Validating free spectral range tracking as a robust method for mitigating artifacts in VSC kinetics

Vibrational strong coupling (VSC) has been reported to modify ground-state chemical reactivity; however, quantitative interpretation of cavity-controlled kinetics depends critically on the spectroscopic observable used to extract rate constants. Here, we systematically compare two commonly employed cavity readout strategies-single-mode frequency shifts (Δf) and free spectral range variations (ΔFSR)-using the VSC-modulated deprotection of 1-phenyl-2-trimethylsilylacetylene (PTA) as a benchmark reaction. Reaction kinetics were monitored in real time inside a Fabry-Pérot cavity, and rate constants were extracted using a normalized first-order formalism. We show that single-mode tracking is sensitive to thermal drift and global cavity perturbations, leading to systematic deviations in the extracted rate constant even for small temperature variations (ΔT = 1 K). Under VSC conditions, ΔFSR-based analysis yields a reaction rate suppression factor of approximately 5.1 relative to the non-cavity case, whereas Δf-based analysis reports only ∼4.25-fold suppression. This discrepancy reflects the higher susceptibility of single-mode observables to non-reactive perturbations. These results demonstrate that differential cavity observables provide improved quantitative reliability for kinetic analysis under VSC and emphasize the importance of rigorous methodological validation in cavity-controlled chemistry.