To confirm this hypothesis, we contract a model reactive potential depicted in Fig. with a lower barrier energy E ‡ , such that there is only one localized vibrational state, |ν L ⟩ (orange) in the reactant side as shown in Fig. . The details of the parameters are provided in Supplemental Note 1 (model 2). The first excited state denoted as |ν 2 ⟩ (green in Fig. ) has a higher energy than the barrier height and is fully delocalized across both reactant and product well. Additionally, the frequency of the spectator mode ω Q is adjusted to match that of the |ν L ⟩ → |ν 2 ⟩ transition. Note that one can still achieve VSC when coupling q c with Q when ω c = ω Q . This is the minimum model that captures the essential features of the CN radical reactions .