Department of Chemistry, Imperial College London, London SW7 2AZ, United Kingdom.
J Chem Phys. 2013 Jul 28;139(4):044110. doi: 10.1063/1.4815914.
Coupled electron-nuclear dynamics, implemented using the Ehrenfest method, has been used to study charge migration with fixed nuclei, together with charge transfer when nuclei are allowed to move. Simulations were initiated at reference geometries of neutral benzene and 2-phenylethylamine (PEA), and at geometries close to potential energy surface crossings in the cations. Cationic eigenstates, and the so-called sudden approximation, involving removal of an electron from a correlated ground-state wavefunction for the neutral species, were used as initial conditions. Charge migration without coupled nuclear motion could be observed if the Ehrenfest simulation, using the sudden approximation, was started near a conical intersection where the states were both strongly coupled and quasi-degenerate. Further, the main features associated with charge migration were still recognizable when the nuclear motion was allowed to couple. In the benzene radical cation, starting from the reference neutral geometry with the sudden approximation, one could observe sub-femtosecond charge migration with a small amplitude, which results from weak interaction with higher electronic states. However, we were able to engineer large amplitude charge migration, with a period between 10 and 100 fs, corresponding to oscillation of the electronic structure between the quinoid and anti-quinoid cationic electronic configurations, by distorting the geometry along the derivative coupling vector from the D6h Jahn-Teller crossing to lower symmetry where the states are not degenerate. When the nuclear motion becomes coupled, the period changes only slightly. In PEA, in an Ehrenfest trajectory starting from the D2 eigenstate and reference geometry, a partial charge transfer occurs after about 12 fs near the first crossing between D1, D2 (N(+)-Phenyl, N-Phenyl(+)). If the Ehrenfest propagation is started near this point, using the sudden approximation without coupled nuclear motion, one observes an oscillation of the spin density--charge migration--between the N atom and the phenyl ring with a period of 4 fs. When the nuclear motion becomes coupled, this oscillation persists in a damped form, followed by an effective charge transfer after 30 fs.
采用 Ehrenfest 方法的电子-核动力学已被用于研究固定核下的电荷迁移，以及核允许运动时的电荷转移。模拟以中性苯和 2-苯乙胺（PEA）的参考几何形状开始，并在阳离子中接近势能面交叉的几何形状开始。阳离子本征态和所谓的突然近似，涉及从中性物种的相关基态波函数中去除一个电子，被用作初始条件。如果 Ehrenfest 模拟（使用突然近似）在接近状态强烈耦合且准简并的锥形交叉点附近开始，则可以观察到没有耦合核运动的电荷迁移。此外，当允许核运动耦合时，仍然可以识别与电荷迁移相关的主要特征。在苯自由基阳离子中，从参考中性几何形状和突然近似开始，人们可以观察到小振幅的亚飞秒电荷迁移，这是由于与更高电子态的弱相互作用所致。然而，我们能够通过沿着从 D6h Jahn-Teller 交叉到较低对称性的导数耦合向量扭曲几何形状来设计大振幅电荷迁移，其中状态不是简并的，从而在醌型和反醌型阳离子电子构型之间产生电子结构的振荡。当核运动变得耦合时，周期仅略有变化。在 PEA 中，在从 D2 本征态和参考几何形状开始的 Ehrenfest 轨迹中，在 D1、D2（N(+)-苯基、N-苯基(+)）之间的第一次交叉附近，大约在 12 fs 后会发生部分电荷转移。如果 Ehrenfest 传播在接近该点的情况下开始，使用没有耦合核运动的突然近似，则可以观察到自旋密度-电荷迁移-在 N 原子和苯环之间以 4 fs 的周期振荡。当核运动变得耦合时，这种振荡以阻尼形式持续存在，之后在 30 fs 后会发生有效的电荷转移。
