We combine quantified natural transition orbital (QNTO) analysis with large-scale linear response time-dependent density functional theory to investigate the concerted [2 + 2] thymine dimerization reaction. This reaction is a main cause of UV-light-induced damage to DNA, but its mechanism has remained poorly understood. QNTO analysis enables the electronic excitations of a molecule to be identified on the basis of their transition origins across a wide range of molecular geometries, allowing the participating excited states to be identified relatively straightforwardly. We identify a barrierless funnel that is responsible for the ultrafast reaction previously indicated in experiments. The reactive state is found to have crossings with several bright excited states, revealing how the initially populated bright states can decay rapidly to the reactive state. We also examine the contribution of environmental factors, such as inclusion of the DNA backbone, which can affect the conformation of the potential energy surfaces of the relevant states.

中文翻译：

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DNA分子的光物理和光化学：导致胸腺嘧啶二聚化的电子激发态。

我们将量化的自然跃迁轨道（QNTO）分析与大规模线性响应时变密度泛函理论相结合，以研究协同的[2 + 2]胸腺嘧啶二聚化反应。该反应是紫外线诱导的DNA损伤的主要原因，但其机理仍知之甚少。QNTO分析使分子的电子激发能够根据其在广泛的分子几何结构中的跃迁起源来识别，从而可以相对直接地识别参与的激发态。我们确定了无障碍漏斗，该漏斗是先前在实验中指出的超快反应的原因。发现反应态与几个明亮的激发态相交，揭示了最初填充的明亮状态如何能够快速衰减到反应状态。我们还研究了环境因素的贡献，例如DNA主链的包含，这可能会影响相关状态的势能面的构象。