  Strong light-matter interaction in cavity environments is emerging as a
promising approach to control chemical reactions in a non-intrusive and
efficient manner. The underlying mechanism that distinguishes between steering,
accelerating, or decelerating a chemical reaction has, however, remained
unclear, hampering progress in this frontier area of research. We leverage
quantum-electrodynamical density-functional theory to unveil the microscopic
mechanism behind the experimentally observed reduced reaction rate under cavity
induced resonant vibrational strong light-matter coupling. We observe multiple
resonances and obtain the thus far theoretically elusive but experimentally
critical resonant feature for a single strongly-coupled molecule undergoing the
reaction. While we do not explicitly account for collective coupling or
intermolecular interactions, the qualitative agreement with experimental
measurements suggests that our conclusions can be largely abstracted towards
the experimental realization. Specifically, we find that the cavity mode acts
as mediator between different vibrational modes. In effect, vibrational energy
localized in single bonds that are critical for the reaction is redistributed
differently which ultimately inhibits the reaction.
