Wolf T J A, Sanchez D M, Yang J, Parrish R M, Nunes J P F, Centurion M, Coffee R, Cryan J P, Gühr M, Hegazy K, Kirrander A, Li R K, Ruddock J, Shen X, Vecchione T, Weathersby S P, Weber P M, Wilkin K, Yong H, Zheng Q, Wang X J, Minitti M P, Martínez T J
Stanford PULSE Institute, SLAC National Accelerator Laboratory, Menlo Park, CA, USA.
Department of Chemistry, Stanford University, Stanford, CA, USA.
Nat Chem. 2019 Jun;11(6):504-509. doi: 10.1038/s41557-019-0252-7. Epub 2019 Apr 15.
The ultrafast photoinduced ring-opening of 1,3-cyclohexadiene constitutes a textbook example of electrocyclic reactions in organic chemistry and a model for photobiological reactions in vitamin D synthesis. Although the relaxation from the photoexcited electronic state during the ring-opening has been investigated in numerous studies, the accompanying changes in atomic distance have not been resolved. Here we present a direct and unambiguous observation of the ring-opening reaction path on the femtosecond timescale and subångström length scale using megaelectronvolt ultrafast electron diffraction. We followed the carbon-carbon bond dissociation and the structural opening of the 1,3-cyclohexadiene ring by the direct measurement of time-dependent changes in the distribution of interatomic distances. We observed a substantial acceleration of the ring-opening motion after internal conversion to the ground state due to a steepening of the electronic potential gradient towards the product minima. The ring-opening motion transforms into rotation of the terminal ethylene groups in the photoproduct 1,3,5-hexatriene on the subpicosecond timescale.
1,3-环己二烯的超快光致开环反应是有机化学中环化反应的典型例子，也是维生素D合成中光生物反应的模型。尽管在众多研究中已经对开环过程中光激发电子态的弛豫进行了研究，但原子间距的伴随变化尚未得到解决。在这里，我们使用兆电子伏特超快电子衍射，在飞秒时间尺度和亚埃长度尺度上直接且明确地观察到了开环反应路径。我们通过直接测量原子间距离分布随时间的变化，跟踪了1,3-环己二烯环的碳-碳键解离和结构开环。我们观察到，由于电子势梯度向产物极小值的变陡，在内部转换到基态后，开环运动有显著加速。在亚皮秒时间尺度上，开环运动转变为光产物1,3,5-己三烯中末端乙烯基的旋转。
