Theoretical breakthrough: A new path for quantum physics to control chemical reactions

Controlling chemical reactions to generate new products is one of the biggest challenges in chemistry. Developments in this area have an impact on industry, for example, by reducing waste generated in the production of building materials or by improving the production of catalysts to speed up chemical reactions.

For this reason, in the field of polariton chemistry, which uses tools from quantum chemistry and optics, in the last 10 years several laboratories around the world have developed experiments in optical cavities to manipulate the chemical reactivity of molecules at room temperature, using electromagnetic fields. Some have succeeded in modifying the products of chemical reactions in organic compounds, but to date, and without significant progress in the last two years, no research group has been able to elaborate a general physical mechanism to describe the phenomenon and reproduce it to obtain the same measures consistently.

Now a team of researchers from the Universidad de Santiago (Chile), part of the Millennium Institute for Research in Optics (MIRO), led by principal investigator Felipe Herrera, and the laboratory of the chemical division of the US Naval Research Laboratory, (United States ), led by researcher Blake Simpkins, they report for the first time manipulating the rate of formation of urethane molecules in a solution contained within an infrared cavity.

The discovery was published on June 16, 2023 in the journal Science and demonstrates, for the first time, both theoretically and experimentally, that it is possible to selectively modify the reactivity of certain bonds in a chemical reaction at room temperature in a liquid solvent, through the influence of the empty electromagnetic field in a narrow range of infrared frequencies . “This theoretical discovery improves our fundamental understanding of the phenomenon compared to other models that simply explain partial aspects of the experimental observations or simply completely refute the experimental evidence,” says researcher Felipe Herrera.

New scientific field for the manipulation of molecules

Why is it so difficult to control chemical reactions? When chemical reactions occur, the bonds that join the atoms in a molecule break down and rearrange, forming new substances known as products. For this process to take place, energy is often needed and various physicochemical principles dating back to the 19th century have helped us to understand how this transfer of energy takes place according to the laws of thermodynamics.

There are also principles of reactivity based on the structures of molecules, such as those proposed by Eyring, Evans and Polanyi in 1935, which are widely used in all fields of chemistry. These basic principles imply that any reaction between two molecules is independent of other chemical reactions that may occur in a solution. “This is very valid in almost all the situations studied in eighty years and more, but the electromagnetic vacuum creates correlations between the different chemical reactions that take place inside the volume of the cavity, and those correlations created by the electromagnetic field, in principle make the traditional assumptions of chemical reactivity questionable,” explains Felipe Herrera.

“The experimental contribution of this study is the confirmation of the modification of the reaction rates through the interaction with the vacuum of the electromagnetic field confined inside the cavity, using a well-studied chemical reaction, and with more significant variations than those found with other types of reactions. In the theoretical part, the contribution is the fact that by modifying the dynamics of the chemical bonds that mainly participate in the reaction, through the infrared vacuum, it is possible to control the products,” adds Johan Triana, a postdoctoral fellow at MIRO and all ‘University of Santiago which participated in the creation of the mathematical model and numerical calculations for the description of the molecular system.

Reproduction and interpretation of measurements

The research began in 2020, when then-postdoctoral fellow at the US Naval Research Laboratory, now a professor at Bilkent University, Dr. Wonmi Ahn, ran the first experiments.

In 2021 Blake Simpkins prepared new samples to ensure measurements were reproducible and improved the liquid cells where chemical reactions take place.

Midway through that year, researcher Felipe Herrera began having regular meetings with Simpkins to investigate possible theoretical answers to support his findings. ‘We decided to start from scratch and build a theory that takes into account all the physical aspects of quantum optics, but which under specific conditions boils down to the standard reactivity theory of theoretical chemistry,’ explains USACH professor Felipe Herrera.

The result of the process is the publication “Modification of ground-state chemical reactivity via light-matter coherence in infrared cavities”, conducted by Simpkins (US Naval Research Laboratory) and Herrera (MIRO, Universidad de Santiago de Chile), with the participation of the researcher Wonmi Ahn, (Bilkent University, Turkey), researcher Johan Triana and Ph.D. student Felipe Recabal, both part of MIRO’s Molecular Quantum Technology group, at USACH.

This early work opens up new scientific possibilities and challenges, explains Dr. Herrera, “We need to develop a sufficiently simple and general mathematical and theoretical framework that any researcher in the world can use to interpret their experiments and hopefully design new types of measurements that no one has yet visualized.”

In this sense, Herrera reflects on his ambitions as a scientist who moves between physics and chemistry: “It would be nice to build a coherent theory that unifies two of the most successful disciplines of modern science: chemical kinetics and quantum physics”.