Instanton theory predicts the lifetime and mechanism for the decay of singlet oxygen
A new instanton method is developed to treat branch-point singularities of the flux correlation function and calculate rate constants. The new method predicts a huge 27 orders of magnitude tunnelling factor for the decay of singlet oxygen in water and shows excellent agreement with the large temperature-dependent kinetic isotope effects.
The first excited electronic state of oxygen (O2), a singlet, is a highly reactive
species that has an unusually long lifetime. Its lifetime is determined by the
rate of its decay to the ground state, a triplet, which takes place nonradiatively
(intersystem crossing) in solution.
This spin crossover is in the deep inverted regime and the singlet and triplet
potential energy surfaces have very different curvatures. In such cases, the
previously developed instanton theory is not valid, as the deformed integration
contour used to obtain the rate from the flux correlation function contains a
branch-point singularity. A new type of instanton theory is developed, revealing
a new class of nonadiabatic tunnelling where an infinite ensemble of equally-
likely reaction paths contribute to the rate process.
When applied to study the decay of singlet oxygen, we find that it cor-
rectly predicts temperature-dependent experimental lifetimes and the large ki-
netic isotope effect. The reaction mechanism involves significant corner cutting
and heavy-atom tunnelling from the oxygen atoms. Compared to a classical
transition-state theory approach, which neglects tunnelling and predicts a life-
time longer than the age of the universe, the instanton mechanism predicts an
enormous 27 orders of magnitude tunnelling speedup.
“Heavy-atom tunnelling in singlet oxygen deactivation predicted by instanton theory with branch-point singularities. Nat Commun 15, 4335 (2024).”Ansari, I.M., Heller, E.R., Trenins, G. and Richardson, J.O.