Reimann Marc, Teichmann Ellen, Hecht Stefan, Kaupp Martin
Theoretische Chemie/Quantenchemie, Institut für Chemie, Technische Universität Berlin, Sekr. C7, Straße des 17. Juni 135, 10623, Berlin, Germany.
Institut für Chemie, Humboldt-Universität zu Berlin, Brook-Taylor-Straße 2, 12489, Berlin, Germany.
J Phys Chem Lett. 2022 Nov 24;13(46):10882-10888. doi: 10.1021/acs.jpclett.2c02838. Epub 2022 Nov 17.
A solution to the azobenzene "entropy puzzle" [ , 2017, 29, 314002] is provided. Previous computational studies of the thermal → (back-)isomerization of azobenzene could not describe the experimentally observed large negative activation entropies. Here it is shown that the experimental results are only compatible with a more complicated multistate rotation mechanism that involves a triplet excited state. Using nonadiabatic transition state theory, close to perfect agreement is achieved between all calculated and experimental Eyring parameters. We also provide new experiments that indicate the presence of a noticeable external heavy-atom effect, which is a direct result of spin-orbit coupling effects being important in the proposed mechanism. These results suggest a reexamination of the mechanisms of related thermal double bond isomerizations in other systems in cases when an excited state of triplet (or other) multiplicity becomes thermally accessible during a rotation process.
本文给出了偶氮苯“熵难题”[ , 2017, 29, 314002]的一个解决方案。先前对偶氮苯热顺反异构化的计算研究无法描述实验观察到的大的负活化熵。本文表明，实验结果仅与一种涉及三重激发态的更复杂的多态旋转机制相一致。使用非绝热过渡态理论，所有计算得到的艾林参数与实验值之间实现了近乎完美的吻合。我们还提供了新的实验，表明存在显著的外部重原子效应，这是所提出机制中自旋轨道耦合效应重要的直接结果。这些结果表明，在旋转过程中三重态（或其他多重性）激发态变得热可及的情况下，需要重新审视其他体系中相关热双键异构化的机制。
