The photochemically induced ring-opening isomerization reaction of 1,3-cyclohexadiene to 1,3,5-hexatriene is a textbook example of a pericyclic reaction and has been amply investigated with advanced spectroscopic techniques. The main open question has been the identification of the single reactive state which drives the process. The generally accepted description of the isomerization pathway starts with a valence excitation to the lowest lying bright state, followed by a passage through a conical intersection to the lowest lying doubly excited state, and finally a branching between either the return to the ground state of the cyclic molecule or the actual ring-opening reaction leading to the open-chain isomer. Here, in a joint experimental and computational effort, we demonstrate that the evolution of the excitation–deexcitation process is much more complex than that usually described. In particular, we show that an initially high-lying electronic state smoothly decreasing in energy along the reaction path plays a key role in the ring-opening reaction.

中文翻译：

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1,3-环己二烯的光化学开环反应：识别真实的反应状态

光化学诱导的 1,3-环己二烯到 1,3,5-己三烯的开环异构化反应是周环反应的教科书示例，并已通过先进的光谱技术进行了充分研究。主要的悬而未决的问题是识别驱动该过程的单一反应状态。普遍接受的异构化路径描述从价激发到最低亮态开始，然后通过锥形交叉点到达最低双激发态，最后在返回到基态之间的分支环状分子或导致开链异构体的实际开环反应。在这里，在联合实验和计算工作中，我们证明了激发-去激过程的演变比通常描述的要复杂得多。特别是，我们表明，最初处于高位的电子态沿反应路径能量平稳降低在开环反应中起着关键作用。