Temperature-Dependent Mechanistic Control of Nonadiabatic Tunnelling in Triplet Carbenes

Experiments on three chemically similar triplet carbenes observed the reaction of one at 10 K, another only when heated to 65 K, whereas the third remained stable despite heating. As the products are singlets, it is clear that the reactions involve intersystem crossing in addition to intramolecular hydrogen transfer. Here, instanton theory is used to study various possible reaction mechanisms, including sequential and concerted pathways. The latter describes a new reaction mechanism which involves changing spin state (a nonadiabatic process) while heavy atoms tunnel underneath a barrier (an adiabatic process). In each case, we find that the concerted pathway dominates the rate at low temperatures, but at higher temperatures it switches to a sequential mechanism. The existence of a crossover temperature is the key to explaining the experimental observations and demonstrates that temperature can control the reactivity of triplet carbenes via nonadiabatic tunnelling.