling’s life was produced for the long-running NOVA series in 1977. By that point, when asked to look back over the decades of significant work that he had done, Pauling still singled out his insights into the chemical bond as being his most significant contribution to chemistry.

Pauling’s initial 1931 paper in particular marked the first time that he published his revolutionary point of view related to the chemical bond, and reflecting on that period Pauling went so far as to call the article “the best work I’ve ever done.” Indeed, the paper marked the first instance in which Pauling began to spell out the ways in which the burgeoning field of quantum mechanics might be applied to fundamental questions in chemistry. And though first in a lengthy series, Pauling placed major significance on this one paper because it profoundly changed the course of his career and set into motion a period of heavy influenced on the trajectory of an entire discipline.


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G.N. Lewis, ca. 1930.

While an undergraduate at Oregon Agricultural College, Pauling had been taught – and went on to teach his own students – the “hooks and eyes” theory of chemical bonding. In this model, each atom was presumed to have a certain number of hooks or eyes that determined how and to what other atoms it could connect. Though it prevailed at the time, the model was deemed to be outdated and insufficient by many of Pauling’s contemporaries, some of whom were concurrently searching for a more satisfactory replacement.

That said, “hooks and eyes” did serve as a useful precursor to the later concept of valence, because it correctly assumed that each atom possessed a concrete number of electrons to contribute to the formation of bonds with other atoms. The theory also suggested that there were rules governing how chemical bonds worked and how likely it was that two or more atoms might form a bond.

While Pauling was still a student in Oregon, he avidly read G.N. Lewis’ ideas about electron structure and also studied Irving Langmuir’s theory of valence, a tutelage that helped propel his own nascent interest in chemical bonding and atomic structure. Lewis proved to be particularly important. A chemist at the University of California, Berkeley and a future mentor of Pauling’s, Lewis proposed that eight electrons provided a maximally stable environment for a molecule, a tenet known today as the octet rule. Working from this idea, Lewis hypothesized that an atom containing, for example, seven electrons would bond more readily with an atom containing nine electrons, and that the bond that was formed consisted of the electron shared between the two atoms.

Lewis wasn’t the first to chase the nature of the chemical bond. Indeed, the problem had been under attack for a few decades. In 1911, Ernest Rutherford created the first modern atomic model, but ran into trouble because it wasn’t compatible with classical physics. Niels Bohr then updated this model. Less than twenty years after J. J. Thompson had discovered the electron, Bohr suggested that electrons orbited the nucleus in a predictable way, emitting quanta when they moved into lower orbits.

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Arnold Sommerfeld, 1928.

From there, others sought to fill gaps in Bohr’s model. In 1915, Arnold Sommerfeld, a physicist with whom Pauling eventually worked, helped to devise what came to be known as the Sommerfeld-Wilson quantization rules. These guidelines provided an explanation for angular momentum by describing electron orbits as ellipses rather than perfect circles.

At the same time, Werner Heisenberg, Erwin Schrödinger and Max Planck (among others) were rapidly embracing a new way to look at physics, expanding the theory and mathematics behind involved in this innovative approach. In 1925, a year before Pauling began an influential trip to Europe, Heisenberg had authored his “Quantum Theoretical Reinterpretation of Kinematic and Mechanical Relations,” which many point to as the true beginning of quantum mechanics. Schrödinger then completed his wave function in 1926. By contrast, the Lewis and Langmuir models were part of an old system that – as soon Pauling discovered – was in the process of being discarded.


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Linus Pauling at the Temple of Neptune, Paestum, Italy, during his legendary Guggenheim trip to Europe. This photo was taken by Ava Helen in April 1926.

An admirer of Sommerfeld’s work, Pauling traveled to Europe on a Guggenheim fellowship to learn as much as he could about quantum mechanics, which at the time was referred to as “new physics.” He was especially keen to learn more about Schrödinger’s equation and to dig into the ways in which the wave function might be applied to a more sophisticated understanding of the chemical bond. Though impressed by Heisenberg, Pauling gravitated toward Schrödinger because he saw more potential for practical application of the wave function, as opposed to Heisenberg’s matrix mechanics. In a note penned during his travels, Pauling wrote specifically that

I think that it is very interesting that one can see the [psi] functions of Schrödinger’s wave mechanics by means of the X-ray study of crystals. This work should be continued experimentally. I believe that much information regarding the nature of the chemical bond will result from it.

These thoughts proved prophetic, as we will see in part 2 of this series.


Pauling returned to Pasadena in the fall of 1927, bursting with new ideas. While he was away, Caltech chemistry chief A.A. Noyes had sent word that a unique position had been created for his promising young faculty member, one that lined up nicely with Pauling’s new interests. Upon his re-arrival at Caltech, Pauling was to begin working under the title of Assistant Professor of Theoretical Chemistry and Mathematical Physics.

Although Noyes eventually dropped the physics course from the appointment, Pauling liked the idea of hybridizing his interests into one name. It was at that point that he began referring to himself as a quantum chemist.

Advocating for Polyspheron Theory Over Two Decades

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[Part 2 of 2]

In the mid-1960s, as he continued to develop his close-packed spheron theory of atomic nuclei, Linus Pauling sought to use the techniques of creative visualization that had served him so well in his past theoretical work. The situation was trickier this time however, as the discipline of nuclear physics had not yet put forth a single agreed-upon understanding of molecular structure on which a visual model might be built. Rather, as Pauling put it:

There are now two ways of considering the structure of a molecule. One of these ways is by application of a highly refined set of ideas about chemical bonds that was largely developed during the period from 1860 to 1900, and was then significantly extended between 1925 and 1935 through consideration of the empirical information about molecular structure in light of basic principles that were introduced by quantum mechanics.

The second way of considering the structure of a molecule is by solving the wave equation for the molecule, describing the state of the various electrons interacting with one another and with the nuclei.

Both methods had been used since quantum mechanics entered the mainstream in the 1920s and began being applied to topics in chemistry. But according to Pauling, very little effort had been made to develop an empirical theory of the structure of atomic nuclei that corresponded nicely with contemporary ideas on chemical structure. The wave equation approach was further complicated by the fact that, by the mid-1960s, reliable quantum mechanical calculations could be carried out for only a small number of molecules.

In Pauling’s estimation, the most useful attempt to date to develop a workable theory was folded into a discussion that conceptualized light nuclei as aggregates of alpha particles – particles that Pauling renamed “helions” and made a centerpiece of his theory. To address the perceived need to move forward with this mode of thinking, Pauling developed his own conceptual framework and promoted it throughout the 1960s.


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Notes from Pauling’s second Robbins Lecture, February 25, 1966

In 1966 Pauling used his platform as Pomona College’s first Robbins Lecturer to begin spreading a more popular word of this theory — he gave five lectures on the subject in February and March of that year. In 1967 he delivered three more talks on the theory, this time at Princeton University under the auspices of the Plaut Lectureship, an honorific which Pauling was again the first person to occupy. Finally, in 1971, he offered a free public lecture at the University of Colorado, Boulder, promoting his theory as the E.U. Condon Chemical-Physics Lecturer.

Throughout all of this public outreach, Pauling continued to draw up new manuscripts for publication on the topic. In these, he began considering how nuclear wave functions and magnetic-moment values could all be accounted for using polyspheron theory. He also continued to emphasize the compatibility of his ideas with the shell and cluster theories that had gained traction within the discipline.


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A figure included with Pauling’s 1976 manuscript submission to Physical Review

Pauling’s fight to push polyspheron theory into the physics mainstream continued in 1976, by which time the term “helion” was generally being used by nuclear physicists to refer to a Helium-3 nucleus. This common understanding proved disadvantageous to Pauling, as he had defined the term as referring instead to alpha particles (although, interestingly, Pauling had earlier pointed out that “his” helions were notably present in Helium-3).

But there was also good news for Pauling: by now, scientists using electron scattering techniques to investigate the electromagnetic properties of atomic nuclei with prolate deformation were publishing data that seemed to support his point of view. In particular, these experiments had derived information about the shapes of the deformed nuclei that showed what appeared to be the formation of spherons out of the objects defined by polyspheron theory as “helions” and “tritons.”

Pauling almost immediately pounced on this encouraging data, writing a paper initially titled “Comment on the Shapes of Deformed Nuclei,” and then later more forcefully retitled as “The Predication of the Shapes of Deformed Nuclei by the Polyspheron Theory.” He submitted his manuscript to Physical Review in Spring 1976, arguing that reports from current nuclear physicists had confirmed many of his model’s assumptions and that, accordingly, polyspheron theory merited wider acceptance within the field.

The referee overseeing Pauling’s manuscript asked that he make several revisions prior to publication. One major request was that Pauling reshape some of the terminology that was so specific to his theory, due to its confusing nature with respect to broader accepted nomenclature. The journal also asked that Pauling support his claims with clearer and more substantial calculations.

Pauling responded that the journal’s editors had apparently misunderstood the purpose and the importance of his work, and then appended a more substantive introduction to the beginning of the paper to clarify some of the specifics that had been requested of him. When the journal later asked Pauling to remove the extended introduction he refused and his submission was withdrawn. In 1982, some six years later, a paper with the same title finally made it into print by way of the Proceedings of the National Academy of Sciences, a journal that had historically published most submissions authored by distinguished Academy members, among whom Pauling could certainly count himself.

In 1987 Pauling again attempted to advocate on behalf of his theory, writing a new paper titled “Properties of the High-Spin Superprolate Structure of 15266Dy86 are Explained by the Polyspheron Theory.” This paper once more claimed that his now thirty-year-old theory was being steadily confirmed by contemporary laboratory research.

As in 1976, the 1987 manuscript was also rejected, this time by Physical Review and by Nature alike. Seven years later, right around Pauling’s ninety-third birthday, an article with a far less forthright title did appear, once again in PNAS: “Analysis of a Hyperdeformed Band of 152 66 Dy86 on the Basis of a Structure with Two Revolving Clusters, Each with a Previously Unrecognized Two-Tiered Structure.”


Despite his periodic appeals to the practicality of his theory over the course of several decades, and disregarding his insistence that the theory had been vindicated by its prediction of novel discoveries, Linus Pauling’s ideas on polyspherons were never generally accepted by his peers in either physics or chemistry.

Today the utility of Pauling’s model of the atomic nucleus remains in doubt. What does seem to be clear is that he did not succeed in grasping one of the scientific world’s most elusive holy grails: a unified theory of quantum physics and chemistry.

Linus Pauling’s Polyspheron Theory

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[Part 1 of 2]

“I consider the polyspheron theory to be a simple statement about the insight into nuclear structure that is provided by the experimental data and to some extent by the quantum mechanical calculations.” 

– Linus Pauling, 1976

In the early twentieth century, when physicists were gaining knowledge of the properties of atomic nuclei in strides, the still young field of quantum mechanical theory was being used to interpret experimental data. At the same time, chemists were beginning to explore the implications of quantum theory as it pertained to molecular structure, a trend that culminated in Linus Pauling’s groundbreaking book, The Nature of the Chemical Bond, published in 1939.

In the following forty years however, no single theory of the structure of the atomic nucleus emerged that could conceptually account for all the results that chemists and physicists were observing. Nor was there a satisfactory theory that could link the behavior of the most infinitesimal and internal parts of the atom with the much larger scale of molecular behavior and chemical reactions. In other words, there was not yet a single accepted unified theory of quantum physics and chemistry. The pursuit of just such a theory stood as a holy grail of sorts, occupying the hopes, dreams and energy of many a twentieth century scientist.

For parts of five decades, Linus Pauling strove to develop just such a theory, one that could account for the basic structural tendencies and behaviors of the atomic nucleus and prove useful not only for atomic physicists but also for chemists. His efforts resulted in what he called “close-packed spheron theory,” simplified later as “polyspheron theory.”


Though he had begun work on the topic much earlier, Pauling first revealed his theory to the world on October 11, 1965 at a meeting of the National Academy of Sciences, held at the University of Washington in Seattle. His talk that day was titled “The Close-Packed-Spheron Theory of the Structure of Nuclei and the Mechanism of Nuclear Fission,” and its contents mirrored a pair of similarly titled papers that he published that same year: “The Close-Packed-Spheron Theory and Nuclear Fission,” published in Science, and “The Close-Packed-Spheron Model of Atomic Nuclei and Its Relation to the Shell Model,” which appeared in the Proceedings of the National Academy of Sciences.

In each of these works, Pauling advanced a theoretical framework that incorporated features of three older theories – the cluster, shell and liquid-drop theories – while also accounting for several observed phenomena of atomic nuclei that were difficult to explain at the time, such as asymmetric nuclear fission. Importantly, the close-packed spheron model of the nucleus differed from past models by declaring “spherons” as its units, rather than nucleons. Pauling described his rationale for this choice as having been an outgrowth of his thinking about nuclear fission.

Twenty five years ago a phenomenon of tremendous importance was discovered, nuclear fission. In the uranium nucleus and other heavy nuclei, fissioning produces a lighter and a heavier nucleus, with mass ratio about 2/3, several hundred times as often as two nuclei with equal mass are produced.

Why is fission asymmetric in this way? Here is a simple reason why this might be: I assume that in nuclei the nucleons may, as a first approximation, be described as occupying localized 1s orbitals to form small clusters. These small clusters, called spherons, are usually helions, tritons, and dineutrons: in nuclei containing an odd number of neutrons, a (Helium-3) cluster or a deuteron may serve as a spheron.

Pauling’s basic assumption here was that, in atomic nuclei, the nucleons were in large part aggregated into clusters that are arranged as closely as allowed by the laws of physics. Nuclei with more neutrons than protons were called tritons or dineutrons by Pauling. Likewise, the clusters of neutrons and protons occupying localized 1s orbitals were called spherons.

The most important spherons in Pauling’s conception were aggregates of two neutrons and two protons, which he called helions, though they were already known to physicists as alpha particles. The localized 1s orbitals that these spherons occupied could also be described mathematically as hybrids of the central-field orbitals that are outlined in shell theory. This process of hybridization of orbitals provided a formal basis for relating the cluster model – of which Pauling’s theory was an extreme version – and the shell model.

Pauling also put forth the idea that the spherons in a nucleus were arranged in a series of concentric layers. For a large nucleus, the outer part of the cavity inside the surface layer was occupied by spherons that were in contact with the inner side of the surface layer. These spherons constituted a layer of their own, within which Pauling believed there might reside yet another layer of spherons. To avoid confusion with the “shells” of the shell model, Pauling referred to his spheron layers as follows: “the mantle” for the surface layer, and the “outer core” and “inner core” for the two additional constituents of a three-layer nucleus.

In an effort to assure the scientific rank and file that he was not seeking to upend their entire understanding of nuclear physics, Pauling promised that the quantum mechanical calculations enabled by his polyspheron theory were essentially the same as those that had been made using various other models in the past.

Perhaps unwittingly, this assurance left many colleagues within the field wondering why Pauling was bothering to develop this theory at all. For many physicists, Pauling’s work seemed redundant, or perhaps merely an attempt to change the names of existing terms to new ones that fit more elegantly into Pauling’s conceptual framework of atomic structure.

Pauling countered this skepticism by suggesting that both qualitative and rough quantitative conclusions could be drawn from his model without the aid of extensive calculations. If these conclusions agreed with the experimental evidence, Pauling argued, then detailed calculations of this sort might not always be required in the future, pushing scientists just that little bit closer to the discovery of their holy grail.

Remembering Richard Marsh

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Richard Marsh, 1960. Credit: Caltech Archives.

At the beginning of this year, on Tuesday, January 3rd, the highly accomplished crystallographer Richard E. Marsh passed away at the age of 94. During his impressive sixty-six-year career at Caltech, Marsh was influenced greatly by Linus Pauling’s work in crystallography, and eventually collaborated with him throughout the 1950s and early 1960s. Colleagues and admirers alike knew Marsh for his rigorous standards in investigating atomic structure, a discipline that resulted in his determination of over one-hundred crystal structures throughout his career and the improvement of at least that many more.

In a Caltech tenure that spanned more than six decades, Marsh also inspired generations of graduates and undergrads alike, teaching valuable techniques in crystallography and instilling in his students the rigor of his own research practice. His course “Methods of Structural Determination” was among the most popular graduate offerings in the Institute’s Chemistry division for a great many years. He leaves behind an impressive legacy for the crystallographers of today.


Marsh, who went by Dick, was born in 1922 in Jackson, Michigan. By the time that he arrived at Caltech as an undergraduate in 1939, Pauling had already helped to established the Institute as among the premiere destinations for budding young crystallographers around the world. In particular, Pauling’s newly published Nature of the Chemical Bond had transformed crystallography from arcane to fundamental.

Though Pauling was certainly well known on campus when Marsh was an undergraduate, it would be another eleven years before Pauling and Marsh formally crossed paths. As a student, Marsh had identified an interest in chemistry, but hadn’t narrowed to a particular focus. He commented later that a technical drawing course at Caltech served as a precursor to his interest in crystallography. He graduated with his BS in applied chemistry in the midst of World War II (1943) and, upon graduation, enlisted in the US Navy, spending the next two years degaussing ships in New Orleans. This is where he met his wife Helena Laterriere, to whom he remained married for nearly seventy years.

Following his discharge, Marsh enrolled in graduate school at Tulane University so that he might remain in close proximity to his fiancée. Most of the courses that he needed were already full at the time of his enrollment, so Marsh signed up for an X-ray crystallography class at the nearby Sophie Newcomb College for women. It was there that he met the teacher who changed his life and cemented his interest in crystallography.

That teacher, Rose Mooney, had previously attempted to enroll at Caltech for graduate studies only to be turned away when she arrived in Pasadena and the administration realized that she was a female. Pauling himself stepped in at this point, giving her a temporary position in his laboratory until she was accepted into the graduate program at the University of Chicago. Her lab at Sophie Newcomb College was quite modest, containing only a Laue film holder and one x-ray tube, but for Marsh it was enough. Inspired, his course was set from then on, though he’d have to travel across the country to continue it.

After marrying Helena on August 11, 1947, Marsh enrolled at UCLA. He later called the 2,000-mile move across the southern United States the beginning of their honeymoon, joking that it was a wedding present to his new bride. At UCLA, Marsh studied crystallography under Jim McCullough and earned his Ph.D. in 1950. Caltech subsequently offered him a post-doctoral research appointment, and he remained at the Institute for his entire career, always in a non-tenured position until his retirement in 1990, when he named an emeritus professor.


In the years immediately following World War II, Caltech was still very much the place to be for crystallographers. Thanks largely to Pauling, who returned to structural chemistry after his own war projects had wrapped up, scientists from all over the world travelled to Pasadena to conduct research and solve structures.

Marsh finally became associated with Pauling in 1950, when he arrived at the Institute as a post-doc. He published his first paper with Pauling, “The Structure of Chlorine Hydrate,” in 1952. A year later, the duo published “The crystal structure of β selenium,” which marked the first time that Marsh issued a correction of someone else’s work. Indeed, over the course of his career, Marsh became increasingly focused on policing the field for errors, always striving for maximum accuracy and precision. Pauling engaged in this work himself from time to time, although the various demands on his attention kept him too busy to make a full-time habit out of it.

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Marsh at the famous Caltech Proteins Conference in 1953. To his right is Francis Crick.

Pauling and Marsh continued to collaborate on a number of other publications related to atomic structure between 1952 and 1955, at which point their interests began to diverge. Nonetheless, the two retained a degree of professional closeness throughout the following decades, often writing to compliment one another on various accomplishments, solicit advice, or suggest future projects. In one instance, Pauling provided the kernel of an idea that resulted in Marsh’s 1982 paper on N, N-Dimethylglycine hydrochloride. Likewise, Marsh helped pave the way for Pauling to publish one of his own articles in Acta Crystallography, where Marsh served as an editor for seven years.

In 1975, presented with the problem of solving of a compound that generates hydrazine from molecular nitrogen, Marsh devised and shared a method for determining the structure. This solution influenced the direction of study into hydrazine formation, creating the opportunity for further study. And although Marsh continued to solve structural problems in the years that followed, he also devoted countless hours – over half his career – to the pursuit and correction of published errors, usually pertaining to inaccurate space groups in important crystal structures. Pauling later described Marsh as the “conscience of crystallography.”

With time, he gained such a reputation that his colleagues in the field were perpetually anxious that they would be “Marshed,” or taken to task, for their errors. Marsh held his colleagues accountable to their calculations and believed firmly in checking a computer’s work, rather than the other way around. He is remembered today as having been responsible for many refinements in crystallographic discipline and for the high standards that make future refinements possible.


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Marsh in 2012. Photo by Rafn Stefansson.

In terms of organizational involvement, Marsh joined the American Crystallographic Association (ACA) shortly after starting at Caltech. Over the duration of his career, he became increasingly active in the group, and ultimately served as its president in 1993. He was also co-editor of Acta Crystallography from 1964-1971.

Marsh’s classroom lectures and his relationships with students were at least as influential as were his publications in crystallography. One colleague, B.C. Wang, recalled that Marsh summoned crystallographers of all stature – be they students, professors, or visiting scientists – to a group coffee at 10:30am every day, to encourage discussion and advancement within the field. Students also remembered him as critical but encouraging, his commitment to student success serving as an inspiration for their own hard work.


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Marsh and Pauling in 1986. Credit: Caltech Archives.

When queried by CRC Press in the early 1990s for his input on future publications, Pauling suggested that the press solicit a monograph from Marsh on the crystal structures that he had corrected thus far, arguing that a volume of this sort might help future crystallographers to avoid these errors. Pauling then wrote to Marsh, inquiring about the total number of crystal structures that Marsh had indeed corrected. Pauling had guessed that Marsh had published fixes for 25 to 30 structures and was surprised to learn that the actual number was between 110 and 120.

Although Marsh didn’t publish this proposed monograph, Pauling’s idea evidently inspired him. In 1995, he authored a substantial article on the subject, titled “Some thoughts on choosing the correct space group.” In the piece, Marsh discussed common types of errors as well as preferable techniques and methodology, including a few tables that documented space group revisions over time.

While at Caltech, Marsh worked closely with Verner Schomaker, another of Pauling’s graduate students. In 1991, the two teamed up to put together a festschrift honoring Pauling’s early work on crystallography. Pauling, a man who, by then, had received basically every award that a scientist can get, was immensely pleased and grateful for this honor.

In 2003, Marsh received the inaugural Kenneth Trueblood Award from the American Crystallographic Association for his outstanding achievements in chemical crystallography. Few other awards could be more fitting for a crystallographer of Marsh’s caliber and commitment. In announcing the prize, the chair of the selection committee identified Marsh as a “rare individual among crystallographers, an outstanding teacher and researcher who has greatly influenced so many students and faculty.” He will be remembered and missed for this indefatigable integrity, dedication, and mentorship.

Ahmed Zewail, 1946-2016

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Earlier this month, on Tuesday, August 2, Ahmed H. Zewail, a world renowned Nobel laureate chemist and Caltech’s Linus Pauling Professor of Chemical Physics, died at 70 years of age. As a major figure in the field of chemistry and a personal friend to Linus Pauling, Zewail’s passing is honored and mourned here at Oregon State University.

Zewail was born and raised in Egypt, where he received his bachelor’s and master’s degrees at Alexandria University before going on to attain his PhD at the University of Pennsylvania. After completing his doctorate in 1974, Zewail joined the faculty at the California Institute of Technology, where he remained for the next forty years.

During his tenure at Caltech, Zewail’s team became the first to directly observe the breaking and formation of atomic bonds, also known as transition states. This was initially accomplished in 1987, but the team’s technique had a long way to go before it could be considered revolutionary, to say nothing of routine. Nonetheless, Caltech saw the potential for greatness in Zewail’s work and, in 1990, it named him the first Linus Pauling Professor of Chemical Physics, a newly endowed chair. Upon receiving this accolade, Zewail wrote to Pauling immediately, confiding, “You are one of my personal heroes in science, and I am honored to be holding your chair.” Zewail remained in this position until his passing, frequently stating that it was an honor just to be compared to Linus Pauling, and that he hoped to do justice to that comparison. Important above all else, however, was that Linus Pauling considered him a friend.


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Zewail and Pauling at the 90th birthday event, Caltech, February 1991.

Zewail played a major role in revitalizing the relationship between Caltech and Pauling during the 1980s and early 1990s. Pauling had left the Institute in 1963 amidst increasingly strained circumstances surrounding his work for peace and his stance against nuclear testing. From 1986 through 1993, Zewail was in regular contact with Pauling, helping to arrange his visits to the Caltech campus for a variety of lectures dedicated to Pauling’s work and time there. In 1986, Caltech’s eighty-fifth birthday “Salute to Linus Pauling” afforded Zewail the opportunity to present Pauling with a portrait depicting his face on the body of a Pharaoh, captioned “King of Kings of Chemistry.”

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“King of Kings in Chemistry”

Later events in which Zewail was involved included Caltech’s first Linus Pauling Lecture in 1989, a second Linus Pauling lecture in 1991, and an additional 1991 symposium on the chemical bond that was held to mark Pauling’s 90th birthday. A year later, Zewail produced an edited volume of the papers presented at this conference, The Chemical Bond: Structure and Dynamics, a work which was the source of much pleasure for Pauling in his final years.

Over time, the two became close friends. Christmas cards were routinely exchanged and Zewail even sent Pauling an announcement on the occasion of the birth of his son. In 1992, Zewail likewise provided Pauling with a manuscript documenting his team’s first successful recording of ultrafast electron diffraction from molecules, a breakthrough that enabled increasingly accurate “pictures” of transition states that had never before been observed by chemists. Pauling responded with praise: this was “a fine piece of work” that would make possible the exploration of previously inaccessible frontiers in the fields of chemistry, physics, and biology.

Zewail won the Nobel Prize for Chemistry in 1999. In continuing to seek out methods to observe transition states, he had pioneered a technique that used laser pulses akin to strobe lights to record the colors of light emitted and absorbed by molecules. This technique was termed “femtosecond spectroscopy.” While chemistry had hitherto inferred specifics of reactions based on the material input and output of a given chemical reaction, Zewail’s work now enabled scientists to see specific changes at the molecular level for the first time.


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Crellin and Linus Pauling with Lynne Martinez and Ahmed Zewail, 1991.

To fully appreciate Zewail’s contributions, one must understand that the breaking and shifting of chemical bonds that he worked to observe typically occur in a space of 10-100 femtoseconds, each femtosecond being a millionth of a billionth of a second. Zewail explained the scale of these observations as follows:

Here is the journey in time… 12 or 15 billion years of the Big Bang, and then you come down to our lifespan, which is about 100 years or so – your heart beats in one second. But to go from here [present day] to there [Big Bang] is about 1015, and I am going to take you from the heart into a molecule inside the heart, or eye specifically, and you have to decrease by 15 orders of magnitude to see the beats of this molecule, as you see the beats of your heart. The timescale is fast… if you go from this age of the universe, and you count back from the age of the Earth to the human lifespan to your heart (1 second), and then you go to the microscopic world (sub-second), into how molecules rotate, vibrate, and how the electrons move… In this whole microscopic world here, we reach 10-15 or so seconds, where on the opposite end you reach 1015.

This is the end of time resolution for chemistry and biology, because if you look here, even molecules that are linking undergo collisions on a time scale of 10-14 seconds. A molecule can break a bond and make a bond on this time scale as well. The eye has a molecule called rhodopsin which divides and allows you to see, and that happens in 200 femtoseconds. The way we get photosynthesis to work, and the electron to transfer inside the green plant, is on the order of femtoseconds. So this is the fundamental time scale, and if we were to understand the dynamics of the chemical bond we must understand this time scale.

In other words, the timespan of one heartbeat is to the age of the universe as the timespan of one molecular bond breaking is to the length of an elderly human’s lifespan; the time required by the event is so infinitesimal as to be practically nonexistent. Yet Zewail found that it was at this scale – the “one heartbeat” of a single bond breaking or forming – upon which our entire reality is formed from its molecular foundations up. Zewail showed that events occurring in femtoseconds are the basis for all the occurrences that we take for granted in everyday life.

The ability to observe these events created a new field of study called femtochemistry. And while femtoscopic experiments provide a method for researchers to determine the amounts of energy that hold together different types of chemical bonds, their impact is not limited to chemistry alone. Since the time of Zewail’s breakthroughs in the 1980s and 1990s, many practical applications have emerged from femtoscopic research, including a better understanding of the mechanics of human vision and of the properties of photosynthesis in plants.  Today, most femtosecond lasers are sold not to chemists or physicists, but to hospitals, because of their ability to image very fine tumors. Likewise, in the technology sector, femtosecond pulses can be used to lift material on the micron scale without dissipating heat into a microchip.


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In more recent years, Zewail was named Director of the National Science Foundation’s Laboratory for Molecular Sciences, and was nominated by President Barack Obama as both the first United States Science Envoy to the Middle East as well as a member of the President’s Council of Advisors on Science and Technology. In February, Caltech held a symposium titled “Science and Society” to celebrate Zewail’s 70th birthday. At the event, the honoree spoke of his efforts to expand scientific research initiatives in his native country and stressed the importance of holding to a scientific vision. Advocating as he was for education and peace across international borders, Zewail’s message was, without doubt, one that would have made Linus Pauling proud.

On February 28, 2001, on what would have been Linus Pauling’s one-hundredth birthday, Zewail delivered the keynote address at the Linus Pauling Centenary Celebration, a day-long symposium organized and hosted by Oregon State University. In his talk, “Timing in the Invisible,” Zewail reflected on the rapid changes that had arisen in the field of chemistry as a result of breakthroughs in femtoscience. In 1950, when asked what he thought chemists would be studying fifty years on, Pauling responded: “We may hope that the chemists of the year 2000 will have obtained such penetrating knowledge of the forces between atoms and molecules that he will be able to predict the rate of any chemical reaction.” Zewail’s work, in effect, accomplished this ambition. It has given chemists insight into the dynamics of chemical bonding, and thus greater predictive knowledge of the forces and rates of these dynamic changes.

Dr. Ahmed Zewail, who held the Linus Pauling chair at the California Institute of Technology for so long, was indeed the right scientist to carry Pauling’s legacy forward. Now, as that chair sits empty, Zewail is remembered and missed for all that he accomplished as a scientist, as an advocate for social change, and as a friend.

A View of Pauling’s Models

In 2010, Oren Eckhaus, a photographer based in New York City, visited our facility to photograph several of the molecular models that remain extant in the Ava Helen and Linus Pauling Papers. He did so in support of Jane Nisselson’s documentary-in-progress, “Unseen Beauty: The Molecule Imagined,” which she was researching with support from the OSU Libraries Resident Scholar Program.

Now Eckhaus is preparing several of his photographs for display in an upcoming art exhibition, and he was kind enough to share a handful of the images with us. He also provided a short artist’s statement:

The idea of photographing the molecular models came as an add-on visual assistant to a movie (that is still in the making) who’s main subject is to show the representation of pure scientific ideas as real tangible forms.

In my profession, along with being a fine art photographer, I also document objects of art for museums and art collectors. Upon seeing the models, I was struck by their beauty. They are important both as art pieces and early science tools.

Therefore, the approach of photographing the pieces was a mix of an artistic and documentary point of view, showing the original scientific intent, along with their artistic beauty.

A book of 32 molecule images is in the making.

Click on any image to open the gallery and to learn more about the molecular models highlighted within.