Using 2D IR spectroscopy, we found that, when polaritons are pumped, they can accelerate the overall 2D IR cross peak dynamics and, more interestingly, makes IVR faster than pseudorotation (Fig. ). In contrast, the dynamics triggered by exciting the dark reservoir modes are similar to the dynamics of molecules outside the cavity. Thus, the fundamental concept of VSC-modified chemistrypolaritons can change reactionsholds. However, because dark modes are statistically dominant, the overall influence of VSC on the dynamics of the Fe(CO)5, when measured without differentiating polariton and dark modes (e.g., without optical pumping at room temperature), should be negligible.