Null VSC results due to low reaction barrier. Recent experimental investigations on CN radical-hydrogen atom abstraction reactions do not reveal any noticeable change in the rate constant, even though the molecular system is under the strong coupling condition. These seemingly null results on the VSC effect will indirectly inform the fundamental mechanism and limitations of the VSC-induced rate constant modifications, and provide insights into when VSC will not be able to change rate constants. As we previously discussed, the reason why cavity is able to modify the rate constant is that k 1 ≪ k 2 , k 3 , such that k ≈ k 1 (see Eq. 7). When the cavity modifies the vibrational excitation rate constant k 1 , this effect manifests into the overall rate of reaction. If the reaction mechanism in Eq. 7 no longer holds, then coupling to the cavity will not modify the rate constant at all. This will indeed be the case when the potential barrier height is even lower than the first vibrational excited state, and ground state tunneling |ν L ⟩ → |ν R ⟩ becomes the dominating reactive channel. This is hypothesized to be the case for the CN radical reactions in Ref. .