College of Chemistry, Beijing Normal University, Beijing 100875, China.
J Org Chem. 2009 Dec 4;74(23):8956-62. doi: 10.1021/jo902080z.
Norrish type reactions for valerophenone in aqueous solution have been investigated by using the combined methods of DFT, CASSCF, and CASPT2 with molecular mechanics. It was found that formation of the intermolecular hydrogen bond in the complex of valerophenone with water results in a blue shift of the n,pi* excited states, while the Coulomb interaction between valerophenone and the bulk surrounding water is mainly responsible for the red shift of the pi,pi* excited states. As a result, the (3)pi pi* state becomes the lowest triplet state and is responsible for the long triplet lifetime observed for aqueous valerophenone. The (1)n pi*, (3)pi pi*, and (3)n pi* states were found to intersect in the same structural region, which appears to be the main reason why the intersystem crossing from (1)n pi* to (3)n pi* is very efficient for aqueous valerophenone and why most aromatic ketones have unique photophysical features such as a short singlet lifetime, high phosphorescence, and weak fluorescence. The Coulomb interaction between valerophenone and the bulk surrounding water has a significant influence on the alpha-C-C cleavage and the 1,5-H shift reaction. The 1,5-H shift is predicted to have a very small barrier on the triplet pathway and the alpha-C-C bond cleavages are not in competition with the 1,5-H shift reaction. This is in good agreement with the experimental findings that Norrish type II quantum yield is close to unity upon photoexcitation of aqueous valerophenone in the wavelength region of 290-330 nm.
已采用密度泛函理论（DFT）、完全活性空间自洽场（CASSCF）和耦合活性空间二级微扰理论（CASPT2）与分子力学相结合的方法研究了正戊基苯酮在水溶液中的 Norrish 型反应。研究发现，正戊基苯酮与水形成的分子间氢键导致 n,pi激发态蓝移，而正戊基苯酮与周围水环境的库仑相互作用主要导致 pi,pi激发态红移。结果，(3)pi pi态成为最低三重态，负责观察到的水溶液中正戊基苯酮的长三重态寿命。发现(1)n pi、(3)pi pi和(3)n pi态在相同的结构区域相交，这似乎是体系间交叉从(1)n pi到(3)n pi非常有效的主要原因，也是为什么大多数芳香酮具有独特的光物理特性，如短单重态寿命、高磷光和弱荧光。正戊基苯酮与周围水环境的库仑相互作用对α-C-C 断裂和 1,5-H 迁移反应有显著影响。预测 1,5-H 迁移在三重态途径上具有非常小的势垒，并且α-C-C 键断裂与 1,5-H 迁移反应不竞争。这与实验结果非常吻合，即在 290-330nm 波长范围内光激发水溶液中正戊基苯酮时，Norrish 型 II 量子产率接近 1。
