The Story of “The Nature of the Chemical Bond”: Coordinating Research & Funding

[Ed. Note: This year marks the 75th anniversary of Linus Pauling’s publication of his landmark text, The Nature of the Chemical Bond.  For the next six weeks we will take a detailed look at the creation, release and impact of a book that changed the scientific world.]

Linus Pauling’s The Nature of the Chemical Bond, first published in 1939, was the product of over two decades of diligence, sacrifice, and collaboration among a broad range of actors that included Pauling’s family, research assistants, professional colleagues and a variety of institutions. Pauling’s prefatory remarks to the book – “For a long time I have been planning to write a book on the structure of molecules and crystals and the nature of the chemical bond” – give an indication of the extent to which this was a long-term objective for Pauling, despite his being only 38 years old.

Looking back at his process, Pauling’s application for a grant from the Carnegie Institute in February 1932 provides a more detailed affirmation of his ambitions. In it, Pauling relayed how his undergraduate research in crystal structures at Oregon Agricultural College between 1917 and 1922 had laid the foundation for his current work by bringing him into contact with contemporary questions in structural chemistry. As a graduate student at Caltech, Pauling began to search for answers to those questions in the newly developing field of quantum mechanics.

In pursuit of those answers, Pauling and his wife Ava Helen, with the support of a Guggenheim Fellowship, left their one-year-old son, Linus Jr., with Ava Helen’s mother in Portland and traveled to Europe in 1926 to study quantum mechanics at its source. There, Pauling deepened his understanding and immersed himself even more by beginning to apply the new physics directly to chemical bonding.

J. Holmes Sturdivant

Upon returning to Caltech in 1927, Pauling began to seek funding so he could continue what he had begun. Let down by the National Research Fund, Pauling supported his work with funding from Caltech and the National Research Council, money which allowed him to hire a full time assistant, J. Holmes Sturdivant, who focused on x-ray crystallography and continued to work with Pauling for many years. Pauling also brought aboard Boris Podolsky for nine months to assist him with the more detailed technical components of connecting quantum mechanics to chemical bonding.

In 1932 Pauling expressed a hope that, with help from the Carnegie Institute, he could expand his work by funding more assistants and purchasing equipment like an “electric calculating machine,” a “specialized ionization spectrometer,” and a microphotometer. The Carnegie Institute was not interested. Luckily for Pauling, the Rockefeller Foundation came through with a general grant of $20,000 per year over two years, to be split between the physics and chemistry departments at Caltech. This allowed Pauling to keep Sturdivant on staff while adding George Wheland, Jack Sherman, and E. Bright Wilson, Jr. to his research team.

This scramble to secure funding and bring new people into the lab came amidst the publication of Pauling’s first four “Nature of the Chemical Bond” articles for the Journal of the American Chemical Society, proof positive that Pauling’s work was bearing fruit. Once the funding was secured and Sherman and Wheland began producing results, Pauling wrote – with Sherman and Wheland as co-authors – three more “Nature of the Chemical Bond” articles the following year, published in the newly established Journal of Chemical Physics. Wheland also worked with Pauling on a monograph discussing the application of quantum mechanics to organic molecules. Wheland finished his part of the book by 1937, but Pauling never got around to his portion: his desire to write a book length treatment of chemical bonds began, more and more, to take center stage.

Warren Weaver

In order to keep the funding coming in through the lean years of the Great Depression, Pauling was compelled to follow the lead of his patrons, the Rockefeller Foundation. Warren Weaver, Director of Natural Sciences for the foundation, told Pauling in December 1933 that the organization was “operating under severe restrictions” and that funding would go to projects “concentrated upon certain fields of fundamental quantitative biology.” That Pauling’s work had “developed to the point where it promises applications to the study of chlorophyll, haemoglobin and other substances of basic biological importance” was key to his potential receipt of continued dollars.

The commitment of Caltech’s chemistry department to continue pursuing the line of research suggested by Weaver helped Pauling to secure funding for the following year. A three-year commitment came after that, providing the Caltech group with a reliable source of support into 1938. Pauling thanked Weaver in February of that year for his direction, writing,

I am of course aware of the fact that our plans for organic chemistry not only have been developed with the aid of your continued advice but also are based on your initial suggestion and encouragement; and I can forsee that I shall be indebted to you also for the opportunity of carrying out on my own scientific work in the future to as great an extent as I have been during the past six years.

Secure funding allowed Pauling to maintain a research group consisting of graduate students and post-doctoral fellows. In his preface to The Nature of the Chemical Bond, Pauling expressed his gratitude to several of these individuals, including Sherman and Sturdivant. Another, Sidney Weinbaum, earned his doctorate under Pauling and continued on afterwards, helping Pauling with quantum mechanical calculations and molecular structures.

Fred Stitt worked as research fellow with Pauling and assisted him in teaching his graduate course on the applications of quantum mechanics to chemistry – an exercise, no doubt, that helped to shape Pauling’s own thoughts on the subject, crystallizing them in preparation for the book.

Charles Coryell and Linus Pauling, 1935.

Charles Coryell and Linus Pauling, 1935.

Charles Coryell worked as a research fellow at the Caltech lab with Pauling on the topic of magnetic susceptibilities, which were central to investigating chemical bonds.  (Coryell also later helped Pauling to construct a magnet for the Caltech labs, based on one already in place at Cornell.)

Edwin H. Buchman, according to a 1985 oral history interview, was self-supporting due to royalties from his synthesis of vitamin B1. Buchman told Pauling in May 1937 that he would assist Pauling “on any problem in which an organic chemist could be useful and for which extra space could be had.”

Once assembled, Pauling’s team helped him to refine his understanding of chemical structures and bonding as the time approached when he could produce a book-length treatment on the subject.


The Business of Detection

Diagram of the Precipitation Apparatus, Smoke Particle-Size Project. approx. 1943.

Our Section L-1 on Aerosols has been set up to handle problems dealing with both offense and defense against toxic smokes. In connection with that program they have naturally run into the old problem of measurement of particle size and particle-size distribution, and have employed two or three of the more promising optical and microscope techniques in this connection… I wonder if you would give some thought to possible new methods of attacking this problem…

James B. Conant, letter to Linus Pauling, June 13, 1941

Following his work with the oxygen meter, Pauling and his right hand man, J. Holmes Sturdivant, were asked to create a carbon monoxide (CO) detection device. This new meter was intended to measure the levels of carbon monoxide in the air.

Carbon monoxide, a colorless, odorless, and tasteless gas, bonds to hemoglobin in the human bloodstream. In the process, carboxyhemoglobin (also known as carbonmonoxyhemoglobin) is created, preventing delivery of oxygen to body tissue, a complication eventually resulting in brain damage and death.

In high temperature environments where there exists an abundance of carbon, the combustion of carbon releases nitrogen and carbon monoxide. Certain tank cabins and airplane cockpits, in which CO could be released by the repeated firing of the vehicle’s weapons systems, were particularly subject to high levels of CO accumulation, sometimes leading to poisoning of the crew. As a result, the U.S. military needed a means of quick-testing air samples for CO saturation.

Pauling and Sturdivant realized early on that traditional chemical indicators like iodine simply weren’t well suited to the apparatus’ requirements. After only a few days of research, they discussed the possibility of using an organic substance as an indicator – hemoglobin seemed a likely candidate. After some calculations and a small amount of experimentation, they were ready to build a prototype.

J. Holmes Sturdivant, 1950.

The CO measurement apparatus contained a sample of hemoglobin molecules bonded with oxygen, otherwise known as oxyhemoglobin. When this sample came into contact with carbon monoxide, carboxyhemoglobin was created. The sample was then measured by a spectrophotometer, providing the user with a reading of the immediate environment’s carbon monoxide saturation in parts per million. Because the conversion of oxyhemoglobin to carboxyhemoglobin was reversible, the apparatus was capable of making consecutive readings without maintenance.

Moving from theory to practice proved a challenge however, as the duo soon found it more difficult than expected to build a working spectrometer that could accurately read carboxyhemoglobin levels. Between October 1942 and November 1943, Pauling and Sturdivant built several spectrometers, attempting to calibrate them in such a way that other environmental changes, including the addition of pure oxygen, would not disrupt the readings.

In December 1943, the pair had taken the project as far as possible. On many levels, the apparatus had proved itself inadequate for use in the field. For one, it was bulky and thus unsuitable for use in confined spaces like tank cabins and cockpits. Worse, it was too fragile to survive transport to the Pacific or European theaters, much less the strain of battlefield conditions. Finally, the device was hopelessly inaccurate unless used under specific, controlled conditions. All of these factors rendered it useless for its intended purpose, and the project was stopped dead in its tracks.

Disappointed, Pauling released his final report and quickly retired the project. In an attempt to find some justification for the hundreds of hours of labor that went into the device, Pauling noted that the apparatus was well-suited for use in the laboratory. Indeed, when deployed in a stable environment, the device was highly accurate and very useful for rapid measurements. As a result, the few meters that were produced before the closure of the project were distributed among researchers at Caltech for their own implementations.

Thanks to his work with the oxygen and carbon monoxide meters, Pauling was considered something of an expert on testing gaseous mixtures. In July 1942, while he was still working with both the oxygen and CO devices, he was asked to develop a field-use apparatus for identifying specific toxins in air samples. The task was complex, but Pauling thought that air contaminants could be identified by their representative particle size. Particle sizes could be determined by electrically charging a group of particles and then drawing them to a condenser plate. According to Stokes’ Law, the rate at which these particles are attracted to the plate is inverse to the radius of the particle. By examining this grouping, Pauling argued that it would be possible to estimate the composition of the smoke being analyzed.

Encouraged by some early calculations, Pauling set a lab assistant, Charles Wagner, to the task of making preliminary measurements. His results were positive and Pauling chose to move the program forward. By 1943 he was overseeing a group of men making calculations, building the apparatus, and creating stable smokes for the testing process, and by early 1944 the team was ready to put the device through its paces.

Diagram of the Filament Charging Device, Smoke Particle-Size Project. approx. 1943.

The initial tests were not good. The condenser plates were causing a bizarre phenomenon in which the largest particles were being grouped with the smallest, resulting in a highly inaccurate reading. And that was just the beginning of the team’s problems. They soon found that unfiltered air, such as that found in standard field conditions, contained a vast range of particles. In addition to the smoke or fog meant to undergo analysis, a typical sample could also contain dust, industrial pollutants, and natural contaminants like pollen. The distribution of precipitated particles was already making analysis difficult with clean samples; adding a host of impurities to the sample so complicated the results that an accurate determination was impossible for professional scientists. It was clear that a soldier in the field would be unable to operate the instrument effectively.

On March 28, 1944, Pauling filed his final report on the Particle Size Measurement Apparatus, number OEMsr-103. The project, he surmised, was a failure. In his final write-up for the Office of Scientific Research and Development, Pauling suggested that the apparatus might be reworked to give a more accurate reading under controlled laboratory conditions. While he frankly admitted that the instrument could “hardly be perfected for field use,” he hoped that his work and that of his fellow researchers had not been conducted in vain.

In hindsight, Pauling’s work with testing gaseous samples was doomed by the available technology. Complex instruments capable of operating in field conditions are difficult to engineer even today. Without access to microchips, solid-state computing equipment, or high-tech manufacturing processes, certain precision instruments were simply unachievable during World War II.

The Crystal Structure of Brookite

Brookite model, side view.

Brookite, TiO2 (titanium = grey, oxygen = red).  An orthorhombic unit constructed by an octahedron of oxygen ions arranged about a single titanium ion. Each octahedron shares three edges with adjoining octahedra.

After returning from a trip to Europe in 1927, Linus Pauling was appointed to the position of Assistant Professor of Theoretical Chemistry at Caltech, and reinitiated his study of crystal structures in Pasadena. During this time, Pauling was focusing his attention on the crystal structures of silicate minerals. He and other scientists were utilizing X-ray analysis for crystal structure determinations, but the limitations of the technique were becoming more and more apparent as it was applied to increasingly complex crystal structures.

To overcome these difficulties, Pauling formulated a new theory which helped him to determine the structure of brookite, topaz and a number of other complex ionic crystals. The theory, and the work that resulted from it, comprised an important step in the development of his most cited and most used crystal structure work.

Pauling’s new development was called coordination theory, and served as a method for predicting the possible structures of ionic compounds. Pauling contrasted the new theory with another method used previously by crystallographers for similar crystal structure determinations. The previous method involved testing and eliminating all but one of the possible arrangements in order to determine the atomic arrangement of particular crystals. Pauling noted that this method was both very certain in its results, and extremely labor intensive, making it difficult to apply to more complex compounds.

Brookite model, top view.

Pauling’s new method utilized sets of rules to both dismiss unlikely potential chemical structures and to hypothesize an atomic arrangement that would likely match the crystal structure of the compound being examined. Pauling and his associates would then compare the hypothesized arrangement to experimental observations. The new theory closely resembled an earlier technique formulated by William Lawrence Bragg, which applied a method called close-packing. (In this letter, Pauling discusses his crystal structure work with Bragg, including his research on the structure of brookite.)

Pauling first used his extended version of the technique to successfully determine the structure of brookite.  Brookite, or titanium oxide, is a minor ore of titanium and a polymorph with two other minerals. It shares a number of similarities with the minerals rutile and anatase, having the same chemistry but a different structure.  The variety of similar structures largely results from exposure of the basic chemical components to different temperature variations. As such, when exposed to higher temperatures, brookite reverts to the chemical structure of rutile.

J. Holmes Sturdivant, 1948

In their examination of brookite, Pauling and J. Holmes Sturdivant used spectral photographs to determine the dimensions of the possible unit cells, and Laue photographs to determine the smallest possible unit and space group symmetry criteria. Using Pauling’s new coordination theory, they predicted two possible structures for brookite. One of these hypothesized structures turned out to have a space-group symmetry and unit cell matching the spectral and Laue photograph observations. From there, Pauling and Sturdivant were able to determine that the basic unit of arrangement in brookite was that of an octahedron of oxygen ions around a titanium ion.

Following his examination of brookite, Pauling later used coordination theory to determine the structure of topaz. After these successful examinations, he was compelled to develop a set of principles which governed the structures of complex ionic crystals. The principles were described in a set of compiled documents known as the Sommerfeld festschrift papers, and would later be known as “Pauling’s Rules”. Pauling used his examinations of brookite and topaz, as well as the principles developed in their determinations, to write a paper that detailed this work. “The Principles Determining the Structure of Complex Ionic Crystals,” [J. Am. Chem. Soc. 51 (April 1929): 1010-1026.] published in 1929, became the most cited and most used of all of Pauling’s crystal structure papers.

For more on Pauling’s achievements in structural chemistry, see Linus Pauling and the Nature of the Chemical Bond:  A Documentary History.

Pauling’s Rules

Studio portrait of Linus Pauling. 1930.

Studio portrait of Linus Pauling. 1930.

“I am enclosing a copy of a manuscript which Mr. Sturdivant and I have prepared, dealing with the structure of brookite. We feel rather confident in our structure, and are pleased to have begun work in the field which you recently opened — the study of complex ionic crystals.”

– Linus Pauling. Letter to William Lawrence Bragg. May 31, 1928.

X-ray diffraction, as discovered by Max Theodore Felix von Laue, is the process of examining a crystalline substance by tracking the scattering of x-rays upon contact with a given material. The process goes something like this: An x-ray photograph is taken, releasing x-rays which then interact with the sample and subsequently interfere with one another. This interference results in an image, known as a Laue photograph, of a diffraction pattern in which the x-rays that have passed through the crystal appear as small black dots. A trained crystallographer can then use this photograph as the basis for deriving the molecular structure of the sample crystal.

In the late 1920s, x-ray diffraction appeared to have reached the peak of its usefulness. Crystallographers had pinpointed the structure of most simple, few-atom crystals and were left to struggle with increasingly complex molecules. Unfortunately, with the addition of only one or two atoms, a crystal’s structure became considerably more difficult to derive. In complex molecules, the diffraction patterns were much more intricate, allowing for a large number of theoretically possible structures. Crystallographers, with the help of their lab assistants, were forced to wade through pages of complex mathematics in search of the correct structure. Pauling and J. Holmes Sturdivant, who were working together on complex crystals, had taken to hiring teams of students to crunch the calculations necessary for this sort of approach.

Pauling was dissatisfied with this process and felt that there had to be another way to attack the problem. He noted that many researchers involved in the field had discovered similar molecular structures and bonding patterns between different crystals, which suggested a limited number of structural possibilities. With this in mind, Pauling believed it possible to develop a guide which would help researchers derive molecular structures of complex crystals via the x-ray diffraction technique.

Supplementing his knowledge of crystalline structures and quantum mechanics with existing research, Pauling attacked the problem. In a short time, he was able to develop five simple guidelines for eliminating scores of theoretical structures, thereby greatly reducing the difficulty of solving molecular structures.

Pauling’s Rules, first published in 1928 as a part of his paper “The Principles Determining the Structure of Complex Ionic Crystals,” are still considered valid by today’s scientific community. They are as follows:

1. A coordinated polyhedron of anions is formed about each cation, the cation-anion distance is determined by the sum of ionic radii and the coordination number (C.N.) by the radius ratio.

2. An ionic structure will be stable to the extent that the sum of the strengths of the electrostatic bonds that reach an anion equal the charge on that anion.

3. The sharing of edges and particularly faces by two anion polyhedra decreases the stability of an ionic structure.

4. In a crystal containing different cations, those of high valency and small coordination number tend not to share polyhedron elements with one another.

5. The number of essentially different kinds of constituents in a crystal tends to be small.

After developing these rules, Pauling began to apply them in his own research. In 1929 and 1930, he worked at solving the structures of groups of silicates, including but not limited to mica and talc. Using his new system of rules, as well as an x-ray powder diffraction apparatus that he had built, Pauling was able to decipher previously unknown bonding patterns. His work with zeolites, for example, uncovered the basis of their unique gas-absorption properties, a problem that had baffled many of his contemporaries.

Pauling’s Rules propelled the young researcher to the forefront of the crystallographic community. In a very short time, he had become a major player in a reputable branch of structural science. Moreover, his use of both the crystallographic and quantum mechanical disciplines hinted at a possible meshing of the fields unlike anything seen before. The young scientist was well on his way to international fame on a grand scale.

Learn more about Pauling’s Rules on the website “Linus Pauling and the Nature of the Chemical Bond: A Documentary History.”