In summary, we have shown that the thermodynamic parameters of a chemical reaction under VSC are indeed dependent on the strong coupling strength. This further illustrates that the polaritonic states are delocalised over all the molecules involved in the strong coupling and act collectively, even though the reactions are ultimately localised on single molecules. The observed changes in chemical dynamics of the present system, despite the relatively weak Rabi splitting energy compared to the TS energy of oncavity conditions. rature under VSC (red diamonds) for the various Rabi splitting energies and the squares). The different values of ħΩ R are shown in the plots. Least-squares method was as assessed based on the coefficient of determination (R 2 ) value. For each fit, R 2 value Figure ) was chosen as the m study. The VSC of the Si-C (ν Si-C ; 860 cm -1 , blue curve in to retard the Si-C bond sciss us to do a detailed thermodyn Vibrational strong coupling o using a Fabry-Pérot cavity a controllable and demountab Specac) equipped with holes filled Fabry-Pérot cavity (em cell by placing a Mylar spacer central region) on top of a (bottom mirror), which is th of Au sputtered on a ZnSe su outlet hole for sample injectio reactants, a 200-nm-thick Si top of the Au mirrors. The IR c ing the screws on the top lid many equally spaced modes is determined by the path len Using a screwdriver, the emp fine-tuned by adjusting the The desilylation of PTA (Supplementary Material, Figure ) was chosen as the model reaction for the present study. The VSC of the Si-C stretching vibrational mode (ν Si-C ; 860 cm -1 , blue curve in Figure ) of PTA has shown to retard the Si-C bond scission dynamics [3], prompting us to do a detailed thermodynamic study, varying the ħΩ R . Vibrational strong coupling of the ν Si-C of PTA is achieved using a Fabry-Pérot cavity assembled in a temperaturecontrollable and demountable IR cell (purchased from Specac) equipped with holes to inject the liquids. An airfilled Fabry-Pérot cavity (empty cavity) is built in the IR cell by placing a Mylar spacer (6 ± 1 µm thick with a hollow central region) on top of an Au-coated ZnSe window (bottom mirror), which is then covered by a top mirror of Au sputtered on a ZnSe substrate having an inlet and outlet hole for sample injection. To protect the Au from the reactants, a 200-nm-thick SiO 2 (glass) was deposited on top of the Au mirrors. The IR cell is then closed by tightening the screws on the top lid. Such an empty cavity has many equally spaced modes in the IR whose periodicity is determined by the path length of the cavity (Figure ). Using a screwdriver, the empty cavity path length is then fine-tuned by adjusting the screws on the cell such that of ∆(∆G ‡ ) with e higher energy C compared to activity up to a orted earlier [3]. ges in the thersplitting, their induced ħΩ R . t only an enerleading to the eaction mechaof ∆H ‡ and ∆S ‡ to see that the which could be and organisastly represents for the formas the degree of conditions, the entacoordinate echanism with f ∆H ‡ (30 ± 4 kJ ∆S ‡ (-54 ± 6 kJ ents agree well TS that is more reactant/intermediate-like as schematically displayed in Figure . Interestingly, the thermodynamic data show that the reaction proceeds through a more product-like TS as shown in Figure under VSC. The dissociative TS requires higher energy as it is more bond-breaking, and it is less strained because of the tetrahedral-like structure. This could mean that the intermediate generated under VSC is easier to form but more stable than the noncavity case either by a charge stabilisation or by anisotropic interactions induced by light-matter coupling. For example, mode selective chemistry experiments have shown that TS control of the reaction can be modulated by anisotropic forces either to enhance or suppress a particular reaction path [38]. In other words, the role of strong coupling could be more important in determining the position and structure of the TS as shown in recent experimental [6, 8] and theoretical studies [23, 24].