Neville Simon P, Wang Yanmei, Boguslavskiy Andrey E, Stolow Albert, Schuurman Michael S
Department of Chemistry, University of Ottawa, 10 Marie Curie, Ottawa, Ontario K1N 6N5, Canada.
State Key Laboratory of Magnetic Resonance and Atomic and Molecular Physics, Wuhan Institute of Physics and Mathematics, Chinese Academy of Sciences, Wuhan 430071, People's Republic of China.
J Chem Phys. 2016 Jan 7;144(1):014305. doi: 10.1063/1.4938561.
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.
我们报告了一项关于丙二烯及其一系列甲基化类似物（1,2 - 丁二烯、1,1 - 二甲基丙二烯和四甲基丙二烯）超快激发态动力学的联合实验和理论研究，以阐明导致超快弛豫到基态电子态的锥形交叉介导的动力学。我们使用飞秒时间分辨光电子能谱（TRPES）来探测在200 nm（6.2 eV）紫外激发后的电子 - 振动耦合动力学。采用从头算多重生成（AIMS）模拟来确定通往基态电子态的两条竞争动力学途径的机理细节。在所有分子中，发现这些途径包括：（i）围绕中心丙二烯C - C - C轴扭转，随后在一个末端碳原子处发生金字塔化，以及（ii）丙二烯部分的弯曲。重要的是，AIMS轨迹数据用于TRPES的从头算模拟，从而可以与实验进行直接比较。对于每个分子，1,2 - 丁二烯、1,1 - 二甲基丙二烯和四甲基丙二烯的TRPES信号衰减分别由短（30 fs、52 fs、23 fs）和长（1.8 ps、3.5 ps、[306 fs、18 ps]）时间常数表征。然而，AIMS模拟表明，这些时间常数仅与电子特性的演变松散相关，实际上与电子激发态上的大幅度运动更密切相关，调节瞬时垂直电离势。此外，观察到完全取代的四甲基丙二烯经历了定性不同的动力学，因为涉及相对较大质量甲基的位移阻碍了直接进入导致在其他物种中观察到的超快弛豫动力学的锥形交叉。这些结果表明，通过丙二烯末端碳原子的选择性甲基化可以改变“扭转”和“弯曲”途径之间的分支。惯性和势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之间的良好一致性为这里呈现的动力学图景提供了额外的可信度。
