Harris S J, Karsili T N V, Murdock D, Oliver T A A, Wenge A M, Zaouris D K, Ashfold M N R, Harvey J N, Few J D, Gowrie S, Hancock G, Hadden D J, Roberts G M, Stavros V G, Spighi G, Poisson L, Soep B
School of Chemistry, University of Bristol, Bristol BS8 1TS, United Kingdom.
Department of Chemistry, Physical and Theoretical Chemistry Laboratory, Oxford University, South Parks Road, Oxford OX1 3QZ, United Kingdom.
J Phys Chem A. 2015 Jun 11;119(23):6045-56. doi: 10.1021/jp511879k. Epub 2015 Feb 18.
The S1((1)ππ*) state of the (dominant) syn-conformer of 2-chlorophenol (2-ClPhOH) in the gas phase has a subpicosecond lifetime, whereas the corresponding S1 states of 3- and 4-ClPhOH have lifetimes that are, respectively, ∼2 and ∼3-orders of magnitude longer. A range of experimental techniques-electronic spectroscopy, ultrafast time-resolved photoion and photoelectron spectroscopies, H Rydberg atom photofragment translational spectroscopy, velocity map imaging, and time-resolved Fourier transform infrared emission spectroscopy-as well as electronic structure calculations (of key regions of the multidimensional ground (S0) state potential energy surface (PES) and selected cuts through the first few excited singlet PESs) have been used in the quest to explain these striking differences in excited state lifetime. The intramolecular O-H···Cl hydrogen bond specific to syn-2-ClPhOH is key. It encourages partial charge transfer and preferential stabilization of the diabatic (1)πσ* potential (relative to that of the (1)ππ* state) upon stretching the C-Cl bond, with the result that initial C-Cl bond extension on the adiabatic S1 PES offers an essentially barrierless internal conversion pathway via regions of conical intersection with the S0 PES. Intramolecular hydrogen bonding is thus seen to facilitate the type of heterolytic dissociation more typically encountered in solution studies.
气相中2-氯苯酚（2-ClPhOH）（主要）顺式构象异构体的S1((1)ππ*)态具有亚皮秒级寿命，而3-氯苯酚和4-氯苯酚相应的S1态寿命分别长约2个和3个数量级。为了解释激发态寿命的这些显著差异，人们使用了一系列实验技术，包括电子光谱、超快时间分辨光离子和光电子能谱、氢里德堡原子光碎片平动光谱、速度映射成像以及时间分辨傅里叶变换红外发射光谱，还进行了电子结构计算（多维基态（S0）态势能面（PES）的关键区域以及前几个激发单重态PES的选定切面）。顺式2-氯苯酚特有的分子内O-H···Cl氢键是关键。在拉伸C-Cl键时，它促进了部分电荷转移以及非绝热(1)πσ势（相对于(1)ππ态）的优先稳定，结果是绝热S1 PES上最初的C-Cl键伸展通过与S0 PES的锥形交叉区域提供了一条基本无势垒的内转换途径。因此，分子内氢键被认为促进了在溶液研究中更常见的异裂解离类型。
