The reactive singlet state of oxygen (O₂) can decay to the triplet ground state nonradiatively in the presence of a solvent. There is a controversy about whether tunnelling is involved in this nonadiabatic spin-crossover process. Semiclassical instanton theory provides a reliable and practical computational method for elucidating the reaction mechanism and can account for nuclear quantum effects such as zero-point energy and multidimensional tunnelling. However, the previously developed instanton theory is not directly applicable to this system because of a branch-point singularity which appears in the flux correlation function. Here we derive a new instanton theory for cases dominated by the singularity, leading to a new picture of tunnelling in nonadiabatic processes. Together with multireference electronic-structure theory, this provides a rigorous framework based on first principles that we apply to calculate the decay rate of singlet oxygen in water. The results indicate a new reaction mechanism that is 27 orders of magnitude faster at room temperature than the classical process through the minimum-energy crossing point. We find significant heavy-atom tunnelling contributions as well as a large temperature-dependent H₂O/D₂O kinetic isotope effect of approximately 20, in excellent agreement with experiment.

氧的反应性单重态 （O₂） 可以在溶剂存在下非辐射地衰变为三重态基态。关于这种非绝热自旋交叉过程是否涉及隧道掘进存在争议。半经典瞬时子理论为阐明反应机理提供了一种可靠且实用的计算方法，并且可以解释核量子效应，例如零点能量和多维隧道。然而，先前开发的瞬时子理论并不直接适用于该系统，因为在磁通量相关函数中出现分支点奇点。在这里，我们为奇点主导的情况推导出了一个新的瞬时子理论，从而得出了非绝热过程中隧道的新图景。这与多参考电子结构理论一起，提供了一个基于第一性原理的严格框架，我们将其应用于计算水中单线态氧的衰变率。结果表明，在室温下，通过最小能量交叉点，一种新的反应机制比经典工艺快 27 个数量级。我们发现了显着的重原子隧穿贡献以及大约 20 的大温度依赖性 H₂O/D₂O 动力学同位素效应，与实验非常吻合。