We depict instantons at a range of temperatures in Fig. (a) alongside the MEPs. This clearly reveals the growing importance of nuclear tunneling upon lowering the temperature. At very high temperatures the instanton is collapsed at the MECP, and as the system is cooled down, it gradually stretches to describe longer tunneling paths and moves away from the MECP towards more symmetric geometries with a substantially reduced dihedral angle. There appears to be a subtle mechanistic change near 200 K. Below this, the pathway converges steadily to a low-temperature limit which extends to the reactant minimum. Note that the instanton mechanism cannot be well described using a reduced-dimensionality approach based on a small subset of modes, as the optimal tunneling path comprises sizeable and temperature-dependent contributions from all normal modes of the triplet minimum, as shown in Fig SI .4. In this sense, the instanton pathways are multidimensional as they can only be found from a full-dimensional treatment.