Department of Physical Chemistry and the Fritz Haber Center for Molecular Dynamics, The Hebrew University , Jerusalem 91904, Israel.
J Phys Chem A. 2013 Nov 21;117(46):11711-24. doi: 10.1021/jp401309b. Epub 2013 Jul 3.
The photochemistry of aldehydes in the gas phase has been the topic of extensive studies over the years. However, for all but the smallest aldehydes the dynamics of the processes is largely unknown, and key issues of the mechanisms are open. In this article, the photochemistry of pentanal is studied by dynamics simulation using a semiempirical MRCI code for the singlet and triplet potential energy surfaces involved. The simulations explore the processes on the triplet state following intersystem crossing from the initially excited singlet. Test simulations show that the photochemistry takes place on the adiabatic triplet surface only and that no nonadiabatic transitions occur to the other triplets. The main findings include the following: (1) Norrish type I and type II reactions and H detachment have been observed. (2) The time scales of Norrish type I and Norrish type II reactions are determined: Norrish type I reaction tends to occur in the time scale below 10 ps, whereas the Norrish type II reaction is mostly pronounced after 20 ps. The factors affecting the time scales are analyzed. (3) The relative yield for Norrish type I and type II reactions is 34% and 66%, which is close to the experimental observed ones. Bond orders and Mulliken partial charges are computed along the trajectories and provide mechanistic insights. The results throw light on the time scales and mechanisms and competition between different channels in aldehyde photochemistry. It is suggested that direct dynamics simulations using semiempirical potentials can be a very useful tool for exploring the photochemistry of large aldehydes, ketones, and related species.
醛类在气相中的光化学反应多年来一直是广泛研究的课题。然而，除了最小的醛类之外，这些过程的动力学在很大程度上是未知的，而且机制的关键问题仍然存在争议。在本文中，使用涉及的单重态和三重态势能面的半经验 MRCI 代码通过动力学模拟研究了戊醛的光化学反应。这些模拟探索了系间穿越后三重态上的过程，最初激发的是单重态。测试模拟表明，光化学反应仅在绝热三重态表面上发生，并且不会发生非绝热跃迁到其他三重态。主要发现包括以下几点：（1）已经观察到 Norrish 型 I 和 II 反应和 H 原子的脱离。（2）确定了 Norrish 型 I 和 Norrish 型 II 反应的时间尺度：Norrish 型 I 反应倾向于在 10 ps 以下的时间尺度上发生，而 Norrish 型 II 反应主要在 20 ps 之后发生。分析了影响时间尺度的因素。（3）Norrish 型 I 和 II 反应的相对产率分别为 34%和 66%，接近实验观察到的产率。沿着轨迹计算了键级和 Mulliken 部分电荷，并提供了机理见解。结果阐明了醛类光化学反应中不同通道之间的时间尺度、机制和竞争。建议使用半经验势能的直接动力学模拟可以成为探索大醛类、酮类和相关物种光化学的非常有用的工具。
