We used 2D IR to monitor the cross-peak dynamics both outside and inside the cavity . Through pseudorotation and IVR, the a2" and e' modes can exchange energy. By measuring the dynamics of the cross and diagonal peaks of the 1->2 transitions (Fig. ) and fitting them to a kinetic model (see SI Section 2.3.1), we determined the energy exchange rate constant kex to be (0.084 ± 0.002) ps -1 at 25 ℃ (Fig. ). Here, unless specifically mentioned, all measurements are done under magic angle conditions to remove contributions from rotational dynamics.