Rudolph A. “Rudy” Marcus, a Canadian-born US chemist whose work on electron transfer helped shape the entire field of chemistry, died on July 16 at age 102. He was a chemist at the California Institute of Technology and a Nobel laureate.
Marcus developed a theory describing how electrons move between molecules. This is known as electron transfer and underlies processes such as photosynthesis, respiration, corrosion, catalysis, and charge transport.
In the 1950s, chemists believed that the rate of a reaction depends on how much energy is released by the reaction (the driving force). The more driving force, the faster the reaction. But some electron-transfer reactions were rapid, while others with similar driving force were unexpectedly slow.
Marcus recognized that when an electron jumps from one molecule to another, it changes the charge distribution of both molecules, and the atoms in the molecules must rearrange in response. This rearrangement costs energy and takes time compared with the instantaneous motion of the electrons. Marcus found that this reorganization of energy was often a dominant factor in controlling the transfer rate.
Marcus theory, as it is known today, also produced a startling prediction: at a certain point, increasing the driving force does not increase the rate of the reaction. Instead, the reorganization energy dominates, and the reaction slows. When Marcus first proposed this so-called inverted region, many scientists found it difficult to believe. In 1984, 3 decades after Marcus developed these concepts, experiments finally confirmed the effect, validating the theory for which Marcus won the 1992 Nobel Prize in Chemistry. The theory is now widely used in research on solar cells, batteries, molecular electronics, proteins, and DNA.
“Rudy Marcus’s contributions were profoundly important to all of chemistry,” says chemist Jacqueline K. Barton, Marcus’s longtime Caltech colleague. “He was an unpretentious and gracious man, and he inspired us all with his unbounded curiosity, always interested to hear the latest experiment and solve the next chemical puzzle.”
Marcus earned his bachelor’s degree and doctorate at McGill University. He developed a chemical kinetics theory while serving as a postdoctoral researcher at the University of North Carolina at Chapel Hill. The Rice-Ramsperger-Kassel-Marcus (RRKM) theory calculates how energy distributed within a molecule controls the rate at which the molecule breaks apart or rearranges. For the RRKM theory and for later work on electron transfer done at the Polytechnic Institute of Brooklyn, Marcus was awarded the Wolf Prize in Chemistry in 1985 for his contributions to chemical kinetics.
“He is the most devoted scientist I have ever seen,” says Yiqin Gao, a theoretical chemist at Peking University, who earned his doctorate under Marcus and then worked with him as a postdoctoral researcher. “Even during his last days, he was working extremely hard on a number of research projects.”
Marcus published more than 500 papers and continued running a research group until his death. “His love for nature, his elegance in doing science, and his sincerity towards people will serve as a great inspiration for generations to come,” Gao says.
In 2024, Caltech established a Center for Theoretical Chemistry in Marcus’s name. He is survived by his family, his friends, and the hundreds of students and postdoctoral scholars he mentored.
