by Thomas Hager
He went back to bed. Pauling kept quiet about his doodles, not even writing Corey about them, filing the idea of spirals away for more exploration when he got back to Caltech. For the moment, he felt, all he had was "just a piece of paper."
The Tortoise and the Hare
The sinus infection hung on for weeks, still bothering Pauling when he arrived in France with his family to give a series of scientific talks. They took an apartment in Paris, where he managed to convince a physician at the American embassy to prescribe some penicillin. He quickly recovered, and as Ava Helen and the children sallied forth to explore parks, museums, bakeries, and cathedrals, Pauling began dazzling the French scientific world.
At one of his meetings, the highlight was to be a full day devoted to comparing the advantages of Pauling's valence-bond approach to molecular chemistry to the molecular-orbital approach championed by the University of Chicago's Robert Mulliken.
For the growing community of quantum chemists, this was something akin to seeing the Pope square off with Martin Luther. Starting with the same gospel, the received wisdom of quantum mechanics, the two men had for nearly twenty years championed different interpretations of its chemical meaning.
In Pauling's approach, derived from the electron-interchange idea of Heitler and London, molecules were aggregates of individual atoms, each linked to its neighbors by bonds formed by electrons localized between the two nuclei. The number of bonds equaled the element's valence, or bonding capacity, and the Pauling school had come to be known as the valence-bond, or VB, approach. In theory, the total quantum-mechanical state of a molecule could be calculated by adding together the wave functions that were involved in each bond, with appropriate adjustments for the effect of each bond upon its neighbors.
During the nearly two decades that Pauling spent promoting the VB approach, Mulliken had patiently worked on his own molecular orbital (MO) theory, an approach predicated on a belief that molecules were not what VB advocates thought they were. Molecules to Mulliken were not aggregates of distinct atoms connected by distinct bonds but things unto themselves, with their own odd behavior explicable only in molecular terms. His experience studying the spectra of light absorbed and emitted by molecules had convinced him that molecules could be more profitably viewed as if their binding electrons were somewhat delocalized and spread across the surface. It was a view anti-intuitive to most chemists, but one that twenty years of work had convinced Mulliken was right. Molecules to Mulliken were what they showed themselves to be, not what nineteenth-century chemists thought they ought to be. Borrowing from Gertrude Stein to encapsulate his philosophy, he said, “A molecule is a molecule is a molecule."
The battle between the VB and MO approaches for the hearts and minds of chemists had been lopsided, in great part because of the force of Pauling's intellect and personality. Pauling knew how to present his VB ideas in ways that appealed to chemists, and that correlated with the way they thought of chemical bonds, as atoms joined to each other one at a time through individual links that could be represented on paper by dashed lines between elemental symbols. Just as important was his ability to figure out shortcuts that simplified the mathematical picture. While the idea of summing a number of separate wave functions to create a quantum-mechanical picture of a molecule was sound in theory, in practice the difficult mathematics made it impossible to demonstrate in precomputer days for any but the simplest molecules. Pauling had gotten around that by improvising semiempirical variations on the VB theme, such as the calculation of resonance energy and the electronegativity scale, devices that worked in the spirit of quantum mechanics but were based as much on Pauling's imaginative way of explaining and organizing laboratory findings as they were on Schroedinger's wave equation. Chemists did not have to know how to add wave functions to make use of Pauling's ideas, and that increased the popularity of his approach through the 1930s and early 1940s, especially after publication of The Nature of the Chemical Bond. Chemists could use his ideas with the assurance that they were grounded in the new physics, but they did not have to learn the physics. They could get the sheen of quantum mechanics without the sweat.
Pauling himself, his brilliance and his personality, were the final and in some ways most important factors in popularizing the VB approach. He was a great teacher, a charismatic proselytizer for this interpretation of the gospel. Wherever he went, wherever his books were read and taught, chemists were converted to his approach. As a result, by the 1940s it seemed that his VB ideas had conquered the chemical world.
Mulliken was poorly equipped to compete. Not only was his basic concept alien to most chemists, not only did he offer it draped in a new and unfamiliar garb, a notation he had helped develop that used Greek symbols, superscripts, and subscripts to describe his molecular orbitals, but he was a terrible communicator—too precise, too mathematical, too qualifying to make his ideas come alive. He bored a generation of chemistry students with the worst-delivered lectures on the University of Chicago campus, shrouding his insights in a fog of timidly delivered, densely packed gobbledygook. His published papers, which appeared in the field's most physics oriented journals, were not much better.
And he watched for years as all the attention and awards went to Pauling. Mulliken saw Pauling's 1930s series of articles on the "Nature of the Chemical Bond" hailed as revolutionary, while his own fourteen-paper "Electronic Structures of Polyatomic Molecules and Valence" series, published during the same period, received relatively little attention. He saw Pauling's lectures crammed, while students avoided his. He saw Pauling receive invitation after invitation, honor after honor, while he stayed in Chicago and toiled.
It was especially galling that Pauling ignored his ideas. It was not that Pauling thought the MO approach was wrong. Slater and Pauling had decided as early as 1931 that the VB and MO approaches both represented acceptable approximations of the wave equation and that both, if taken far enough, led to the same conclusions. Pauling had used some MO ideas himself in several early papers. But he was certain that his version of the VB approach was more usable by chemists and less confusing to teach. "One picture is enough," Pauling wrote. "Molecular orbitals just confuse the student." He devoted a substantial amount of space to discussing the VB approach in his 1935 book An Introduction to Quantum Mechanics, for instance, while brushing the MO approach off in a single paragraph. In The Nature of the Chemical Bond he noted Mulliken's ideas only in passing.
Mulliken for his part saw Pauling's popularized version of the VB approach doing real damage. Pauling was a "showman," he said, who "made a special point in making everything sound as simple as possible and in that way making it very popular with the chemists but delaying their understanding. . . . He left them with a pretty crude idea and made them feel that was satisfactory, whereas something better could have been done."
By the late 1940s an increasing number of chemists were beginning to listen to Mulliken. Quantum chemistry was now moving out of its introductory phase, thanks in large part to Pauling's work. Advanced chemistry students at top schools were now expected to learn basic quantum mechanics and a good deal of mathematics as part of their preparatory work for a career in chemistry. The more the new generation understood, the less they needed Pauling's shortcuts. They were hungry for a more quantitative, less intuitive approach to the field, and they found it in Mulliken's MO approach.
Some things seemed a toss-up—in the 1930s, for instance, the MO analysis of the hydrogen molecule gave a better bond length but a poorer dissociation constant than the VB method—but it was becoming clear that improved MO methods were making it the better tool for studying more complex molecules. Complaints were also surfacing about some of Pauling's VB-based ideas, such as the electronegativity scale, a useful practical device in most situations, critics said, but one with a shaky theoretical basis and a suspicious weakness in addressing metallic elements. Then there was the way Pauling used resonance between contributing structures to explain the properties of molecules. In
practice, this depended on coming up with the right set of starting structures—canonical structures, they were called—between which resonance occurred, then properly weighting the contribution of each to get the final product. In general, the larger the molecule and the more atoms involved, the greater the crowd of canonical structures required to explain its character. While Pauling, blessed with peerless chemical intuition, seemed able to come up with the right mix and balance, other chemists had trouble. George Wheland, a Pauling student who had gone on to become successful in applying the VB approach to organic chemistry, further complicated matters by invoking "excited" structures—purely imaginary constructs that could not reasonably be expected to exist in nature—as resonance contributors, opening the door, some chemists thought, to any manner of wild speculation about what sorts of odd ingredients might be thrown into the canonical mix. Used this way, the VB approach seemed unnecessarily arbitrary. There was a growing sense that Pauling and his followers could pull out of their hats whatever resonance combinations they needed to explain the properties of a given molecule.
By 1947, even Wheland agreed that while the overall idea of resonance was vital in understanding chemistry, the assignment of quantitative values to canonical structures was "quite arbitrary and not at all reliable. . . . Nevertheless, I do not feel that they are therefore completely worthless; if one admits, as he must, that a rigorous treatment is impossible, and if one therefore adopts an approximate procedure, then some arbitrariness must be introduced if any progress at all is to be made. As long as one knows what he is doing, and does not take the results too seriously, he is not likely to get into serious trouble, and he may gain some insights into the problem."
That was not good enough for postwar researchers eager to make quantum chemistry a more strictly quantitative science. As Mulliken said, "The valence-bond method required great flocks of resonance structures when the molecule got complicated, and to make any calculations with those is still just about hopeless." During the 1930s he found a small but influential group of British quantum chemists who thought the same way. Led by John Lennard-Jones, the first man in Britain to hold a chair of theoretical chemistry, and Christopher Longuet-Higgins, the Anglo-MO group actively promoted Mulliken's ideas and extended the reach of his methods. Just before Pauling's and Mulliken's scheduled meeting in France, for instance, Lennard-Jones devised a simple way of using the MO approach to explain the directional nature of bonds, a gap in the method that had been one of its major drawbacks.
By the late 1940s, as a result of the effort put into quantum chemistry by Pauling, Mulliken, and their followers, two things became increasing clear: The MO and VB approaches were at their core essentially the same; and the MO camp had developed simpler and more useful tools for the quantitative study of molecules. The tide was turning from VB to MO.
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That Mulliken was given equal time with Pauling in their French debate confirmed the growing influence of his MO ideas. The full-day event also confirmed that Pauling was the more engaging and convincing speaker. But it was clear at the end that, other than the new insights he had gained into metals, Pauling had not done a great deal of important work with his VB approach in the past ten years. He had charged off to other things, while Mulliken had labored patiently, perfecting his MO approach, making it work in ways that the new chemists needed. There was no immediate shift, no massive change in allegiance, as a result of the day's speeches. But the debate was a confirmation of a trend that would make the MO approach the preferred one for quantum chemists over the decade to come. The tortoise was overtaking the hare.
After the discussion, Mulliken and his wife went to the Paulings' Paris apartment for a party. They sat quietly as Pauling held court among a throng of chemists and physicists. It was a pleasant evening, with laughter, jokes, good-natured ribbing about chemical bonding ideas, and, Mulliken remembered, "endless bottles of champagne." Pauling's daughter, Linda, did an impromptu dance solo. It went on late into the night. Mulliken, however, slipped away early. There was still time to do a little work before bed.
A Sickled Cell
When he returned to Britain in May, Pauling delivered three lectures at Cambridge, which gave him an opportunity to assess firsthand Bragg's labs at the Cavendish. Max Perutz, who thought Pauling was one of the outstanding figures of world science, was happy to be his guide. When Perutz had been a poor student, he remembered borrowing enough money from his girlfriend to buy a used copy of The Nature of the Chemical Bond, a book that, he said, "transformed the chemical flatland of my earlier textbooks into a world of three-dimensional structures." Pauling was impressed by Perutz's work on hemoglobin, which indicated that the molecule had an overall oval shape but, more importantly, was made of what looked like stacks of protein cylinders, each about ten or eleven angstroms across, running parallel to the molecule's long axis. Pauling noted that this might fit the dimensions of the spiral he had folded out of paper in his sickbed a few weeks before.
But he kept that observation to himself. "I didn't say anything to [Perutz] about it," Pauling said. "I thought there is still a possibility that there is a real joker, you know, eluding me—that there is something wrong." Perutz's x-ray patterns from hemoglobin showed the 5.1angstrom reflection that could not exist with Pauling's spiral. There was no sense muddying the water with unproved guesses—or even a new idea that could lead the Cavendish group more quickly to the final details of protein structure.
Pauling was, in fact, worried by what he saw at the Cavendish. Bragg—who received Pauling graciously but still refused to talk science—had turned it into a crystallographic showplace, with all the latest equipment and most talented researchers using it. Pauling's x-ray outfit at Caltech, by comparison, seemed shabby. "They have about five times as great an outfit as ours, that is, with facilities for taking nearly 30 x-ray pictures at the same time," he wrote his assistant Eddie Hughes. "I think that we should expand our x-ray laboratory without delay."
His competitive streak was showing. Pauling felt that he was in a race with Bragg again, this time for a greater prize, and he was concerned to see that Bragg's group had a good chance of winning. "I am beginning to feel a bit uncomfortable about the English competition," Pauling wrote Corey, describing the work Perutz and others had been doing with protein structures. "They have been driving straight at the heart of a problem, and getting its solution by hook or crook. . . . The progress that has been made seems to me to be truly astounding." In response, Pauling asked Corey to try a new tack. He had seen the British using protein-digesting enzymes to split large proteins into middle-sized pieces—strings of about twenty-six amino acids—which were thought to provide more reasonable targets for x-ray analysis; now he told Corey to do the same thing. Dutiful as always, Corey wrote back, "I am terribly impatient about getting into the protein work with a force that will really give the British some competition."
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Their remaining time in England was very happy. Linda and Peter enjoyed their schools and their new friends, and Crellin surprised everyone by topping his schoolmates in Latin tests at London's Dragon School. Pauling continued his lectures at Oxford through the rest of the spring. In May, he heard the good news of the Rockefeller Foundation's $700,000 grant for his grand plan to launch with Beadle a combined chemical-biological attack on protein structure and other problems. In June, he and Ava Helen celebrated their twenty-fifth wedding anniversary with their children and friends at Oxford.
In July he presented his new ideas on the structure of metals at a huge symposium in Amsterdam, amusing the crowd by filling blackboards with data and then lecturing, hidden, behind them. While walking in Amsterdam, Pauling also impressed his friends when he ran after a woman he saw being dragged down the street, her coat caught in a streetcar door. Pauling held her up while pounding on the car door until the conductor stopped and freed her. He was still something of a daredevil. The family then traveled to Switzerland before returning to France for two week
s, where Pauling received yet another honorary degree, this one from the University of Paris.
By the time he was ready to return to Caltech, Pauling was buzzing with new ideas. His time away had given him both the stimulus of conversations with the most outstanding scientists in Europe and the time to mull things over. The VB approach to metallic bonds and his ideas of protein spirals had been satisfying, the debate with Mulliken, he thought, had gone well, and he had a raft of other ideas jostling for attention in his mind. "I think that it has really been very much worthwhile for me to get away for this period of time, under circumstances favorable to my thinking over questions and trying to find their solution," he wrote Corey.
The results of his European journey showed when he got back to Pasadena. During the next months he wrote reviews of his ideas on complementarity; an important set of publications describing the new theory of the structure of metals that he had presented in Paris; and more on antibody action, the structure of uranium hydride, the stability of fibrous sulfur, the valence-state energy of bivalent oxygen, the structure of hemoglobin, and the action of x-rays on fruit flies. Subjects ranged from the very general "Chemistry and the World of Today" to the highly specific "The Cis-Trans Isomerization of the Carotenoids." He published in French, German, British, and American journals. The number of papers, between 1948 and 1949, topped thirty.
