The Guggenheim Trip, Part II: The Growth of a Scientist

Linus Pauling, Werner Kuhn, and Wolfgang Pauli traveling by boat in Europe. 1926.

Linus Pauling, Werner Kuhn, and Wolfgang Pauli traveling by boat in Europe. 1926.

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. The Chemical Bond: Structure of Dynamics, Ahmed Zewail, ed. 1992.

After his and Ava Helen’s stay in Italy, Linus Pauling was itching to return to the lab. The couple arrived in Munich in the last week of April and the first item on Pauling’s agenda was a meeting with Arnold Sommerfeld.

Sommerfeld, in association with Niels Bohr, was responsible for the Bohr-Sommerfeld model of the atom, a precursor to modern quantum mechanical ideas on atomic structure. At the time of Pauling’s European trip, Sommerfeld was serving as the director of the Institute of Theoretical Physics in Munich. He had spent the past decade building Germany’s community of physicists, nuturing many of Europe’s best scientists on a steady diet of cutting edge research. His lectures, famous by the time Pauling reached Europe, were known for their new and innovative content. As Thomas Hager, a Pauling biographer, explains, “[Sommerfeld] knew everyone in theoretical physics, had collaborated with many of them and corresponded regularly with the rest.” He knew exactly what was happening in his field and made sure his students did too.

Pauling’s first Munich meeting with Sommerfeld was something of a disappointment for the young scientist. Rather than being allowed to continue the work he had begun at Caltech, Sommerfeld chose to assign Pauling mathematical research relating to electron spin – an area that held little interest for him.

After a spell of half-hearted devotion to the electron spin problem, Pauling convinced Sommerfeld to allow him to study the motion of polar molecules. Pauling believed he could clarify portions of the Bohr-Sommerfeld model by introducing the effects of a magnetic field to the existing equations. This caught Sommerfeld’s attention and Pauling was subsequently instructed to continue his research under the stipulation that he provide Sommerfeld with the details of his work for presentation at an upcoming conference in Zurich. Pauling did so, and a few days after Sommerfeld had departed for the conference, he received an order to appear in Zurich to discuss his work.

Once at the conference, Pauling found himself surrounded by the leading physicists of Europe. Wolfgang Pauli, a young German physicist famous for his development of the revolutionary Pauli Exclusion Principle, was among those in attendance. On a whim, Pauling approached his colleague and began explaining his recent work on the Bohr-Sommerfeld model. Pauli was unimpressed. The paradox-riddled Bohr-Sommerfeld model, and Pauling’s work supporting it, was on its way out with the new ideas of quantum mechanics soon to take its place. Pauling’s research was too late to be of any value and Pauli was not shy about telling him so.

After finishing his summer vacationing with Ava Helen in Switzerland, Pauling returned to Munich for the fall semester. It was at this time that Pauling really began to prove himself, developing a reputation for his extensive knowledge and concentrated enthusiasm. Pauling’s most important accomplishment, however, was not his ability to make friends. Instead, it was gaining both the attention and the esteem of Arnold Sommerfeld. Pauling did so by discovering a mathematical error in the work of Gregor Wentzel, a protégé of Sommerfeld. The discovery and correction of this mistake garnered Pauling a great deal of respect in Sommerfeld’s eyes.

As it turned out, Pauling’s discovery of Wentzel’s error resulted in more than just Sommerfeld’s acclaim. It allowed Pauling to apply Wentzel’s work to the calculation of energy levels, which in turn provided the platform for a series of calculations on the energy values for complex atoms. This was a totally new approach to deriving atomic properties and Pauling took full advantage of his discovery, publishing his findings in a paper titled “The Theoretical Prediction of the Physical Properties of many-Electron Atoms and Ions.”

In a matter of months, Pauling had evolved from a star-struck young American to a legitimate player in the European field of quantum mechanics. Fortunately for him, his rise to scientific prominence had only just begun.

Read about Arnold Sommerfeld in “The Duelist” or learn more about this entire story on the website “Linus Pauling and the Nature of the Chemical Bond: A Documentary History.”


Our Newest Addition: Pauling-Goudsmit Letters

Portrait of Samuel Goudsmit, 1937.

Portrait of Samuel Goudsmit, 1937.

Goudsmit and I were never together, I think, during the period when [The Structure of Line Spectra] was written. He would write a draft of some material that he thought ought to go in the book and then using that as a basis I wrote the corresponding sections of the book.”
– Linus Pauling. AHQP (Archive for the History of Quantum Physics), interview transcript part 2. Interview by John Heilbron. March 27, 1964.

The Oregon State University Libraries Special Collections is pleased to announce an important addition to the Ava Helen and Linus Pauling Papers — the donation, by history of science scholar and dealer Jeremy Norman, of a series of letters between Linus Pauling and Samuel Goudsmit.

This correspondence, originally a part of Goudsmit’s personal papers, relates primarily to Pauling’s first book publication, The Structure of Line Spectra, a work largely-derived from Goudsmit’s original paper of the same name and co-authored by Goudsmit himself. The Pauling-Goudsmit donation includes 14 autographed letters, 5 typed signed letters, 1 typed signed note and 3 unsigned carbons, concerning the scientists’ collaboration on The Structure of Line Spectra and other topics.

This fascinating series of letters between Pauling and Goudsmit reflects their long scientific and personal association. Most of the letters were written during the 1930s and roughly half focus on The Structure of Line Spectra. While the line spectra textbook had its origins in Goudsmit’s doctoral thesis, it was translated from the German by Pauling and extensively reworked by both Pauling and Goudsmit for nearly three years before its publication in 1930. The pioneering text was the first work to be published in book form by either author.

Samuel Goudsmit, born in the Netherlands in 1902, became famous for his 1925 work with Eugene Uhlenbeck in which the physicists introduced the concept of electron spin to the scientific community. Pauling and Goudsmit met in 1926 in Europe, where Pauling had traveled on a Guggenheim fellowship to study quantum mechanics. At the time, Goudsmit was continuing his investigations into complex spectra and the Zeeman effect. The two men formed a strong friendship during their work together and, in a 1931 letter to Goudsmit, Pauling described their month of collaboration in Copenhagen as “the happiest period of scientific cooperation in my life, and the most profitable for me.”

In 1927, after obtaining his doctorate, Goudsmit accepted a professorship at the University of Michigan, where he taught until 1946. Much the correspondence from the Norman donation dates from Goudsmit’s time in Michigan, during which Pauling served first as an assistant professor and then as a full professor at Caltech. During World War II, Goudsmit was a member of the Alsos mission, a part of the Manhattan Project, in which he and other scientists were charged with assessing the German nuclear weapons development project.

After the war, Goudsmit took a position at the Brookhaven National Laboratory and served as editor-in-chief of the Physical Review, a prominent physics journal. Goudsmit was also an amateur Egyptologist, occasionally publishing in his work in archaeological journals. He passed away in at the age of seventy-six in Reno, Nevada.

The Pauling-Goudsmit letters are sprinkled with references to other famous or noted physicists, including but not limited to Sir William Lawrence Bragg (1890-1971), co-recipient of the 1915 Nobel Prize for physics for his studies in x-ray crystallography; Robert Millikan (1868-1953), Nobel laureate in 1923 for his work on electron charges and the photoelectric effect; Arthur Amos Noyes (1866-1936), professor of chemistry at Caltech and Pauling’s mentor; and Richard Tolman (1881-1948), thermodynamics expert and co-author of the first American commentary on relativity theory. Many of these men were associates of Pauling’s at Caltech, where the majority of the letters in this collection were written.

The OSU Libraries Special Collections is very grateful to Jeremy Norman of Jeremy Norman’s for his incredibly generous donation of the Pauling-Goudsmit letters. Norman is a collector and seller of historical documents relating to science, medicine and technology whose blog can be found here.

Read more about Samuel Goudsmit’s work on the website “Linus Pauling and the Nature of the Chemical Bond: A Documentary History.”

The Heisenberg Uncertainty Principle

Werner Heisenberg

Werner Heisenberg

I learned mathematics from Born and physics from Bohr, and from Sommerfeld I learned optimism.”
– Werner Heisenberg

While the Bohr-Sommerfeld atom had proved revolutionary in the mid-1910s, a decade later the model was considered disordered and highly paradoxical. For years, researchers had tried to rebuild mathematics to fit the atomic model of the day.

Instead of struggling along the same path as his contemporaries, Werner Heisenberg, a young German physicist, chose to entirely ignore visual models and focus on the mathematics of spectral data. Over the course of several days, by limiting himself to hard, verifiable data, Heisenberg created the basis for matrix mechanics. In cooperation with Max Born and Pascual Jordan, he was able to refine his work, allowing scientists to approach particles as evolving matrices rather than stale, immobile ball-and-stick models. Through his study of particles using matrix mechanics, he was able to develop a detailed theory suggesting that it was impossible to pinpoint both the momentum and the exact location of any given particle at a specific point in time. Instead, he argued, it was possible to create a probability distribution which could be used to calculate the likelihood of a particle achieving an exact momentum and position at a particular moment.

In late March of 1927, Heisenberg published a manuscript entitled “On the perceptual content of quantum theoretical kinematics and mechanics.” The paper detailed the terms of his probability theory, eventually known as the indeterminacy principle, or more commonly, the Heisenberg Uncertainty Principle. According to David Cassidy, author of Uncertainty: The Life and Science of Werner Heisenberg, Heisenberg’s paper, coupled with Bohr’s complementarity principle and Born’s statistical interpretation of Schrodinger’s wave function, formed an integral part of the Copenhagen interpretation of quantum mechanics. Cassidy calls the Copenhagen interpretation “an explication of mechanics that fundamentally altered our understanding of nature and our relation to it,” and an event that “marked the end of a profound transformation in physics that has not been equaled since.” In this way, Heisenberg was able to reshape scientists’ understanding of the world at the molecular level.

Linus Pauling had the great fortune of touring Europe on a Guggenheim Fellowship during the time of Heisenberg’s discovery. During his stay in Germany, Pauling visited the Göttingen Institute of Physics, the home of Max Born, Arnold Sommerfeld, and of course, Werner Heisenberg. The institute’s renowned scientists, determined to educate their students on the newest developments in their fields, were known for presenting cutting-edge research in their day-to-day lectures. In true Göttingen fashion, Max Born, the famed physicist and mathematician, presented the young visitor with a pre-publication copy of Heisenberg’s paper. We are pleased to note the final pre-publication proof sheets, item corr155.1, is a part of the Ava Helen and Linus Pauling Papers.

Listen: Pauling discusses his contacts with some of Europe’s finest scientists in the mid-1920s

As groundbreaking as the Heisenberg Uncertainty Principle was, Pauling and many of his fellow scientists found the matrix approach to be frustratingly mathematical. Much of Pauling’s work was heavily influenced by Heisenberg’s discoveries and he commonly introduced some of the concepts in his lectures, but ultimately he struggled with the abstract, intangible aspects of the math-based matrix mechanics.

“Uber den anschauclichen Inhalt der quantentheoretischen Kinematik und Mechanik.” March 23, 1927.”

Erwin Schrödinger’s work, which complemented Heisenberg’s complex mathematics, was comparatively simple and conducive to visual representation. As such, it was much more widely adopted by the researchers of the day. Both individuals quickly became known as titans of twentieth-century science.

Learn more at the website “Linus Pauling and the Nature of the Chemical Bond: A Documentary History,” or by clicking on the multimedia link below.

“Valence and Molecular Structure”

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.”

A Classic of Twentieth-Century Science: The Nature of the Chemical Bond

Portrait of Linus Pauling, 1930s

Portrait of Linus Pauling. 1930s.

“I have just returned from a short vacation for which the only books I took were a half-dozen detective stories and your ‘Chemical Bond’. I found yours the most exciting of the lot.”
– G.N. Lewis. Letter to Linus Pauling. August 25, 1939.

In the fall of 1930, Pauling began work on a determination of the structure of the carbon tetrahedron, implementing the simplified version of the Schrödinger wave equation as modified by John C. Slater. Pauling’s goal was to define the atomic architecture of the carbon tetrahedron, as it was understood by chemists, in terms of quantum mechanics. Success in this venture carried with it the potential to unify chemists and physicists in their understanding of molecular bonding.

Pauling worked through the fall without any major breakthroughs until finally, in December of 1930, he made the decision to remove the radial function from his equation in an attempt to simplify the mathematics of the project. Having done so, Pauling found that the resulting wave functions, (which might be thought of as mathematical models of atomic structures derived from x-ray studies of substances) when mathematically combined, resulted in four hybrid orbitals oriented at the angles of a tetrahedron. Moreover, the mathematics demonstrated that bonds strengthened as the overlap between orbitals increased.

Pauling was elated to find that all variables could be accounted for using his new mathematical method. It was immediately clear to the 29-year-old scientist that this was a major discovery. In his own words,

“I was so excited and happy, I think I stayed up all night, making, writing out, solving the equations, which were so simple that I could solve them in a few minutes. Solve one equation, get the answer, then solve another equation about the structure of octahedral complexes such as the ferrocyanide ion in potassium ferrocyanide, or square planar complexes such as in tetrachloroplatinate ion, and various other problems. I just kept getting more and more euphorious as time went by.”

In February of 1931, Pauling mailed his paper to the Journal of the American Chemical Society. Boldly 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,” Pauling’s publication was promptly recognized to be an instant classic of twentieth-century science.

The paper defined six rules for the shared electron bond and presented his findings in uncomplicated terms, allowing his colleagues to examine his research without becoming lost in the mathematics. Pauling’s write-up was distributed in J.A.C.S. only six weeks after the manuscript arrived in the journal’s offices. The incredible speed of this turn-around from manuscript to print was abetted by the fact that the paper had not been refereed, as J.A.C.S. editor Arthur Lamb could think of no individual properly-qualified to review the revolutionary content of Pauling’s work.

Pauling’s unique combination of chemistry and quantum mechanics reinforced the accumulated knowledge of both fields and, in an invaluable breakthrough, pushed researchers toward an understanding of completely new atomic structures and properties. The importance of Pauling’s discovery only grew with time — that, and the publication by Pauling of six more papers on the topic.

In 1939, Pauling collected his research into an extremely popular textbook titled The Nature of the Chemical Bond and the Structure of Molecules and Crystals: An Introduction to Modern Structural Chemistry. The text was a huge success on many levels: not only did it describe research of fundamental importance to the study of chemistry, but it did so in a lucid style that was understandable to a wide range of users. As Max Perutz would later note, The Nature of the Chemical Bond proved that “chemistry could be understood rather than being memorized.” A contemporary of Pauling’s, Dr. Charles P. Smyth, would echo this perspective in a 1939 letter, writing:

“I have been very much interested by your new book and have assigned several of the chapters for reading in connection with a graduate course. As evidence of my interest in it, I can cite the fact that it is the first scientific book which I can remember reading during the course of a fishing trip, although I have carried many with me in the past.”

It is likewise worth noting that various editions of The Nature of the Chemical Bond can still be found for sale online, nearly eighty years after its first printing and fifteen years after it’s author’s death. While some of the research presented in its pages is now outdated, the clarity and impact of its writing insures its status as one of the great scientific publications of all time.

Read the original manuscript for the first “Nature of the Chemical Bond” paper and learn much more about this important story on the website “Linus Pauling and the Nature of the Chemical Bond: A Documentary History.”

Featured Website: Linus Pauling and the Nature of the Chemical Bond

“I consider that the field of work in which Dr. Pauling is engaged, namely the study of the chemical bond and of valence from the standpoint of modern physics, is the most important line of research in theoretical chemistry today; and I venture to believe that there is no one in the world who in the same degree has the chemical background and at the same time has the physical knowledge, mathematical power, and originality required for the handling of this problem.”
– A. A. Noyes. Letter to William Foster. October 15, 1931.

For the next month, the PaulingBlog will proudly feature a newly-updated website devoted to Linus Pauling’s research on the nature of the chemical bond. “Linus Pauling and the Nature of the Chemical Bond: A Documentary History” includes over four hours of audio tracks and video snippets, more than 2,500 manuscript pages, a chronological narrative that details the story of Pauling’s research, and nearly 100 photographs and illustrations.

The site discusses the beginnings of Pauling’s structural chemistry studies, the importance of quantum mechanics, the development and publication of “Pauling’s Rules,” and Pauling’s eventual receipt of the 1954 Nobel Prize for Chemistry, granted for “research into the nature of the chemical bond and its application to the elucidation of complex substances” and the first chemistry prize awarded for a body of work, rather than one singular discovery.

The Narrative feature of the website has been designed with an eye toward leading users through Pauling’s research in a logical and accessible fashion. The narrative provides a series of links showcasing important or interesting documents, materials related to the achievements and contributions of Pauling’s colleagues, and media integral to understanding the evolution of the chemist’s view of the chemical bond.

The unique Linus Pauling Day-by-Day calendar is likewise included. For the entirety of the 1930s as well as Pauling’s first Nobel year (1954), Linus Pauling Day-by-Day provides an organized overview of Pauling’s correspondence, containing summaries of the thousands of letters and papers that document the daily life of the scientist, his colleagues and his family.

Users may note that “Linus Pauling and the Nature of the Chemical Bond” may also be used in conjunction with the Linus Pauling Research Notebooks website as a means of accessing a more comprehensive series of documents related to Pauling’s structural chemistry research.

Finally, in technical terms, it is worth adding that the revised and expanded “Linus Pauling and the Nature of the Chemical Bond,” which was originally launched in slimmer form in 2004, is the second of the OSU Special Collections documentary history websites to operate on a METS/MODS-driven platform. A future post on the PaulingBlog will describe this back-end process in much greater depth.