Senior Caltech Nobelist Reflects on His Prize-Winning Work

It's a good thing Rudy Marcus loves libraries. Otherwise, the Noyes Professor of Chemistry at Caltech might never have stumbled across the problem that he solved to win the 1992 Nobel Prize in Chemistry.

These days, researchers can read all the leading journals online, but when Marcus was a young associate professor at the Polytechnic Institute of Brooklyn, he spent hours in the library, leafing through the chemical literature. One day in 1955, he happened across a symposium issue of the Journal of Physical Chemistry in which chemist Bill Libby laid out a theory to explain some of the puzzling observations chemists had made in the lab—namely, why some chemical reactions that involve a simple transfer of an electron happen quickly, while others take much longer to transpire.

Marcus was intrigued by Libby's explanation, which was that electrons are so light compared to the nuclei of reactants that they should be able to jump from one reactant to another before the nuclei have a chance to change. "I thought, 'That's fantastic!'" Marcus recalled recently. "Libby was taking the Franck-Condon principle—something that was devised in the 1920s for a totally different application, for interpreting the spectra of molecules—and applying it to the rate of chemical reactions." But after thinking about Libby's theory for a while, he says, "Something didn't seem quite right." That uneasy feeling launched a month-long flurry of work that yielded a different explanation—an equation and the beginnings of the Marcus theory of electron transfer that many years later won Marcus his trip to Stockholm.

Marcus realized that Libby's explanation didn't feel right because it violated the law of conservation of energy—if an electron were transferred without the nuclei changing, the system would end up with far more energy after the transfer than before. To get away from that violation, the Marcus theory says that the molecular structure of some of the nuclei of the reactant and solvent molecules have to change positions before an electron can transfer, and then adjust again afterward. Therefore, an energy barrier has to be overcome in order for an electron-transfer reaction to proceed. And since that barrier varies depending on the structure of the molecules involved, it makes sense that some reactions take longer than others. Marcus worked out a mathematical model to describe such electron-transfer reactions and to calculate the expected values for their energy barriers.

"It took one month from start to finish to produce that equation," Marcus says. "For the record, it was the fastest thing I've ever done before or since."

In addition to being completely engrossed by the problem, in many ways Marcus was prepared to attack it. Early on, as both a graduate student at McGill University, in Montreal, and as a postdoctoral fellow at the National Research Council, he had worked in the lab, measuring rates of chemical reactions. But equally critical to his success was the fact that by the time he was pondering Libby's article, Marcus had developed the ability to approach problems from a theoretical standpoint.

He hadn't always had that ability. When Marcus was in school, there were no theoretical chemists in Canada. He had taken a course in theoretical chemistry at McGill, but the professor didn't teach where the theories came from or how they were developed. So Marcus says, "It never occurred to some of us to go into theoretical chemistry." But he had always been very interested in mathematics. In fact, Marcus says he probably took more math courses at McGill than any other chemistry student at the time. So after grad school, sometime during his postdoctoral fellowship, he says, "I became very dissatisfied because I wasn't using the kind of math that I enjoyed so much." It occurred to him that theoretical chemistry might provide the blend of chemistry and mathematics he was looking for.

So he and a friend at the Research Council, Walter Trost, formed a two-man seminar. They took turns describing theoretical papers to each other and then tried to apply the findings to their own experimental work. As simple as it may sound, that preparation encouraged Marcus to take a rather bold step—to apply for a postdoctoral position in theoretical chemistry in the United States. Though Marcus had no formal training on the theoretical side, one professor, Oscar Rice from the University of North Carolina, invited the eager young chemist to join his group.

As it turned out, Marcus's decision to head to Chapel Hill was a good one for more than one reason. Within a couple weeks of his arrival, Marcus met the love of his life, Laura Hearne, a graduate student in sociology and cultural anthropology, whom he married six months later and who passed away in 2003. He was also able to nurture and develop his knowledge of theoretical chemistry. After a few months of sitting in on lectures and reading every theoretical paper he could get his hands on, and after some gentle prodding by Rice, Marcus started working on a theoretical problem that dealt with what are called unimolecular reactions. "I gradually put together the bits of a theory," Marcus says. The theory predicts how long a molecule that has acquired a lot of energy will survive in such a state before breaking up or becoming stabilized, by colliding with another molecule, for example. "Before I realized it—after being there for six months—I had developed a theory of unimolecular reactions that is still used today." That theory is referred to in textbooks as the RRKM theory—the "M" stands for Marcus.

So by the time he joined the faculty at the Polytechnic Institute of Brooklyn in 1951, Marcus had proven his theoretical chops. But sensing that there wouldn't be enough experimental results in the area of unimolecular reactions to continue on that path, he needed a new problem to focus on. Eventually, it was a student's question about electrolytes that got Marcus interested in electrostatics. He published two papers in the field before coming across Bill Libby's symposium paper in the library.

"One often hears something along the lines of, 'Discoveries come to those with a prepared mind,'" Marcus says. "Here, my preparation was that I had published something about treating electrostatic interactions. I combined that background with elements of the work I had read about that were going on in physics at the time . . . It was really a matter of putting a bunch of little ideas together."

Marcus may downplay his accomplishment, but in the Nobel award-ceremony speech, Lennart Eberson of the Royal Swedish Academy of Sciences addressed Marcus, saying, "Your theory is a unifying factor in chemistry, promoting understanding of electron-transfer reactions of biochemical, photochemical, inorganic, and organic nature and thereby contributing to science as a whole."

Marcus received his Nobel medal 19 years ago for work he started more than 35 years before that. He says that the honor changed his life in some ways—more invitations and requests came his way—but that his interest in and enthusiasm for solving problems has never waned. Today, Marcus is 88 years old and still actively working on problems in theoretical chemistry while advising postdocs and grad students.

He's also planning a return to his beloved ski slopes this winter after a couple of seasons off. In his speech at the Nobel Banquet in 1992, Marcus drew comparisons between the sport of skiing and doing theoretical work in science, offering insight into the rush he gets from both. He described "the challenge and sense of excitement when the slope is a little more difficult than one feels comfortable with."