Ultrafast dynamics of CN radical reactions with chloroform solvent under vibrational strong coupling

TL;DR

This study investigates whether vibrational strong coupling (VSC) can alter the ultrafast reaction dynamics of CN radicals with chloroform, finding no significant changes in reaction rates under VSC conditions, thus providing insights into the mechanisms of cavity-modified chemistry.

Contribution

First experimental examination of an elementary bimolecular reaction under vibrational strong coupling, revealing the absence of rate modifications and informing the understanding of cavity effects on reactivity.

Findings

VSC of CHCl₃ C-H stretch does not alter reaction rates.

Ultrafast transient absorption shows no significant rate changes.

Provides insights into mechanisms of cavity-modified ground state reactivity.

Abstract

Polariton chemistry may provide a new means to control molecular reactivity, permitting remote, reversible modification of reaction energetics, kinetics, and product yields. A considerable body of experimental and theoretical work has already demonstrated that strong coupling between a molecular vibrational mode and the confined electromagnetic field of an optical cavity can alter chemical reactivity without external illumination. However, the mechanisms underlying cavity-altered chemistry remain unclear in large part because the experimental systems examined previously are too complex for detailed analysis of their reaction dynamics. Here, we experimentally investigate photolysis-induced reactions of cyanide (CN) radicals with strongly-coupled chloroform (CHCl$_3$) solvent molecules and examine the intracavity rates of photofragment recombination, solvent complexation, and hydrogen abstraction. We use a microfluidic optical cavity fitted with dichroic mirrors to facilitate vibrational strong coupling (VSC) of the C-H stretching mode of CHCl$_3$ while simultaneously permitting optical access at visible wavelengths. Ultrafast transient absorption experiments performed with cavities tuned on- and off-resonance reveal that VSC of the CHCl$_3$ C-H stretching transition does not significantly modify any measured rate constants, including those associated with the hydrogen abstraction reaction. This work represents, to the best of our knowledge, the first experimental study of an elementary bimolecular reaction under VSC. We discuss how the conspicuous absence of cavity-altered effects in this system may provide insights into the mechanisms of modified ground state reactivity under VSC and help bridge the divide between experimental results and theoretical predictions in vibrational polariton chemistry.