Stanford PULSE Institute, SLAC National Accelerator Laboratory, 2575 Sand Hill Road, Menlo Park, CA, 94025, USA.
Department of Physics and Astronomy, Stony Brook University, Stony Brook, NY, 11790, USA.
Nat Commun. 2023 May 18;14(1):2795. doi: 10.1038/s41467-023-38513-6.
Electrocyclic reactions are characterized by the concerted formation and cleavage of both σ and π bonds through a cyclic structure. This structure is known as a pericyclic transition state for thermal reactions and a pericyclic minimum in the excited state for photochemical reactions. However, the structure of the pericyclic geometry has yet to be observed experimentally. We use a combination of ultrafast electron diffraction and excited state wavepacket simulations to image structural dynamics through the pericyclic minimum of a photochemical electrocyclic ring-opening reaction in the molecule α-terpinene. The structural motion into the pericyclic minimum is dominated by rehybridization of two carbon atoms, which is required for the transformation from two to three conjugated π bonds. The σ bond dissociation largely happens after internal conversion from the pericyclic minimum to the electronic ground state. These findings may be transferrable to electrocyclic reactions in general.
电环化反应的特点是通过环状结构协同形成和断裂 σ 和 π 键。这种结构被称为热反应的周环过渡态和光化学反应的激发态周环极小值。然而，周环几何结构尚未在实验中观察到。我们使用超快电子衍射和激发态波包模拟的组合，通过分子α-萜品烯中光化学反应电环开环反应的周环极小值来成像结构动力学。进入周环极小值的结构运动主要由两个碳原子的重新杂化主导，这是从两个到三个共轭 π 键的转变所必需的。σ 键的解离主要发生在从周环极小值到电子基态的内转换之后。这些发现可能适用于一般的电环化反应。
