by Thomas Hager
Pauling quickly decided against relocating. But before turning the offer down, he used it to get one more thing: the chance to work closely with G. N. Lewis. A few days after he returned from his eastern trip, the Caltech executive council granted Pauling, upon his request, an annual leave of absence of at least one month during the academic year "for the purpose of giving advanced courses of lectures or seminars at the University of California or elsewhere," expandable to a full academic term in alternate years—along with five hundred dollars per year to cover travel costs. The next day, Pauling declined Harvard's offer, then wrote G. N. Lewis, "In deciding to stay at the Institute the prospect of coming occasionally to Berkeley probably had some effect, as had perhaps also your own opinion regarding the matter. At any rate, I have regained my peace of mind."
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Pauling was not the only sought-after young professor. Another was J. Robert Oppenheimer, the young American physicist Pauling had met in Munich. In 1928, Millikan talked Oppenheimer into teaching physics for part of the year at Caltech, the remaining time to be spent at Berkeley, much like Pauling's new deal.
Oppenheimer made an immediate impression in Pasadena. Thin, almost frail in appearance, with strikingly large, wide-set eyes and a head of thick, dark hair, he was attractive as well as brilliant. Although raised in New York, he seemed exotically European, Bohemian, poetic, chain-smoking, prone to exotic literary and philosophical references. His only shortcoming seemed to be that he was a dismal lecturer, mumbling, scattering cigarette ashes, talking over the heads of his listeners, and packing the blackboard with cramped, barely readable equations. Despite that, he soon attracted a devoted band of acolytes, some of the West Coast's finest students, who were able to cut through the obscurity to the essentials of the new physics and who began following him on his annual trek between Pasadena and Berkeley. He was pursued, too, by scandalous rumors (which he seemed disinclined to squelch), hints of free love—perhaps homosexuality—and radical politics.
Pauling and Ava Helen found him witty, attractive, and a welcome antidote to the deadly dullness of most Caltech faculty members.
They were all the same age, all young and brilliant, and all on the way up. The Paulings and the young physicist quickly became close friends. They shared dinners and jokes, talked about European physics, and gossiped about Caltech and Berkeley professors. Oppenheimer came to Pauling for advice on how to become a better lecturer, and Pauling sought him out to talk about quantum mechanics. The two of them began to consider mounting a joint attack on the chemical bond, with Oppenheimer working on the mathematics and Pauling providing the chemical insights.
Perhaps they became too close too fast. Something began to seem odd to Pauling. Oppenheimer not only adopted some of Pauling's lecturing style; he began wearing an old fedora around campus, much like one that Pauling wore. He started to give Pauling gifts, sometimes little ones, a favorite ring on one occasion, and on another, a magnificently extravagant one, Oppenheimer's large boyhood mineral collection, the crystal treasury that had first spurred Oppenheimer's interest in science, a thousand fine specimens, including some fine calcites in which Pauling took special interest. Then there were the poems Oppenheimer gave Pauling, verse that Pauling found both obscure and troubling, mixing classical allusions with lines about mineralogy, Dante, and pederasty. Pauling had never had a friendship like this.
Neither had Ava Helen. She enjoyed Oppenheimer enormously, took pleasure in talking with him and flirting a little with him, as she did with almost everybody on social occasions. Perhaps she flirted a little more than usual, for Oppenheimer was unusually intriguing. Perhaps he felt her interest went beyond a casual friendship. It all went a little too far, in any case, when Oppenheimer approached her one day in 1929 when Pauling was at work and proffered a clumsy invitation to join him on a tryst in Mexico. Surprised and flattered, Ava Helen told him no, of course not, she was married and took it seriously. That night, she reported the whole thing to Pauling. "I think she was somewhat pleased with herself as a femme fatale," Pauling said. Perhaps she was a little too pleased. Pauling cut off his relationship with Oppenheimer, ending any chance of collaboration on the chemical bond and initiating a coolness between the two men that would last the rest of their lives.
Years later, Ava Helen told her husband, "You know, I don't think Oppenheimer was in love with me. I think he was in love with you." After mulling it over, Pauling concluded that she might be right.
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The loss of Oppenheimer left Pauling without help in his quest to make mathematical sense of the tetrahedral bonds of carbon. The way to solve the problem—the way all problems involving the application of quantum mechanics to multiatomic systems had to be solved—was to find a set of shortcuts, approximations of terms in the wave equation, that would simplify the mathematics enough to allow progress without distorting the results too much. Pauling tried again and again to batter through the mathematics, but nothing worked.
In late 1929, stymied with the chemical bond problem, he decided to return to Europe, visit old friends in Munich, tour crystallography laboratories and get some advice on the carbon problem. After his application for a Guggenheim Foundation fellowship was turned down, he negotiated the needed travel funds from Caltech.
He and Ava Helen landed in England in May 1930, this time with five-year-old Linus junior in tow, and set off for their first scientific stop, the world's foremost center of x-ray crystallography, the Manchester laboratory of Lawrence Bragg.
Pauling had great hopes for his first meeting with Bragg. Their correspondence had been cordial, even warm, following the publication of Pauling's rules, with Bragg writing, "Your method certainly led you to the ideal structure! ... I like your way of looking at these coordination compounds very much indeed," and Pauling becoming uncharacteristically buttery: "My wife and I think of you often. Our favorite daydream has for its theme a visit to Manchester."
The dream soured when they arrived. While Bragg was personally accommodating—setting up a flat for them, finding a maid, and arranging care for Linus junior—he was professionally distant. To Pauling's surprise, during the weeks they were there he was never asked to speak with Bragg about scientific matters, nor was he invited to give a seminar on his work, as would have been standard for a visiting professor at Caltech. "I had essentially no contact with Bragg," Pauling said, summing up his Manchester visit as "a disappointment." Although he felt confused and slighted, he tried to shrug it off, blaming it initially on Bragg's busy schedule.
Later, though, he heard from other researchers that Bragg, despite his flattering letters, thought of the young American as an unwelcome poacher in his scientific domain. Bragg had been beaten at what he thought was his own game, and he was chagrined by the experience. "I did not know then, and in fact not for many more years, that Bragg thought of me as a competitor," Pauling later wrote. There was more than that, too. Bragg's personal worries and the weight of his responsibilities were bringing him to a crisis point. A few months after Pauling left, he would suffer a nervous breakdown. If he was distant from Pauling, it was in part because he was involved in a battle with himself.
Whatever the reasons, this first visit would cast a pall over their future relationship. After Pauling returned to America, correspondence between the two fell off rapidly. He and Bragg would spend the rest of their lives more cordial competitors than close colleagues.
Not every English researcher, however, treated Pauling as Bragg had. His hurt feelings were somewhat assuaged when the mercurial English crystallographer John Desmond Bernal asked him to give a seminar at Cambridge on the rotational motion of molecules in crystals. Then it was on to Germany, where Pauling had a pleasant time visiting friends and catching up on the latest developments. He settled down for almost three months of work in Munich, hacking away at his attempt to ease the use of quantum mechanics to explain the chemical bond, getting some help from Sommerfeld but not succeeding in making any major advances.
Pauling, however, was overwhelmed by what he had heard. For some time he had been looking for a way to examine the structure of individual molecules without having to worry about the sometimes very complicated way in which they arranged themselves into crystals. Mark and Wierl's apparatus, focusing on separate molecules in a gas rather than ones welded together in a larger structure, offered a way to eliminate one level of complexity in calculating structures. Because the exposure time for electron-diffraction photographs was a few tenths of a second—rather than the hours sometimes required for x-ray crystallography—the range of substances available for structural study could be expanded to include volatile substances, especially small organic compounds difficult to hold in crystalline form. "As the impact of the significance of this discovery burst upon me I could not contain my enthusiasm, which I expressed to Mark—my feeling that it should be possible in a rather short time, perhaps ten years, to obtain a great deal of information about bond lengths and bond angles in many different molecules," Pauling said. Mark, a bit surprised by the impression his device seemed to have made, gave Pauling a set of plans for its construction and his blessings for its use.
When he returned to Pasadena in the fall of 1930, Pauling immediately put a new graduate student, Lawrence Brockway, to work building an electron-diffraction machine. It took two years to get it running properly, but it eventually became a workhorse of Pauling's laboratory and one of the most important scientific tools at Caltech. Over the next twenty-five years, Pauling, and his students and coworkers, would use it to work out the structures of some 225 molecules.
"Euphorious"
Back home in the fall of 1930, Pauling returned to the problem of the tetrahedral carbon atom. His European trip had not helped him make any important steps forward, but he found something when he returned that did.
That year, a young American physicist named John C. Slater had found an important simplification of the Schroedinger wave equation that made it possible to better picture carbon's four binding electrons. Spurred by Slater's work, Pauling picked up his pen and started making calculations again in earnest. In order to match the chemists' reality of a carbon tetrahedron, the physicists' two sets of electron subshells had to be broken and mixed together somehow in a new, equivalent form. The central problem was finding appropriate mathematical approximations of the wave function, shortcuts that would make manageable the equations for combining the subshells' wave functions.
For weeks through the fall, though, none of Pauling's shortcuts worked. Then, on a night in December 1930, sitting at his desk in the study of his home, he tried one more approximation. This time in trying to combine the two subshells' wave functions, he chose to ignore a part of the mathematics called the radial function, a simplification that Slater's papers indicated might work. By stripping away that layer of complexity, Pauling was surprised to find that "the problem became quite a simple one from the mathematical point of view"—at least, for a Sommerfeld-trained quantum physicist.
He could now, with the right coefficients, combine the wave functions of the physicists' two carbon subshells into a mathematical description of a new hybrid form: four equal orbitals oriented precisely at the angles of a tetrahedron. Not only that, but his new hybrid orbitals were more highly directed away from the nucleus, capable therefore of overlapping more with the orbitals of other electrons from other atoms. And here was a basic insight: The greater the overlap of orbitals from two atoms, the more exchange energy was created and the stronger the bond.
He had a sudden rush of energy. From the principles and equations of quantum mechanics he had formed a tetrahedral carbon atom. The calculated angles between bonds were right; the bond lengths looked right; the energy needed to change the electron subshell orbitals into their new shapes was more than accounted for by the energy of the electron exchange.
He kept working for hours. Using the same basic approach, he found he could add more electrons to his calculations and derive the features of more complex molecules. The ability to hybridize the physicists' subshells into new orbitals opened the door to explaining the structure of a number of molecules, such as the bonding pattern found in certain cobalt and platinum compounds. One by one, under Pauling's pen, the physicists' new mechanics was proving out the chemists' ideas. "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," he remembered. "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."
Over the next two months he worked hard polishing and expanding his findings into what would become one of the most important papers in the history of chemistry. In it he presented six rules for the shared electron bond. The first three, restatements of Lewis's, Heitler's, London's, and his own earlier work, noted that the electron-pair bond was formed through the interaction of an unpaired electron on each of two atoms; that the spins of the electrons had to be opposed; and that once paired, the two electrons could not take part in additional bonds. His last three rules were new. One stated that the electron exchange terms for the bond involved only one wave function from each atom; another, that available electrons in the lowest energy levels would form the strongest bonds. Pauling's final rule asserted that of two orbitals in an atom, the one that could overlap the most with an orbital from another atom would form the strongest bond and that the bond would tend to lie in the direction of that concentrated orbital. This allowed the prediction and calculation of bond angles and molecular structures.
Appropriately for his audience of mathematics-shy chemists, Pauling did not present lengthy mathematical proofs of his rules, for, as he wrote in the paper, "even the formal justification of the electron-pair bond in the simplest cases . . . requires a formidable array of symbols and equations." But he outlined the way others could work through the proofs and presented a number of examples of his reasoning at work. From the principles of quantum mechanics he was now able to derive everything from the strengths and arrangements of bonds to a complete theory of magnetism in molecules and complex ions. Using his new system, Pauling was also able to predict new electronic structures and properties for atoms. Quantum mechanics, in other words,
did not just confirm what was already known; it pointed the way to new insights. In mid-February 1931, Pauling mailed his work to the JACS. He titled the paper, somewhat grandly, "The Nature of the Chemical Bond."
His happiness was compounded by the nearly simultaneous birth of his second son, Peter Jeffress (his middle name honoring Pauling's boyhood friend), on February 10.
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Then came a shock. On March 1—two weeks after Pauling had submitted his new work but a month before it was published—a paper appeared in the Physical Review that covered much of the same ground, including the idea of maximum overlapping of wave functions to create the most stable bonds; a discussion of the relationship between ionic and covalent bonds; a description of how, in compounds where there are several ways of drawing valence bonds, it was likely that "the real situation is ... a combination of the various possibilities, and on account of resonance the energy is lower than it would otherwise be"; and, most importantly, an explanation of the tetrahedral bond in carbon. The author was the physicist whose work had helped inspire Pauling's breakthrough, John Slater.
