We study the bimolecular nucleophilic substitution (SN2) reaction of 1-phenyl-2-trimethylsilylacetylene (PTA) under vibrational strong coupling (VSC) from the perspective of high-level ab initio quantum and polaritonic chemistry. Specifically, we address conflicting mechanistic proposals, cavity-induced electronic corrections under VSC and the relevance of a previously debated Si-C-stretching motion of PTA for vibrational polariton formation. We first provide computational evidence for a two-step mechanism based on density functional theory and high-level coupled cluster results, identify new encounter and product complexes and illustrate the relevance of diffuse basis functions for a qualitatively correct description of anionic reactive systems. We subsequently show that cavity-induced dipole fluctuation corrections of electronic energies can be significant at the level of cavity Born-Oppenheimer coupled cluster theory and discuss their qualitative impact on the proposed two-step mechanism taking into account cavity-induced molecular reorientation. We finally show that the Si-C-stretching contribution to the experimentally relevant double-peak feature of PTA exhibits a dominant dipole character, which renders it central for a linear IR response and vibrational polariton formation despite the presence of CH3-rocking contributions. The dipole character along the cleaving Si-C-bond is eventually shown to rationalize Rabi splittings throughout the proposed two-step mechanism. Our work refines the microscopic perspective on the SN2 reaction of PTA under VSC and highlights recent developments in ab initio polaritonic chemistry for the VSC regime.
Frerick et al. (Thu,) studied this question.