The sharp contrast between the VSC dynamics starting in UP and a2" dark reservoir modes is interesting because, even when UP is excited, the population relaxes to the dark modes on a much shorter time scale than pseudorotation and IVR. The difference then lies in the relaxation processes available to the initial states. Several mechanisms could explain the faster IVR upon pumping UP. For example, the decay from UP to the a2" dark modes is accompanied by excitation of low-frequency vibrations (i.e., phonons), and some of these phonons could be further excited during the energetically downhill IVR from a2" to e' modes. It follows that IVR would be accelerated by the first scattering process, and this enhancement would not occur if the system were initialized in the a2" dark modes. A limitation of this hypothesis is that, at room temperature, the phonons should have a high occupation number (≈10), which should not change significantly when UP relaxes to dark modes (through one-or few-phonon excitation). Another possibility is that the VSC-induced speedup in the dynamics reflects a polariton-induced intermolecular vibrational energy transfer. In this case though, the observed anisotropy dynamics should practically be zero, or have a very fast decay , instead of starting from a negative value, due to the lack of orientational correlation between donor and acceptor molecules. On the other hand, the mild suppression of pseudorotation is surprising, as it is conventionally thought that these high frequency vibrational modes do not drive the reaction. However, the co-existence of IVR enhancement and pseudorotation suppression suggests otherwise. By quickly going through IVR, molecules may lose their driving force for pseudorotation, leading to its slowdown. It is also possible that the pseudorotation motion is hindered by the phonons excited by the transition from UP to the dark modes. The temperature-dependent measurements further showed that VSC shifts the thermodynamic parameters of activation in the same direction (SI Section 2.4), which has also been observed-and rather consistently-in reports of reaction kinetics altered by VSC . This correspondence supports that the insights obtained here should be relevant to understanding the previous experiments .