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首页> 外文期刊>The Journal of Chemical Physics >Excited state dynamics of acrylonitrile: Substituent effects at conical intersections interrogated via time-resolved photoelectron spectroscopy and ab initio simulation
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Excited state dynamics of acrylonitrile: Substituent effects at conical intersections interrogated via time-resolved photoelectron spectroscopy and ab initio simulation

机译：丙烯腈的激发态动力学：通过时间分辨光电子能谱和从头算起在圆锥形交叉点处的取代基效应

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摘要

We report a joint experimental and theoretical study on the photoinitiated ultrafast dynamics of acrylonitrile (AN) and two methylated analogs: crotonitrile (CrN) and methacrylonitrile (MeAN). Time-resolved photoelectron spectroscopy (TRPES) and ab initio simulation are employed to discern the conical intersection mediated vibronic dynamics leading to relaxation to the ground electronic state. Each molecule is pumped with a femtosecond pulse at 200 nm and the ensuing wavepackets are probed by means of one and two photon ionization at 267 nm. The predominant vibrational motions involved in the de-excitation process, determined by ab initio trajectory simulations, are an initial twisting about the C=C axis followed by pyramidalization at a carbon atom. The decay of the time-resolved photoelectron signal for each molecule is characterized by exponential decay lifetimes for the passage back to the ground state of 60 +/- 10, 86 +/- 11, and 97 +/- 9 fs for AN, CrN, and MeAN, respectively. As these results show, the excited state dynamics are sensitive to the choice of methylation site and the explanation for the observed trend may be found in the trajectory simulations. Specifically, since the pyramidalization motion leading to the conical intersection with the ground state is accompanied by the development of a partial negative charge at the central atom of the pyramidal group, the electron donation of the cyano group ensures that this occurs exclusively at the medial carbon atom. In this way, the donated electron density from the cyano group "directs" the wavepacket to a particular region of the intersection seam. The excellent agreement between the experimental and simulated TRPES spectra, the latter determined by employing trajectory simulations, demonstrates that this mechanistic picture is consistent with the spectroscopic results.
机译：我们报告了对丙烯腈（AN）和两个甲基化类似物：巴豆腈（CrN）和甲基丙烯腈（MeAN）的光引发超快动力学的联合实验和理论研究。时间分辨光电子能谱（TRPES）和从头算起的模拟被用来识别圆锥形相交介导的振动动力学，导致弛豫到基态电子状态。每个分子用200 nm的飞秒脉冲泵浦，随后的波包通过267 nm处的一个和两个光子电离探测。通过从头开始的轨迹模拟确定，去激励过程中涉及的主要振动运动是围绕C = C轴的初始扭曲，然后在碳原子上进行锥体化。每个分子的时间分辨光电子信号的衰减的特征在于，AN，CrN返回基态时的指数衰减寿命分别为60 +/- 10、86 +/- 11和97 +/- 9 fs。和MeAN。如这些结果所示，激发态动力学对甲基化位点的选择很敏感，并且可以在轨迹模拟中找到观察到的趋势的解释。具体地，由于导致与基态的圆锥形相交的锥形化运动伴随着在锥形基团的中心原子上的部分负电荷的发展，所以氰基的电子给予确保了它仅在中间碳上发生。原子。这样，来自氰基的给定电子密度将波包“引导”到相交接缝的特定区域。实验和模拟的TRPES光谱之间的极佳一致性（后者通过使用轨迹模拟确定）表明该机理图与光谱结果一致。

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