Photodissociation of 2-, 3-, and 4-hydroxyacetophenone (2-, 3-, and 4-HAP) has been studied by analyzing excited-state potential energy surfaces (PES). For that, ab initio CASSCF(12,12)/6–31++G(d,p) calculations of low-lying excited states have been performed. Molecular beam study of photodissociation of 2-, 3-, and 4-HAP under laser excitation at 193 nm revealed that there are three possible dissociation channels of HAP: separation of H atom, CH₃ and COCH₃ fragments. In 2-HAP isomer the H separation channel is quenched. In this study we explain the quenching mechanism of the H separation channel and suggest a new model of CH₃ and COCH₃ fragment elimination reactions. Calculations of the excited-state PES reveal that the most of H, CH₃ and COCH₃ fragments arise after relaxation into the second, optically “dark” ²A″ state, while some of the reaction products result from the ¹A″ state dissociation. Dissociation of 2-HAP through H separation in the ²A″ state is deemed impossible, which explains the absence of deprotonated fragments in the 2-HAP mass spectrum. Breaking of intramolecular hydrogen bond in 2-HAP needs much more energy than the CH₃ detachment, which makes relaxation through the S₁ − S₀ conical intersection unfavorable.

通过分析激发态势能面（PES），研究了2-，3-和4-羟基苯乙酮（2-，3-和4-HAP）的光解离。为此，已经进行了从头开始的低层激发态的CASSCF（12,12）/ 6–31 ++ G（d，p）计算。在193 nm激光激发下对2-HAP，3-HAP和4-HAP进行光解离的分子束研究表明，HAP存在三种可能的解离通道：H原子，CH ₃和COCH ₃片段的分离。在2-HAP异构体中，H分离通道被淬灭。在这项研究中，我们解释了H分离通道的猝灭机理，并提出了CH ₃和COCH ₃的新模型。片段消除反应。激发态PES的计算表明，大部分H，CH ₃和COCH ₃片段在弛豫进入第二个光学“暗” ² A''状态后出现，而某些反应产物是由¹ A''状态解离产生的。通过H分离以² A''的状态分离2-HAP被认为是不可能的，这解释了2-HAP质谱图中不存在去质子化的片段。2-HAP中分子内氢键的断裂比CH ₃脱离需要更多的能量，这不利于通过S ₁  -S ₀圆锥形交点的弛豫。