Heavy‐Atom Quantum Tunnelling in Spin Crossovers of Nitrenes**

We simulate two recent matrix-isolation experiments at cryogenic temperatures, in which a nitrene undergoes spin crossover from its triplet state to singlet via quantum tunnelling. detail the failure of commonly applied weak-coupling method (based on linear approximation potentials) describing these deep-tunnelling reactions. The more rigorous approach semiclassical golden-rule instanton theory conjunction with double-hybrid density-functional and multireference perturbation does, however, provide rate constants kinetic isotope effects good agreement experiment. In addition, calculations locate optimal tunnelling pathways, molecular picture reaction mechanism. reactions involve substantial heavy-atom carbon, nitrogen oxygen atoms, unexpectedly even continues play role room temperature.