杂原子位置控制噁唑和异噁唑的超快光动力学

中文摘要 AI 辅助

杂环化合物的超快光化学是光生物学和材料化学的核心，然而对其结构动力学的直接实验观测仍然罕见。本文利用MeV超快电子衍射（UED）技术，结合非绝热分子动力学模拟，首次实现了异构体对噁唑和异噁唑中光诱导开环及后续碎裂过程的实时结构表征。在200 nm光激发下，两种异构体均经历开环并随后碎裂为多种产物，但由杂原子位置控制的不同动力学表现出显著差异。轨迹表面跳跃模拟重现了实验衍射特征——这两种异构体的衍射特征惊人地相似——并揭示了两种异构体不同的机理路径。对于异噁唑，所有轨迹在40 fs内专一性地发生N-O键断裂，随后在数百飞秒时间尺度上依次碎裂为HCN +乙烯酮和HCO +乙烯基氮烯通道。相比之下，噁唑表现出显著较慢的开环过程（290 fs），效率仅为85%，主要通过O-C键断裂进行，并进入更丰富的中间体景观，包括腈叶立德和O-锥体化结构。由轨迹系综导出的模拟衍射图样，经与实验仪器响应函数卷积后，与UED观测到的分支比一致。UED与轨迹表面跳跃之间的协同作用提供了原子级图像，展示结构异构体中杂原子连接性的简单互换如何从根本上重塑激发态势能面、锥形交叉的可达性以及光化学结果。

英文摘要

The ultrafast photochemistry of heterocyclic compounds is central to photobiology and materials chemistry, yet direct experimental observation of their structural dynamics remains rare. Here we present the first real-time structural characterization of photoinduced ring opening and subsequent fragmentation in the isomeric pair oxazole and isoxazole using MeV ultrafast electron diffraction (UED), complemented by non-adiabatic molecular dynamics simulations. Upon photoexcitation at 200 nm, both isomers undergo ring opening followed by fragmentation into various products, but with strikingly different dynamics governed by heteroatom positioning. Trajectory Surface Hopping simulations reproduce the experimental diffraction signatures, which are surprisingly similar for both isomers, and reveal distinct mechanistic pathways for the two isomers. For isoxazole, all trajectories exclusively undergo N-O bond cleavage within 40 fs, followed by sequential fragmentation into HCN + ketene and HCO + vinyl nitrene channels on the hundreds-of-femtoseconds timescale. Oxazole, by contrast, shows significantly slower ring opening (290 fs) with only 85% efficiency, proceeding primarily through O-C cleavage and accessing a richer landscape of intermediates including nitrile ylide and O-pyramidalized structures. Simulated diffraction patterns derived from trajectory ensembles, convolved with the experimental instrument response function, are in agreement with the branching ratios observed by UED. This synergy between UED and trajectory surface hopping provides an atomistic picture of how the simple interchange of heteroatom connectivity in structural isomers fundamentally reshapes excited-state potential surfaces, conical intersection accessibility, and photochemical outcome.

发表机构

• University of Missouri（密苏里大学）
• Kansas State University（堪萨斯州立大学）
• SLAC National Accelerator Laboratory（斯坦福直线加速器中心）
• University of Nebraska-Lincoln（内布拉斯加大学林肯分校）
• University of Science and Technology of China（中国科学技术大学）

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