The Crystal Structure of Molybdenite

[Ed note: This is the first in a six part series of posts featuring models of crystal structures solved by Linus Pauling during the early years of his career.  The models were built by two undergraduate students working in the OSU Libraries Special Collections.  More about their work can be found on page 10 of this PDF link.]

Molybdenite model, side view.

Molybdenite, MoS₂ (molybdenum = pink; sulfur = yellow).  A hexagonal crystal system constructed of molybdenum ions bonded to two layers of sulfur atoms through ionic bonding. The sulfur layers do not present strong bonds with other sulfur layers, which creates perfect cleavage.

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After moving to Pasadena to begin his graduate studies in 1922, it was decided that Linus Pauling would begin working as Roscoe Dickinson’s sole graduate student. Dickinson received the first doctorate from the California Institute of Technology in 1920, having joined the staff there after completing his undergraduate studies at M.I.T. in 1917.

In late September of 1922, Pauling and Dickinson were working on structure determinations of crystals with the use of X-ray diffraction, and Pauling was initially encouraged to determine the structure of the lithium hydride crystal. However, after three weeks of synthesizing the crystals and setting up his photographic apparatus, he was forced to abandon his work upon receiving word that the structure had recently been determined in Holland.

Pauling then created crystals of fifteen different organic substances. He subjected some of them to preliminary stages of X-ray analysis, but none of the samples proved sufficient for the work. After these disappointments, Dickinson helped Pauling work through the process of determining the complete structure of the molybdenite crystal.

Molybdenite, or molybdenum sulfide, is a very soft metallic mineral. Its properties include a bluish-silver color and a greasy feel that can leave marks on fingers. It has a very high melting point, so it is often alloyed with steel to make it stronger and more heat resistant. It is also an important material for the chemical and lubricant industries, and can be used as a catalyst in some chemical applications.

Molybdenite model, top view

Molybdenite crystals bend easily but are not elastic, making X-ray spectral photo analysis, the method used by Pauling and Dickinson, somewhat difficult. Despite these issues, Pauling and Dickinson managed to take a suitable photograph and were able to determine the crystal’s structure. Molybdenite was expected to have an octahedral atomic structure. Pauling and Dickinson discovered that it was instead comprised of six atoms surrounding the corners of a trigonal prism. This surprise demonstrated, among other things, the potential for unpredictability and excitement in chemical experimentation.

Fresh off their laboratory success, Pauling co-authored his first scientific paper with Dickinson. It appeared in a 1923 issue of the Journal of the American Chemical Society under the title “The Crystal Structure of Molybdenite.” The success renewed Pauling’s confidence in his capacity to carry out professional scientific analysis, and instilled in him an understanding of the value of well-planned experimentation. The process likewise provided him with a valuable framework that he began shaping and developing to fit his particular temperament. Throughout the rest of the 1920s and 1930s, he would use this method to write and publish multiple crystal structure papers nearly every year.

Pauling notebook entry documenting work on molybdenite, 1922.

Much of Pauling’s early work on the structure of molybdenite is detailed in his Research Notebook 2.  For more on his amazing achievements in structural chemistry, see the website Linus Pauling and the Nature of the Chemical Bond: A Documentary History.

The Paving Inspector Job

Linus Pauling (second from right) with a highway work crew, Sutherlin, Oregon. Summer 1922.

A unique chapter of Linus Pauling’s life played out over the summers of his undergraduate years at Oregon Agricultural College. A theme that had shadowed much of his young adult life – problems with finances – would continue to follow him into his graduate studies. The absence of a steady source of income, as well as short periods of more intensified financial hardship, significantly shaped the transition years between his start as an undergraduate and the beginning of his rigorous studies at the California Institute of Technology.

Pauling worked odd jobs on campus to make ends meet during the school year, but during most summers he was employed by the Oregon State Highway Commission as a paving plant inspector, living in a tent and charged with monitoring the quality of the bitumen-stone mixes used in the building of roads. His employment at the highway commission would stretch from the end of his sophomore year to the beginning of his doctoral studies. Over this course of time, particularly his final summer, distinguishing themes and aspects of Pauling’s professional life began to blossom.

Though it was not glorified work, and at times very boring, Pauling did enjoy his time working outdoors. He wrote of his love for the sun, and the benefits of spending a substantial portion of the year outside of a laboratory. Though Pauling would go on to work three additional summers for the highway commission, his first year was not without conflict. At this time he worked under the partial jurisdiction of a man named E.W. Lazell, a chemical and efficiency engineer stationed in Portland. A series of letters and reprimands from Mr. Lazell, as well as consultations with third parties, became common toward the end of Pauling’s first summer at the commission. In early September Pauling replied to department official Leland Gregory, apparently in regard to a complaint lodged against his handling of paving material temperatures. The “misinformed informant,” as Pauling referred to the unnamed complainant (Lazell), could apparently have been better informed had he referred to Pauling’s reports.

At the end of his first season with the commission, Pauling’s mother Belle informed him that she had been forced to use the money he had been sending her over the summer. The money had been meant to pay his school expenses for the following year, and with no additional funds at his disposal, Pauling chose to continue working into the fall.

Luckily, in late autumn of the same year, Pauling was offered a job by the chemistry department at O. A. C. Though it entailed a $25 per month pay cut, Pauling returned to the college as a full-time assistant instructor in quantitative analysis. The following summer he began work once again for the highway commission, and saved enough money to continue his studies as an undergraduate.

As has been well-documented, it is during Pauling’s stint as “boy professor” that he met Ava Helen Miller, his future wife, while teaching chemistry to her and twenty-four other home economics students. The two began dating toward the end of the school year, and the exchange of letters between them during Pauling’s last summer as a paving plant inspector gives one of the clearest and most intimate views of the future Nobel Prize winner’s advancing train of thought. All in all Pauling received 94 letters over the summer from Ava Miller, and replied in kind every day, sometimes two or three times.

You are my own darling girl, and your love is my only priceless possession. I shall try to make my life perfect in order that it may be good enough for you. I love your beautiful big blue eyes, your dainty little ears, your adorable own darling self. I love you.

-Linus Pauling to Ava Helen Miller, June 14, 1922.

The elements that generally defined Pauling’s correspondence with his future wife were a) their wish to be engaged, and b) the strong opposition to marriage that the two faced from their respective families. Always the romantic, Pauling was accused by some of Ava’s friends as being consistently “too mushy,” and indeed there is much written between the two about marriage, children and love.

However, over the course of their exchanges, Pauling likewise discussed much of his evolving personal philosophy. Both suggested reading materials to one another, with the bulk of the books suggested by Ava generally being metaphysical or philosophical in nature. As a result, Pauling discussed, in great detail, his perceptions of the soul, his conflicted feelings between animism and materialism, and his predisposition towards pacifism.

Money, a common theme for the duration of his undergraduate experience, also makes its presence felt throughout their correspondence. At times Pauling secretly mailed money to Ava to help finance trips to see him. He also devoted a substantial portion of his energies to trying to acquire the funds that would allow the two to marry after the summer’s end, with or without help from their parents.

Through youthful confessions, bouts of jealousy, and bold declarations, much can be gleaned about the budding relationship between Pauling and his wife-to-be. Other precursors such as Ava’s influence on Pauling’s diet, as well as his developing fascination with fruits, hint at patterns that would come to define important periods of his future life.

Hand-tinted photo of Pauling at the Sutherlin work site, 1922.

Pauling also read from his own selection of books, and took quite a liking to David Copperfield among others. Far and away, however, a major defining characteristic of his summer evenings was the time that he spent working through proof sheets of the first nine chapters of a newly revised chemistry textbook, Chemical Principles, sent to him by Arthur Amos Noyes, the head of the Division of Chemistry and Chemical Engineering at the California Institute of Technology.

Worked while stationed near the Pacific Coast at Astoria, Pauling devoured all 500 of the listed problems. After discussing his other interests with Noyes by mail, Pauling also began reading books on x-ray crystallography, a new technique being used to study the structure of crystals.  (One of these texts was X-rays and Crystal Structures by W. H. and W. L. Bragg, the latter of whom would eventually become a chief scientific rival of Pauling’s.)  Having completed his reading, and prompted by some nudging from Noyes, Pauling would begin his career as an x-ray crystallographer under the direction Professor Roscoe Dickinson at Caltech the following year.

It is clear by the end of his final summer with the highway commission that Pauling had grown weary of his summer occupation. (In an August 1922 letter to Ava Helen he writes: “I really hate working in a paving plant.  I do it just because I earn more than I would elsewhere.”) Bored, lonely and finished with the problem sets given to him by Professor Noyes, it appears that Pauling was left in an ideal state of mind to begin his graduate studies, and start what would become a brilliant career as an academic, a scientist and an activist for peace.

For more information on Linus Pauling in Oregon, check out our Oregon 150 series. For general information on Linus Pauling, please visit the Linus Pauling Online portal.

Pauling’s Methodology: X-ray Crystallography

X-ray apparatus at Linus Pauling's desk, Gates Laboratory, California Institute of Technology. 1925.

“I was very fortunate in having A.A. Noyes suggest to me, or tell me, that I was to work with Roscoe Dickinson on x-ray crystallography, determination of the structure of crystals by x-ray diffraction. This technique gave for the first time detailed information about how atoms are related to other atoms in a crystal and how far apart they are from the other atoms.“
- Linus Pauling, 1988.

As a graduate student, well before Pauling began to research hemoglobin in earnest, he spent a great deal of his time using the technique of X-ray crystallography to determine the crystalline structure of a number of inorganic compounds. Pauling recalled that at that time X-ray crystallography “was a new technique, ten years old when I began. Quite a number of structures had been determined but there was a tremendous field open, a tremendous amount of work that could be done.”

Listen: Pauling discusses the importance of X-ray crystallography to his early structural chemistry research

The young Pauling obviously reveled in the excitement of being able to use a new and powerful technology. “We have a pretty extensive collection of apparatus” he once wrote to William Lawrence Bragg, the senior author of a 1922 textbook that started Pauling on X-ray crystallographic research. Any one of Bragg’s student’s, Pauling remarked, “no matter how physical his training,” need not “be frightened at coming to a chemical laboratory” so well-stocked with mechanical apparatus.

Initially Pauling used the technique of X-ray diffraction to determine the structures of fairly simple inorganic compounds, but later, as his own expertise grew and as he discovered new sources of funding, Pauling oriented this new technology toward complex organic compounds, including hemoglobin.

What was ultimately important to Pauling was not what X-ray crystallography could tell him about the size, structure, or relative placement of atoms within a molecule, but rather, what broader theories that information could then be used to support. His growing allegiance to structural chemistry, his developing ideas about the nature of the chemical bond, and his still nascent interest in biochemical interaction were all fed by his experience of rigorously determining molecular structure through new technological methods.

Pauling’s manuscript notes concerning his early experiments with hemochromogen, for instance, indicate the wide spectrum of experimental results he had to assimilate in order to create a coherent picture of the hemoglobin molecule.

"Outline of Experiments on Hemochromagen," pg. 1. June 25, 1935.

The difficulties presented by the need to combine the information he had obtained from x-diffraction with information from other kinds of experimentation, including solubility and more traditional experimental methods, are readily apparent in Pauling’s notes.  Indeed, the impressive new technology of X-ray crystallography is relegated to just one entry in a list of experimental results.

Ultimately it wasn’t the technology at Pauling’s disposal that helped him become such a successful researcher, but rather his attitude in approaching technology and his ability to use the results it gave him to construct more broadly-applicable and intellectually-powerful theories.

To learn more about Linus Pauling’s use of x-ray crystallography, see the websites Linus Pauling and the Nature of the Chemical Bond: A Documentary History and It’s in the Blood!  A Documentary History of Linus Pauling, Hemoglobin and Sickle Cell Anemia.

The Paulings’ Wedding Anniversary

Linus and Ava Helen Pauling, wedding photo, June 17, 1923.

“I suppose that I am responsible to some degree for Linus’s deciding to put so much of his effort into peace activities. In talking with him, I said I thought that it was of course important that he do his scientific work. But if the world were destroyed, then that work would not be of any value — so he should take part of his time and devote it to peace work.”
- Ava Helen Pauling. Interview. June 1977.

Today marks the eighty-fifth wedding anniversary of Linus and Ava Helen Pauling. In honor of the occasion, we would like to briefly share the story of their meeting, courtship and marriage.

On January 6, 1922, Linus Pauling, still an undergraduate, entered a classroom as instructor rather than student. Oregon Agricultural College, now Oregon State University, had hired him to teach a freshman level chemistry course to a class of home economics majors. Thomas Hager, a Pauling biographer, tells us:

[H]e knew the best way to avoid any “boy professor” sniggering was to get right to the subject. This was the second term of a three-term course, and he decided to start by measuring the class’s basic knowledge. “Will you tell me all you know about ammonium hydroxide, Miss…” He ran his finger down the registration sheet, looking for a name he couldn’t possibly mispronounce. “Miss Miller?” He looked up and into the eyes of Ava Helen Miller. She was a small, delicate, strikingly pretty girl with long, dark hair. She was barely eighteen years old. She was a flirt. And, as it turned out, she knew quite a bit about ammonium hydroxide. (Force of Nature: The Life of Linus Pauling. New York: Simon & Schuster, 1995. 69.)

In the months that followed, a relationship between the two blossomed and, at the end of the term, Pauling asked Ava Helen to marry him. She accepted. That fall, Pauling departed for Caltech where he continued his education and served as a teaching assistant. The couple corresponded regularly, but the distance between them grew unbearable. Against the wishes of both mothers, they chose to cut their engagement short and marry in the spring of 1923.

To make the trip up from southern California for the ceremony, Pauling purchased a Model T Ford from Roscoe Dickinson, a Caltech professor, and headed north for Oregon. Unfortunately, Pauling’s driving experience was limited to just a few minutes of practice and, come nightfall, he crashed into a roadside pit in the Siskiyou Mountains, resulting in an injured leg and a wrecked car. After waiting all night for help, Pauling was able to get his car repaired and arrived at the wedding on time.

Over the next six decades, the couple only grew closer. Together they raised four children, were leading activists for world peace, and were extremely instrumental in the creation of legislation banning the above-ground testing of nuclear weapons. Despite the pressures of Pauling’s work and activism, the couple remained inseparable until Ava Helen’s death in 1981.

In interviews, Pauling often cited his wife’s intelligence, good sense, patience and kindness as the foundation for many of his greatest achievements.

A plaque now hangs in Education Hall Room 201 on the Oregon State University campus, marking the location where Ava Helen Miller and her future husband first met.

For further information, visit the Linus and Ava Helen Pauling Papers.