Substituent effects on dynamics at conical intersections: Allene and methyl allenes.
Substituent effects on dynamics at conical intersections: Allene and methyl allenes.
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取代基对圆锥形交叉点动力学的影响：丙二烯和甲基丙二烯。
DOI：
10.1063/1.4938561
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发表时间：
2016
期刊：
影响因子：
--
通讯作者：
M. Schuurman
中科院分区：
文献类型：
--
作者：
S. Neville;Yanmei Wang;A. Boguslavskiy;A. Stolow;M. Schuurman
我们报告了一个联合的实验和理论研究的超快激发态动力学的丙二烯和一系列的甲基化的类似物（1，2-丁二烯，1，1-二甲基丙二烯，四甲基丙二烯），以阐明圆锥交叉介导的动力学，引起超快弛豫的基态电子状态。我们使用飞秒时间分辨光电子能谱（TRPES）探测在200 nm（6.2 eV）的UV激发后的耦合电子振动动力学。采用从头算多重产卵（AIMS）模拟来确定两个竞争的动力学途径的机制细节，以地面电子状态。在所有的分子中，这些途径被发现涉及如下：（i）围绕中心丙二烯C-C-C轴的扭曲，随后在末端碳原子之一处的扭曲和（ii）丙二烯部分的弯曲。重要的是，大西洋国际监测系统的轨道数据被用于TRPES的从头计算模拟，从而可以与实验进行直接比较。对于每个分子，TRPES信号的衰减的特征在于短（30 fs，52 fs，23 fs）和长（1.8 ps，3.5 ps，[306 fs，18 ps]）的时间常数分别为1，2-丁二烯，1，1-二甲基丙二烯，和四甲基丙二烯。然而，AIMS模拟表明，这些时间常数只是松散的电子字符的演变，实际上更密切相关的电子激发态上的大幅度运动，调制瞬时垂直电离势。此外，观察到完全取代的四甲基丙二烯经历定性不同的动力学，因为涉及相对大量的甲基的位移阻碍直接进入圆锥形交叉点，从而引起在其他物种中观察到的超快弛豫动力学。这些结果表明，“扭曲”和“弯曲”路径之间的分支可以通过丙二烯末端碳原子的选择性甲基化来修饰。惯性效应和势能效应之间的相互作用是理解这些动力学分支途径的关键。模拟和测量的TRPES之间的良好协议赋予额外的信心，这里提出的动态图片。
We report a joint experimental and theoretical study on the ultrafast excited state dynamics of allene and a series of its methylated analogues (1,2-butadiene, 1,1-dimethylallene, and tetramethylallene) in order to elucidate the conical intersection mediated dynamics that give rise to ultrafast relaxation to the ground electronic state. We use femtosecond time-resolved photoelectron spectroscopy (TRPES) to probe the coupled electronic-vibrational dynamics following UV excitation at 200 nm (6.2 eV). Ab initio multiple spawning (AIMS) simulations are employed to determine the mechanistic details of two competing dynamical pathways to the ground electronic state. In all molecules, these pathways are found to involve as follows: (i) twisting about the central allenic C-C-C axis followed by pyramidalization at one of the terminal carbon atoms and (ii) bending of allene moiety. Importantly, the AIMS trajectory data were used for ab initio simulations of the TRPES, permitting direct comparison with experiment. For each molecule, the decay of the TRPES signal is characterized by short (30 fs, 52 fs, 23 fs) and long (1.8 ps, 3.5 ps, [306 fs, 18 ps]) time constants for 1,2-butadiene, 1,1-dimethylallene, and tetramethylallene, respectively. However, AIMS simulations show that these time constants are only loosely related to the evolution of electronic character and actually more closely correlate to large amplitude motions on the electronic excited state, modulating the instantaneous vertical ionization potentials. Furthermore, the fully substituted tetramethylallene is observed to undergo qualitatively different dynamics, as displacements involving the relatively massive methyl groups impede direct access to the conical intersections which give rise to the ultrafast relaxation dynamics observed in the other species. These results show that the branching between the "twisting" and "bending" pathways can be modified via the selective methylation of the terminal carbon atoms of allene. The interplay between inertial and potential effects is a key to understanding these dynamical branching pathways. The good agreement between the simulated and measured TRPES confers additional confidence to the dynamical picture presented here.