Polaritonic normal modes in transition state theory

Abstract

A series of experiments demonstrate that strong light-matter coupling between vibrational excitations in isotropic solutions of molecules and resonant infrared optical microcavity modes leads to modified thermally-activated kinetics. However, Feist and coworkers [Phys. Rev. X., 9, 021057(2019)] have recently demonstrated that, within transition state theory, effects of strong light-matter coupling with reactive modes are electrostatic, and essentially independent of light-matter resonance or even of the formation of vibrational polaritons. To analyze this puzzling theoretical result in further detail, we revisit it under a new light, invoking a normal mode analysis of the transition state and reactant configurations for an ensemble of an arbitrary number of molecules in a cavity, obtaining simple analytical expressions that produce similar conclusions as Feist. While these effects become relevant in optical microcavities if the molecular dipoles are anisotropically aligned, or in cavities with extreme confinement of the photon modes, they become negligible for isotropic solutions in microcavities. It is concluded that further studies are necessary to track the origin of the experimentally observed kinetics.

On the SN2 reactions modified in vibrational strong coupling experiments: reaction mechanisms and vibrational mode assignments

“…7,8,29,31,32 On the other hand, since there is no theoretical explanation of the mechanism yet, it is not understood whether this is true. 20,[22][23][24] However, the phenomenological evidence for it relies on the correct identification of vibrational modes in the experimental IR spectra.…”

“…22 In contrast, in VSC experiments with Fabry-Pérot cavities, rates of S N 2@Si reactions are only found to be modified when polaritons are formed, in particular, when a cavity mode is in resonance with a molecular vibration involved in the reaction coordinate. 7,8 For the purposes of this work, we would like to highlight that at the present time, there is no clear physical explanation as to how vibrational strong coupling can slow down or accelerate thermal reactions in solution taking place inside Fabry-Pérot cavities, 20,[22][23][24] and that further theoretical work in this direction is highly desirable to rationalize the experimental observations. This being said, considering (i) the intricacies of S N 2 reactions that will be discussed in the following lines, and (ii) that vibrational strong coupling experiments have reported modified rates of S N 2@Si reactions, we believe that prior to deciphering how the reaction path is modified in the complex strong-coupling situation, the reaction mechanism under normal conditions should be fully understood.…”

Recent Progress in Vibropolaritonic Chemistry

“…In addition, a number of theoretical studies have been developed since the first observation of the VSC‐modified reaction kinetics. Many theoretical works continue to appear which do not predict appreciable reactivity change in experimentally relevant scenarios . Much work lies ahead before routine prediction of VSC‐modulated chemistry is possible.…”

Non-adiabatic molecular dynamics of molecules in the presence of strong light-matter interactions

“…In addition, recent developments have found that vibrational strong coupling (VSC) can resonantly enhance thermally-activated chemical reactions via the formation of vibrational polaritons. 35,36 Currently, the majority of microscopic models for strong coupling in polaritonic chemistry employ few-level model systems. Some studies further add nuclear degrees of freedom using effective damping rates, 37,38 or by explicitly including vibrational modes 39 to examine their influence on the polariton properties.…”