Abstract
<jats:p>We present an experimentally and analytically rigorous reexamination of the urethane addition reaction under vibrational strong coupling (VSC) initially reported by Ahn & Simpkins in 2023. We find that, while the observation of distinct cavity-coupled and extracavity chemical reaction rates is reproduced qualitatively using a comparable idealized approach, these apparent differences vanish when spatial and temporal pathlength variations are properly accounted for. When such variations are present and neglected when inferring molecular concentration, systematic errors can emerge which produce the erroneous appearance of distinct chemical reactivity under strong vibrational coupling. Counterintuitively, we find that reaction rate constants extracted from cavity-coupled transmission spectra can be more reliable than those obtained from extracavity (i.e., control) reaction spectra because pathlength information is more readily accessible under cavity-coupled conditions. In extracavity samples, however, inconspicuous changes in pathlength upon solution injection and the subsequent mechanical relaxation of the cell can bias the determination of concentration depending on the sign, magnitude, and rate of pathlength change. We demonstrate that modeling the solution with an expanded dielectric function that provides pathlength information encoded in solvent vibrational bands reduces the difference between observed chemical kinetics in extracavity and cavity-coupled samples. Additionally, we investigate the effect of pathlength nonuniformity and find that the analytical approach used by Ahn & Simpkins, which mimics the observed spatial/temporal broadening of cavity modes by artificially reducing the simulated mirror reflectivity, can systematically overestimate molecular concentration. We show that when cavity transmittance spectra are modeled in a way which faithfully represents the underlying pathlength distribution, correct molecular concentrations are obtained. Properly accounting for pathlength instability and nonuniformity using these analytical methods is vital for making an accurate comparison between, and drawing conclusions from, cavity-coupled and extracavity reaction rates.</jats:p>