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On the SN2 Reactions Modified in Vibrational Strong Coupling Experiments: Reaction Mechanisms and Vibrational Mode Assignments

preprint
submitted on 20.08.2020 and posted on 21.08.2020 by Claudia Climent, Johannes Feist

Recent experiments have reported modified chemical reactivity under vibrational strong coupling (VSC) in microfluidic Fabry-Pérot cavities. In particular, the reaction rate of nucleophilic substitution reactions at silicon centers (SN2@Si) has been altered when a vibrational mode of the reactant was coupled to a confined light mode in the strong coupling regime. In this situation, hybrid light-matter states known as polaritons are formed and seem to be responsible for the modified chemical kinetics. These results are very encouraging for future applications of polaritonic chemistry to catalyze chemical reactions, with the ability to manipulate chemical phenomena without any external excitation of the system. Still, there is no theory capable of explaining the mechanism behind these results. In this work we address two points that are crucial for the interpretation of these experiments. Firstly, by means of electronic structure calculations we report the reaction mechanism in normal conditions of the two recently modified SN2@Si reactions, obtaining in both cases a triple-well PES where the rate-determining step is due to the Si-C and Si-O bond cleavage. Secondly, we characterize in detail the normal modes of vibration of the reactants. In the VSC experiments, reaction rates were modified only when specific vibrations of the reactants were coupled to a cavity mode. We find that these vibrations are highly mixed among the different fragments of the reactants leading to a completely new assignment of the IR peaks coupled to cavity modes in the original experimental works. Our results are fundamental for the interpretation of the VSC experiments given that in the absence of a theory explaining these results, the current phenomenological understanding relies on the assignment of the character of the vibrational IR peaks.

Funding

ERC-2016-StG-714870

RTI2018-099737-B-I00

PCI2018-093145

MDM-2014-0377

History

Email Address of Submitting Author

claudia.climent@uam.es

Institution

Universidad Autónoma de Madrid

Country

Spain

ORCID For Submitting Author

0000-0002-5302-1660

Declaration of Conflict of Interest

There are no conflicts to declare.

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