Chemical Engineering Department, Al-Hussein Bin Talal University, Ma'an, Jordan.
J Phys Chem A. 2010 Jan 21;114(2):1098-108. doi: 10.1021/jp9091706.
Semiquinones are persistent free environmental radicals formed as important initial products from the decomposition of dihydroxylated benzene isomers. This study develops detailed decomposition pathways for the thermal decomposition of the three isomeric semiquinone radicals. Branching ratios based on the calculated high-pressure limit reaction rate constants predict that p-benzoquinone is a major product from the unimolecular decomposition of the p-semiquinone radical, while the formation of o-benzoquinone from the o-semiquinone radical corresponds to a minor channel. This finding is consistent with the absence of o-benzoquinone from the thermal degradation of the 1,2-dihydroxybenzene isomer and the abundance of p-benzoquinone from the thermal decomposition of 1,4-dihydroxybenzene. Ring contraction/CO elimination is shown to be the dominant sink pathway for the o-semiquinone and m-semiquinone radicals. Thermochemical properties, in terms of enthalpies of formation, entropies, and heat capacities for dihydroxylated benzene isomers, semiquinone radicals, and benzoquinones, are evaluated by quantum chemical calculations. Values of the enthalpies of formation calculated by the B3LYP/GTLarge method show good agreement with those obtained at the G3B3 level of theory.
半醌是由二羟基苯异构体分解形成的持久性游离环境自由基，是重要的初始产物。本研究发展了三种同分异构半醌自由基热分解的详细分解途径。基于计算得到的高压极限反应速率常数的分支比预测，p-苯醌是 p-半醌自由基单分子分解的主要产物，而邻苯醌的形成则对应于次要通道。这一发现与 1,2-二羟基苯异构体热降解中不存在邻苯醌以及 1,4-二羟基苯热降解中邻苯醌的丰度相一致。环收缩/CO 消除被证明是邻半醌和间半醌自由基的主要消亡途径。通过量子化学计算，评估了二羟基苯异构体、半醌自由基和苯醌的热化学性质，包括生成焓、熵和热容。B3LYP/GTLarge 方法计算的生成焓值与 G3B3 理论水平的结果吻合较好。
