Geng Ting, Ehrmaier Johannes, Schalk Oliver, Richings Gareth W, Hansson Tony, Worth Graham, Thomas Richard D
Department of Physics, AlbaNova University Centre, Stockholm University, Roslagstullsbacken 21, 106 91 Stockholm, Sweden.
Department of Chemistry, Technical University of Munich, Lichtenbergstr. 4, 85748 Garching, Germany.
J Phys Chem A. 2020 May 21;124(20):3984-3992. doi: 10.1021/acs.jpca.9b11788. Epub 2020 May 8.
The excited state relaxation pathways of isoxazole and oxazole upon excitation with UV-light were investigated by nonadiabatic ab initio dynamics simulations and time-resolved photoelectron spectroscopy. Excitation of the bright ππ*-state of isoxazole predominantly leads to ring-opening dynamics. Both the initially excited ππ*-state and the dissociative πσ*-state offer a combined barrier-free reaction pathway, such that ring-opening, defined as a distance of more than 2 Å between two neighboring atoms, occurs within 45 fs. For oxazole, in contrast, the excited state dynamics is about twice as slow (85 fs) and the quantum yield for ring-opening is lower. This is caused by a small barrier between the ππ*-state and the πσ*-state along the reaction path, which suppresses direct ring-opening. Theoretical findings are consistent with the measured time-resolved photoelectron spectra, confirming the timescales and the quantum yields for the ring-opening channel. The results indicate that a combination of time-resolved photoelectron spectroscopy and excited state dynamics simulations can explain the dominant reaction pathways for this class of molecules. As a general rule, we suggest that the antibonding σ*-orbital located between the oxygen atom and a neighboring atom of a five-membered heterocyclic system provides a driving force for ring-opening reactions, which is modified by the presence and position of additional nitrogen atoms.
通过非绝热从头算动力学模拟和时间分辨光电子能谱研究了异恶唑和恶唑在紫外光激发下的激发态弛豫途径。异恶唑的明亮ππ态激发主要导致开环动力学。最初激发的ππ态和解离的πσ态都提供了一条无势垒的联合反应途径，使得开环（定义为两个相邻原子之间的距离超过2 Å）在45 fs内发生。相比之下，对于恶唑，激发态动力学大约慢两倍（85 fs），开环量子产率较低。这是由于沿着反应路径在ππ态和πσ态之间存在一个小势垒，它抑制了直接开环。理论结果与测量的时间分辨光电子能谱一致，证实了开环通道的时间尺度和量子产率。结果表明，时间分辨光电子能谱和激发态动力学模拟相结合可以解释这类分子的主要反应途径。一般来说，我们认为位于五元杂环系统中氧原子和相邻原子之间的反键σ轨道为开环反应提供了驱动力，这会因额外氮原子的存在和位置而改变。
