Climent Clàudia, Feist Johannes
Departamento de Física Teórica de la Materia Condensada and Condensed Matter Physics Center (IFIMAC), Universidad Autónoma de Madrid, E-28049 Madrid, Spain.
Phys Chem Chem Phys. 2020 Nov 7;22(41):23545-23552. doi: 10.1039/d0cp04154h. Epub 2020 Oct 16.
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 (S2@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 S2@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.
最近的实验报道了在微流控法布里-珀罗腔中振动强耦合（VSC）下化学反应活性的改变。特别是，当反应物的一种振动模式在强耦合 regime 中与受限光模式耦合时，硅中心（S2@Si）的亲核取代反应速率发生了改变。在这种情况下，形成了被称为极化激元的混合光物质态，似乎是它导致了化学动力学的改变。这些结果对于极化激元化学未来催化化学反应的应用非常令人鼓舞，因为它有能力在不对外界系统进行任何激发的情况下操纵化学现象。然而，仍然没有理论能够解释这些结果背后的机制。在这项工作中，我们解决了对于解释这些实验至关重要的两个问题。首先，通过电子结构计算，我们报道了最近两个被修改的 S2@Si 反应在正常条件下的反应机制，在这两种情况下都得到了一个三阱势能面，其中速率决定步骤是由于 Si-C 和 Si-O 键的断裂。其次，我们详细表征了反应物的正常振动模式。在 VSC 实验中，只有当反应物的特定振动与腔模式耦合时，反应速率才会改变。我们发现这些振动在反应物的不同片段之间高度混合，这导致了在原始实验工作中与腔模式耦合的红外峰有了全新的归属。鉴于在没有解释这些结果的理论时，目前的现象学理解依赖于对振动红外峰特征的归属，我们的结果对于解释 VSC 实验至关重要。
