Department of Chemistry, University of California, Berkeley, CA 94720, USA.
Chemical Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, CA 94720, USA.
Science. 2017 Apr 7;356(6333):54-59. doi: 10.1126/science.aaj2198. Epub 2017 Apr 6.
The ultrafast light-activated electrocyclic ring-opening reaction of 1,3-cyclohexadiene is a fundamental prototype of photochemical pericyclic reactions. Generally, these reactions are thought to proceed through an intermediate excited-state minimum (the so-called pericyclic minimum), which leads to isomerization via nonadiabatic relaxation to the ground state of the photoproduct. Here, we used femtosecond (fs) soft x-ray spectroscopy near the carbon K-edge (~284 electron volts) on a tabletop apparatus to directly reveal the valence electronic structure of this transient intermediate state. The core-to-valence spectroscopic signature of the pericyclic minimum observed in the experiment was characterized, in combination with time-dependent density functional theory calculations, to reveal overlap and mixing of the frontier valence orbital energy levels. We show that this transient valence electronic structure arises within 60 ± 20 fs after ultraviolet photoexcitation and decays with a time constant of 110 ± 60 fs.
1,3-环己二烯的超快光致环开反应是光化周环反应的基本原型。通常，这些反应被认为是通过中间激发态最小值（所谓的周环最小值）进行的，通过非绝热弛豫到光产物的基态导致异构化。在这里，我们使用台式设备在碳 K 边附近的飞秒（fs）软 X 射线光谱（约 284 电子伏特）直接揭示了这种瞬态中间态的价电子结构。实验中观察到的周环最小的核心到价电子光谱特征与时间相关的密度泛函理论计算相结合，揭示了前沿价轨道能级的重叠和混合。我们表明，这种瞬态价电子结构在紫外光激发后 60 ± 20 fs 内出现，并以 110 ± 60 fs 的时间常数衰减。
