The spin-crossover reaction of thiophosgene has drawn broad attention from both experimenters and theoreticians as a prime example of radiationless intramolecular decay via intersystem crossing. Despite multiple attempts over 20 years, theoretical predictions have typically been orders of magnitude in error relative to the experimentally measured triplet lifetime. We address the T₁ → S₀ transition by the first application of semiclassical golden-rule instanton theory in conjunction with on-the-fly electronic-structure calculations based on multireference perturbation theory. Our first-principles approach provides excellent agreement with the experimental rates. This was only possible because instanton theory goes beyond previous methods by locating the optimal tunneling pathway in full dimensionality and thus captures “corner cutting” effects. Since the reaction is situated in the Marcus inverted regime, the tunneling mechanism can be interpreted in terms of two classical trajectories, one traveling forward and one backward in imaginary time, which are connected by particle–antiparticle creation and annihilation events. The calculated mechanism indicates that the spin crossover is sped up by many orders of magnitude due to multidimensional quantum tunneling of the carbon atom even at room temperature.

中文翻译：

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通过多维重原子量子隧道进行硫光气的自旋交叉

作为通过系统间交叉进行无辐射分子内衰变的主要例子，硫光气的自旋交叉反应引起了实验者和理论家的广泛关注。尽管 20 多年来进行了多次尝试，但相对于实验测量的三重态寿命，理论预测通常存在几个数量级的误差。我们解决了 T ₁ → S ₀通过首次应用半经典的黄金法则瞬子理论结合基于多参考微扰理论的动态电子结构计算来实现过渡。我们的第一性原理方法与实验速率非常吻合。这是唯一可能的，因为瞬时子理论超越了以前的方法，通过在全维中定位最佳隧道路径，从而捕获“切角”效应。由于反应位于马库斯倒置区，隧道机制可以解释为两条经典轨迹，一条在虚时间中向前行进，一条向后行进，它们通过粒子 - 反粒子的产生和湮灭事件相连。