  Strong light-matter interaction in cavity environments has emerged as a
promising and general approach to control chemical reactions in a non-intrusive
manner. The underlying mechanism that distinguishes between steering,
accelerating, or decelerating a chemical reaction has, however, remained thus
far largely unclear, hampering progress in this frontier area of research. In
this work, we leverage a combination of first-principles techniques, foremost
quantum-electrodynamical density functional theory, applied to the recent
experimental realization by Thomas et al. [1] to unveil the microscopic
mechanism behind the experimentally observed reduced reaction-rate under
resonant vibrational strong light-matter coupling. We find that the cavity mode
functions as a mediator between different vibrational eigenmodes, transferring
vibrational excitation and anharmonicity, correlating vibrations, and
ultimately strengthening the chemical bond of interest. Importantly, the
resonant feature observed in experiment, theoretically elusive so far,
naturally arises in our investigations. Our theoretical predictions in
polaritonic chemistry shine new light on cavity induced mechanisms, providing a
crucial control strategy in state-of-the-art photocatalysis and energy
conversion, pointing the way towards generalized quantum optical control of
chemical systems.
