Department of Chemistry, Universität Hamburg, Harbor Bldg. 610, Luruper Chaussee 149, 22761Hamburg, Germany.
The Hamburg Centre of Ultrafast Imaging, Luruper Chaussee 149, 22761Hamburg, Germany.
J Phys Chem B. 2023 Feb 16;127(6):1399-1413. doi: 10.1021/acs.jpcb.2c07992. Epub 2023 Feb 2.
Electron transfer reactions can be strongly influenced by solvent dynamics. We study the photoionization of halides in water as a model system for such reactions. There are no internal nuclear degrees of freedom in the solute, allowing the dynamics of the solvent to be uniquely identified. We simulate the equilibrium solvent dynamics for Cl, Br, I, and their respective neutral atoms in water, comparing quantum mechanical/molecular mechanical (QM/MM) and classical molecular dynamics (MD) methods. On the basis of the obtained configurations, we calculate the extended X-ray absorption fine structure (EXAFS) spectra rigorously based on the MD snapshots and compare them in detail with other theoretical and experimental results available in the literature. We find our EXAFS spectra based on QM/MM MD simulations in good agreement with their experimental counterparts for the ions. Classical MD simulations for the ions lead to EXAFS spectra that agree equally well with the experiment when it comes to the oscillatory period of the signal, even though they differ from the QM/MM radial distribution functions extracted from the MD. The amplitude is, however, considerably overestimated. This suggests that to judge the reliability of theoretical simulation methods or to elucidate fine details of the atomistic dynamics of the solvent based on EXAFS spectra, the amplitude as well as the oscillatory period need to be considered. If simulations fail qualitatively, as does the classical MD for the aqueous neutral halogen atoms, the resulting EXAFS will also be strongly affected in both oscillatory period and amplitude. The good reliability of QM/MM-based EXAFS simulations, together with clear qualitative differences in the EXAFS spectra found between halides and their atomic counterparts, suggests that a combined theory and experimental EXAFS approach is suitable for elucidating the nonequilibrium solvent dynamics in the photoionization of halides and possibly also for electron transfer reactions in more complex systems.
电子转移反应可以强烈地受到溶剂动力学的影响。我们以卤化物在水中的光致电离为模型体系来研究此类反应。溶质中没有内部核自由度，从而可以唯一地确定溶剂的动力学。我们模拟了卤化物及其各自中性原子在水中的平衡溶剂动力学，比较了量子力学/分子力学（QM/MM）和经典分子动力学（MD）方法。基于所得到的构型，我们严格地根据 MD 快照计算了扩展 X 射线吸收精细结构（EXAFS）谱，并与文献中可用的其他理论和实验结果进行了详细比较。我们发现，我们基于 QM/MM MD 模拟的 EXAFS 谱与离子的实验对应物非常吻合。对于离子，经典 MD 模拟得到的 EXAFS 谱在信号的振荡周期方面与实验结果吻合得很好，尽管它们与从 MD 中提取的 QM/MM 径向分布函数不同。然而，振幅却被大大高估了。这表明，为了判断理论模拟方法的可靠性，或者根据 EXAFS 谱阐明溶剂原子动力学的细节，不仅需要考虑振幅，还需要考虑振荡周期。如果模拟在定性上失败，就像水合中性卤原子的经典 MD 那样，那么得到的 EXAFS 谱在振荡周期和振幅方面都会受到强烈影响。基于 QM/MM 的 EXAFS 模拟的可靠性良好，以及在卤化物与其原子对应物之间发现的 EXAFS 谱存在明显的定性差异，表明一种结合理论和实验的 EXAFS 方法适合阐明卤化物光致电离过程中溶剂的非平衡动力学，并且可能也适合阐明更复杂体系中的电子转移反应。
