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

Automated summary

This study simulates two recent matrix-isolation experiments at cryogenic temperatures, revealing the failure of the commonly used weak-coupling method in describing deep-tunneling reactions. However, the more rigorous approach of semiclassical golden-rule instanton theory combined with double-hybrid density-functional theory and multireference perturbation theory successfully reproduces rate constants and kinetic isotope effects in good agreement with experiment. Additionally, these calculations identify the optimal tunnelling pathways, providing a molecular picture of the reaction mechanism.