Solvation or not solvation: tunneling reactions of molecules embedded in cryogenic matrices

Abstract

Noble gas cryogenic matrices have been widely employed to study the tunneling reactivity of embedded molecules. We regard cryogenic matrices as an elementary type of solvent, because the interactions between solvent molecules and between the solvent and the solute are limited to weak, non-directional forces. Nevertheless, they remain very interesting for fundamental solvation studies, as the tunneling reactivity varies considerably for different noble gases. Here, we present a multidimensional nuclear quantum approach based on the semiclassical transition state theory approximation with a quantum mechanics/molecular mechanics potential to investigate the effect of different cryo-matrix environments on the H-tunneling CO bond rotamerization reaction. Specifically, we show how the rotamerization reaction rate of glycine, formic acid, acetic acid, and their deuterated variants changes with different types of cryo-matrix embedding and in the gas phase. After reproducing the available experimental results, we show that cryogenic matrices indeed play a crucial role in the tunneling process, which is not limited to washing out or quenching tunneling. We conclude that condensed phase matrices can enhance the reaction rate by interacting with substituents at the α-carbon site when present. Our approach opens the possibility for future studies of more complex solvation scenarios to gain physical insight into the effect of solvation on tunneling-dominated reactions.