Nucleophilic substitution at silicon under vibrational strong coupling: Refined insights from a high-level ab initio perspective

TL;DR

The paper investigates the SN2 reaction of PTA with fluoride under vibrational strong coupling using high-level ab initio methods to reconcile competing mechanistic proposals. It combines DFT and CC approaches with cavity Born-Oppenheimer theory to reveal a robust two-step mechanism, emphasizes the necessity of diffuse basis functions for anionic response, and shows that cavity-induced dipole fluctuations can meaningfully alter electronic energies, particularly for z-polarized modes. The Si-C stretching contribution, characterized by a strong dipole along the Si-C bond, emerges as central to both IR response and vibrational polariton formation, correlating with observed Rabi splittings across the reaction coordinates. Together, these results advance ab initio vibro-polaritonic chemistry and clarify how electronic polarizability and mode dipole character shape reactivity under VSC, while noting open challenges such as solvent effects and collective strong coupling.

Abstract

We study the bimolecular nucleophilic substitution (S$_\mathrm{N}$2) reaction of 1-phenyl-2-trimethylsilylacetylene (PTA) under vibrational strong coupling (VSC) from the perspective of high-level ab initio quantum and polaritonic chemistry. Specifically, we address conflicting mechanistic proposals, cavity-induced electronic corrections under VSC and the relevance of a previously debated Si-C-stretching motion of PTA for vibrational polariton formation. We first provide computational evidence for a two-step mechanism based on density functional theory and high-level coupled cluster results, identify new encounter and products complexes and illustrate the relevance of diffuse basis functions for a qualitatively correct description of anionic reactive systems. We subsequently show that cavity-induced dipole fluctuation corrections of electronic energies can be significant on the level of cavity Born-Oppenheimer coupled cluster theory and discuss their qualitative impact on the proposed two-step mechanism taking into account cavity-induced molecular reorientation. We finally show that the Si-C-stretching contribution to the experimentally relevant double-peak feature of PTA exhibits a dominant dipole character, which renders it central for linear IR response and vibrational polariton formation despite the presence of CH$_3$-rocking contributions. The dipole character along the cleaving Si-C-bond is eventually shown to rationalize Rabi splittings throughout the proposed two-step mechanism. Our work refines the microscopic perspective on the S$_\mathrm{N}$2 reaction of PTA under VSC and highlights recent developments in ab initio polaritonic chemistry for the VSC regime.

Figures (3)

• Figure 1: Analysis of the S$_\mathrm{N}$2 reaction mechanism hypothesis. a) Stable structure of reactant (R) and products (P), encounter, product and transition complexes (EC, PC and TC) as well as transition states (TS$_1$ and TS$_2$) with species-specific body-fixed axis system diagonalizing the respective polarizability tensor. b) Electronic and activation energies, $\Delta_i,\,i=1,2$, relative to reactants obtained on a B3LYP(D4)/CPCM level of theory for basis sets with (def2-TZVPPD) and without (def2-TZVPP) diffuse basis functions and an implicit solvation model (CPCM) for methanol. c) Electronic and activation energies relative to reactants obtained on a DLPNO-CCSD(T)/def2-TZVPPD/CPCM level of theory besides Gibbs free energies/activation free energies, $\Delta G_i,\,i=1,2$, with CC electronic components and thermodynamic corrections obtained on a CAM-B3LYP(D4)/def2-TZVPPD/CPCM level of theory.
• Figure 2: Electronic energies on DLPNO-CCSD(T)/def2-TVZPPD/CPCM level of theory, $E^{(e)}_\mathrm{cc}$, relative to reactants corrected by cavity-induced dipole fluctuation contributions obtained on lCRP-CCSD/def2-SVPD level of theory for a $z$-polarized cavity mode and mean-value equally accounting for $x$-, $y$- and $z$-polarization.
• Figure 3: a) Normal modes contributing to the double-peak feature of PTA with (from left to right) two CH$_3$-rocking modes and a mode of mixed CH$_3$-rocking/Si-C-stretching character. b) Theoretical linear IR spectra for different reactive species containing the relevant Si-C-bond and related effective cavity transmission spectra for VSC involving a single cavity mode at frequency $862\,\mathrm{cm}^{-1}$ (vertical line) for polarizations $\lambda=x$ (green) and $\lambda=z$ (blue) at light-matter interaction strength, $g_0=0.015\,\sqrt{E_h}/ea_0$. Effective transmission spectra were obtained via the CBO-PT(2) linear response approach and vibrational polariton peaks are indicated by vertical bars. c) Rabi splittings for polarizations $\lambda=x$ (green) and $\lambda=z$ (blue) as function of reactive species obtained via the CBO-PT(2) linear response approach.