Vibrational strong coupling (VSC) offers a promising route for modifying ground-state chemical reactivity without altering molecular composition or structure. Yet, the controllability of cavity-mediated kinetic modulation remains limited, and existing characterization methods are insufficient for providing direct measurements under localized VSC conditions. Here, we develop a spatially resolved in situ FT-IR methodology using hybrid cavity/non-cavity windows to directly map reaction kinetics across the cavity boundary using the previously reported deprotection reaction of 1-phenyl-2-trimethylsilylacetylene. This approach enables simultaneous spectroscopic monitoring and kinetic extraction at defined spatial coordinates with micrometer-level precision. We observe a continuous transition of rate constants from suppressed values near the cavity region to non-cavity behavior farther away, demonstrating that the VSC-induced reactivity modulation extends beyond the physical cavity boundary via diffusion mechanism. By introducing a controlled gradient in cavity length, we reveal a two-dimensional spatial inhomogeneity of reaction rate constants correlated with both cavity detuning and distance to the cavity boundary. These results show that even submicron variations in cavity thickness can significantly impact kinetic reproducibility, underscoring the importance of spatially resolved real-time measurements for disentangling intrinsic VSC effects from cavity heterogeneity. Our findings establish a robust framework for quantifying spatially localized polaritonic reactivity and highlight the need to carefully control cavity uniformity in future studies of polariton chemistry.
Nie et al. (2026) studied this question.