The Guggenheim Trip, Part III: Unexpected Colleagues

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

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

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

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

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

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

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

Fritz London

Fritz London

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

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

Walter Heitler

Walter Heitler

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

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

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Linus Pauling and the Birth of Quantum Mechanics

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

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

My year in Munich was very productive. I not only got a very good grasp of quantum mechanics — by attending Sommerfeld’s lectures on the subject, as well as other lectures by him and other people in the University, and also by my own study of published papers — but in addition I was able to begin attacking many problems dealing with the nature of the chemical bond by applying quantum mechanics to these problems.”
– Linus Pauling, 1992

In the spring of 1926, funded by a Guggenheim Fellowship, Linus and Ava Helen Pauling embarked on their first trip to Europe, scientific tourists beginning a journey that would revolutionize modern chemistry and physics.

The Paulings travelled through the continent, stopping at the famed institutes of modern science in Munich, Göttingen, and Zurich, among others, and meeting with scientific giants including Arnold Sommerfeld, Max Born and Erwin Schrödinger. It was at this time that quantum mechanics, the branch of science devoted to the study of the atom’s physics, was being revolutionized by the ideas of Schrödinger and Werner Heisenberg. It wasn’t until Pauling left Germany for Switzerland however, that he was introduced to a ground-breaking idea – the combination of Schrödinger’s wave mechanics with the study of structural chemistry.

In Zurich, the German researchers Walter Heitler and Fritz London explained to Pauling the concept of “electron resonance” as developed by Heisenberg. At its core, the theory suggested that electrons could be attracted to one another, to the point where they would eventually switch back and forth between two given atoms. This exchange of electrons would, in turn, release energy, in the process drawing the two atoms together and creating a chemical bond. This revolutionary concept agreed with certain known principles of the hydrogen atom – the atom on which Heitler and London were conducting their calculations – and appeared to support the Pauli exclusion principle which, as Pauling later put it, “states that no two electrons in the universe can be in exactly the same state.”

After his return to Caltech in September of 1927, Pauling worked on several projects, including his first published book and a class on the Heitler-London work. In the process of defining his research program as a young member of the Caltech faculty, Pauling decided that, rather than continuing to dabble in theoretical physics, he would instead return to his roots in chemistry. With that, he set out to combine what he had learned in Europe with his continuing interests in structural chemistry.

He began his work on the chemical bond, figuring calculations and comparing his results to existing experimental data. He affirmed that Heitler and London’s work meshed comfortably with G. N. Lewis‘ theory of the shared electron pair and he published articles on the subject, in the process introducing many chemists to the notion of using quantum mechanics as a tool for the study of non-physics problems. In early 1928, he suggested that quantum mechanics could answer the question of carbon bonding – a revolutionary idea at the time. Unfortunately, while the preliminary mathematics were promising, the sheer mathematical computing power required did not exist for Pauling to fully solve the problem.

In 1930 M.I.T. physicist John C. Slater succeeded in simplifying Schrödinger’s mathematical description of the types of changes experienced by any quantum system over time — an important mathematical model known as the Schrödinger Wave Equation. By slightly restructuring Slater’s set of simplified equations, Pauling was able to utilize the concept of wave functions to describe new orbitals that matched the known traits of the carbon-tetrahedron bond. Not only did these new methods allow Pauling to calculate the data for basic tetrahedral bonds, they also provided stable footing for detailing the precise structures of a series of complex molecules. This was the genesis of valence-bond theory — a hugely important marriage of quantum physics and structural chemistry.

In early 1931, Pauling released a paper detailing six rules, later known as “Pauling’s Rules,” that dictated the basic principles governing the molecular structure of any given molecule. He presented his findings in the simplest language possible, avoiding complex mathematics in order to make the concepts accessible to his fellow chemists. This paper, of course, was titled “The Nature of the Chemical Bond” and would serve as the basis for his immensely popular textbook of the same name.

In 1954 Pauling won the Nobel Prize in Chemistry “For research into the nature of the chemical bond and its application to the elucidation of complex substances.” The award was granted in recognition of the work that began during his first trip to Europe and blossomed in the decade that followed. Pauling’s innovative application of quantum mechanics had resulted in his receipt of the highest possible scientific honor and the subsequent worldwide recognition of his talents.

Learn more about this story by visiting the website “Linus Pauling and the Nature of the Chemical Bond: A Documentary History.”