  In vibrational strong coupling (VSC), molecular vibrations strongly interact
with the modes of an optical cavity to form hybrid light-matter states known as
vibrational polaritons. Experiments show that the kinetics of thermally
activated chemical reactions can be modified by VSC. Transition-state theory,
which assumes that internal thermalization is fast compared to reactive
transitions, has been unable to explain the observed findings. Here, we carry
out kinetic simulations to understand how dissipative processes, namely those
that VSC introduces to the chemical system, affect reactions where internal
thermalization and reactive transitions occur on similar timescales. Using the
Marcus-Levich-Jortner type of electron transfer as a model reaction, we show
that such dissipation can change reactivity by accelerating internal
thermalization, thereby suppressing nonequilibrium effects that occur in the
reaction outside the cavity. This phenomenon is attributed mainly to cavity
decay (i.e., photon leakage), but a supporting role is played by the relaxation
between polaritons and dark states. When nonequilibrium effects are already
suppressed in the bare reaction (the reactive species are essentially at
internal thermal equilibrium throughout the reaction), we find that reactivity
does not change significantly under VSC. Connections are made between our
results and experimental observations.
