A new scientific breakthrough involving researchers at the University of Szeged is the focus of a recent article published in Nature Communications, a prestigious journal in the Nature portfolio. Among the co-authors are Dr. Gábor Czakó, Associate Professor and head of the Theoretical Reaction Dynamics Research Group at the Department of Physical Chemistry and Materials Science, and Dr. Dóra Papp, Assistant Professor and former member of the group. Working in collaboration with researchers from the University of Innsbruck (Austria) and Huazhong University of Science and Technology (China), the team used computer simulations to demonstrate that quantum tunneling can influence the dynamics of a chemical reaction – particularly when it involves a light hydrogen isotope.
A new article co-authored by Dr. Gábor Czakó and Dr. Dóra Papp – as part of an international research collaboration - has been published in Nature Communications. The study offers a quantum tunneling-based explanation for a long-standing experimental finding – namely, that the dynamics of a chemical reaction can vary depending on the hydrogen isotope involved.
The experimental phenomenon addressed in the study was first noted by Dr. Roland Wester of the University of Innsbruck, a long-time collaborator of Dr. Gábor Czakó’s research group. As in previous cases, Wester shared the data with the Szeged team, knowing that they had developed a unique potential energy surface model capable of simulating chemical reactions at the classical atomic and molecular level. During the computer simulations, Dr. Dóra Papp – then a postdoctoral researcher with the SZTE group and now an Assistant Professor at the university – observed that when the heavier hydrogen isotope, deuterium, was involved, the results matched classical expectations. In contrast, reactions involving the lighter isotope deviated from those predictions. This led the researchers to suspect that the dynamics of the lighter isotope’s reaction were being influenced by a quantum effect – specifically, quantum tunneling. To confirm this hypothesis through quantum dynamical simulations, Dr. Czakó brought in the theoretical chemistry group from Huazhong University of Science and Technology in Wuhan.
“In our simulations, we examined the reaction between a fluoride ion and methyl iodide molecule. This molecule consists of a carbon atom bonded to an iodine atom and three hydrogen atoms, making it particularly suitable for isotope substitution. In contrast, in the original experiment – conducted by our colleagues in Innsbruck – both the light hydrogen isotope, which contains only a proton, and deuterium, which has an additional neutron, were used. Comparing the two cases revealed an intriguing and lesser-known isotope effect that appears to influence reaction dynamics. While kinetic isotope effects – in which isotope substitution alters the reaction rate – have long been recognized, our research went a step further by investigating how the reaction unfolds at the atomic level. This allowed us to identify not just a kinetic isotope effect, but also what is known as a dynamic isotope effect. In this case, the effect is reflected in the directional scattering of the reaction products. Specifically, we found that when methyl iodide contains light hydrogen, the products tend to scatter forward – a preference that does not appear when the heavier isotope, deuterium, is used,” explained Dr. Gábor Czakó.
Dr. Dóra Papp and Dr. Gábor Czakó performed the classical dynamics simulations using a software platform developed within the reaction dynamics research group and in use for more than ten years. This tool has consistently produced results that align well with experimental data. That was again the case for reactions involving methyl iodide with deuterium, the heavier hydrogen isotope. However, when the lighter isotope was used, the classical model – based on classical physics – failed to reproduce the dynamic isotope effect observed in the experiment.
“This led us to suspect that a quantum mechanical effect might be at play,” said Dr. Czakó. “So we brought in our Chinese colleagues, who carried out a quantum mechanical description of the system based on a simplified model of nuclear motion. By comparing the quantum and classical simulations, we were ultimately able to explain the phenomenon through tunneling. This means that atoms can pass through an energy barrier – allowing the reaction to proceed along a pathway that classical mechanics would not permit. The lighter the nuclei, the more pronounced their quantum behavior becomes. Deuterium is twice as heavy as light hydrogen, and that difference already significantly reduces the impact of the tunneling effect.”
Dr. Dóra Papp and Dr. Gábor Czakó, co-authors of the Nature Communications publication, A dynamic isotope effect in the nucleophilic substitution reaction between F− and CD3I
Photo by István Sahin-Tóth
Although identifying the quantum phenomenon required the additional quantum mechanical model, the key insight is rooted in the classical simulation framework developed by the Theoretical Reaction Dynamics Research Group at the University of Szeged.
“The disadvantage of the quantum model is that it requires an unfeasibly large amount of computational power,” emphasized Dr. Papp. “For that reason, our Chinese colleagues' model could only account for a limited number of degrees of freedom – specifically, those most relevant to the reaction. They couldn’t describe all reaction pathways, and the amount of data they could generate was much smaller. Their results, for instance, are not suitable for visualization; you can’t display a full reaction mechanism on a screen using their method alone. They also weren’t able to calculate scattering angle distributions – something we could do with the classical model. And that’s where the differences between the two isotopic reactions became clearly visible. That said, they were, in fact, able to calculate reaction probabilities and cross sections, which were directly comparable to our results – and those also revealed significant differences.”
According to Dr. Papp, future research should explore whether quantum effects might also be present in systems where they would not be expected at first glance. Dr. Czakó adds that the results are likely to draw attention from both the experimental and theoretical communities, as this work marks an important first step toward identifying quantum effects in the dynamics of more complex chemical reactions.
“We expect that similar observations will emerge in the future, as researchers begin to investigate increasingly complex systems – and in doing so, they may be able to build on our results,” said Dr. Czakó. “These findings could also motivate theoretical chemists to develop new methods capable of accounting for quantum mechanical effects. In fact, this line of inquiry partly inspired our second Momentum (Lendület) grant proposal. We believe that the dynamic isotope effect we observed with light hydrogen is not a one-off case. Rather, as more experiments explore such systems, quantum-mechanical phenomena of this kind will likely become more common – and appropriate methods will be needed to simulate them.”
This is not the first time Dr. Gábor Czakó has been recognized for his contributions to theoretical chemistry. We recently spoke with him on the occasion of receiving multiple honors. In 2022, he was awarded the “Publication of the Year” prize by the University of Szeged. In 2024, he received the prestigious Momentum (Lendület) Grant from the Hungarian Academy of Sciences for the second time – a grant that supports exceptional early-career researchers.
Dr. Dóra Papp originally planned to become a biochemist, but an early lecture on quantum chemistry changed her path. She completed her PhD at Eötvös Loránd University (ELTE), then joined Dr. Gábor Czakó’s group in Szeged in 2018, focusing on gas-phase reaction dynamics. After a short postdoctoral stay at the University of New Mexico, she became an Assistant Professor at the University of Szeged in 2024. Her current research on interfacial reaction dynamics is supported by a Starting Grant from Hungary’s National Research, Development and Innovation Office (NKFIH).
Dr. Gábor Czakó and Dr. Dóra Papp are not only co-authors of the Nature Communications publication, but also a married couple – and they are currently expecting their first child.
Original Hungarian article by Sándor Panek
Feature photo: Dr. Dóra Papp and Dr. Gábor Czakó, co-authors of the publication in Nature Communications
Photo by István Sahin-Tóth
