Garner Scott M, Haugen Eric A, Leone Stephen R, Neuscamman Eric
Department of Chemistry, University of California, Berkeley, California 94720, United States.
Chemical Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, United States.
J Am Chem Soc. 2024 Jan 31;146(4):2387-2397. doi: 10.1021/jacs.3c08002. Epub 2024 Jan 18.
We theoretically investigate the influence of diradical electron spin coupling on the time-resolved X-ray absorption spectra of the photochemical ring opening of furanone. We predict geometry-dependent carbon K-edge signals involving transitions from core orbitals to both singly and unoccupied molecular orbitals. The most obvious features of the ring opening come from the carbon atom directly involved in the bond breaking through its transition to both the newly formed singly occupied and the available lowest unoccupied molecular orbitals (SOMO and LUMO, respectively). In addition to this primary feature, the singlet spin coupling of four unpaired electrons that arises in the core-to-LUMO states creates additional geometry dependence in some spectral features with both oscillator strengths and relative excitation energies varying observably as a function of the ring opening. We attribute this behavior to a spin-occupancy-induced selection rule, which occurs when singlet spin coupling is enforced in the diradical state. Notably, one of these geometry-sensitive core-to-LUMO transitions excites core electrons from a backbone carbon not involved in the bond breaking, providing a novel nonlocal X-ray probe of chemical dynamics arising from electron spin coupling.
我们从理论上研究了双自由基电子自旋耦合对呋喃酮光化学开环的时间分辨X射线吸收光谱的影响。我们预测了与几何结构相关的碳K边信号，这些信号涉及从核心轨道到单占据分子轨道和未占据分子轨道的跃迁。开环最明显的特征来自直接参与键断裂的碳原子，它通过跃迁到新形成的单占据分子轨道和可用的最低未占据分子轨道（分别为SOMO和LUMO）来实现。除了这个主要特征外，在核心到LUMO态中出现的四个未成对电子的单重态自旋耦合在一些光谱特征中产生了额外的几何结构依赖性，随着开环过程，振子强度和相对激发能都有明显变化。我们将这种行为归因于自旋占据诱导的选择规则，这种规则在双自由基态中强制单重态自旋耦合时出现。值得注意的是，这些对几何结构敏感的核心到LUMO跃迁之一会激发来自未参与键断裂的主链碳的核心电子，这为电子自旋耦合产生的化学动力学提供了一种新颖的非局域X射线探测手段。
