  Polariton chemistry holds promise for facilitating mode-selective chemical
reactions, but the underlying mechanism behind the rate modifications observed
under vibrational strong coupling is not well understood. Using the recently
developed quantum transition path theory, we have uncovered a mechanism of
resonant suppression of a thermal reaction rate in a simple model polaritonic
system, consisting of a reactive mode in a bath confined to a lossless
microcavity with a single photon mode. This mechanism was uncovered by
resolving the quantum dynamical reactive pathways and identifying their rate
limiting transitions. Upon inspecting the wavefunctions associated with the
rate limiting transition, we observed the formation of a polariton and
identified the concomitant rate suppression as due to hybridization between the
reactive mode and the cavity mode, which inhibits bath-mediated tunneling
during the reaction. The transition probabilities that define the quantum
master equation can be directly translated into a visualisation of the
corresponding polariton energy landscape. This landscape exhibits a double
funnel structure, with a large barrier between the initial and final states.
This mechanism of resonant rate suppression is found to be robust to model
parameters and computational details, and thus expected to be general.
