Quantum Tunnelling in Triplet Carbenes Explained by Instanton Theory

The temperature-dependent reactivity of three triplet carbenes (denoted as C1, C2 and C3) were investigated using instanton theory. Experiments showed that C1undergoes an intramolecular reaction at very low temperatures, while C2 requires heating, and C3 remains stable despite heating. The reactions studied involved both hydrogen transfer and intersystem crossing, and therefore we considered sequential and concerted processes as possible candidates for the reaction mechanism. Calculations of instanton tunnelling pathways in conjunction with double-hybrid density functional theory showed that the sequential mechanism dominates the reaction at high temperatures while the concerted mechanism is the predominant channel at low temperatures. The observed temperature-dependent reactivity can thus be explained in terms of a crossover temperature where the mechanism switches. This study suggests a powerful way to control the reactivity of triplet carbenes solely by tuning temperature.