Prendergast Matthew B, Kirk Benjamin B, Savee John D, Osborn David L, Taatjes Craig A, Hemberger Patrick, Blanksby Stephen J, da Silva Gabriel, Trevitt Adam J
School of Chemistry, University of Wollongong, Wollongong, NSW 2522, Australia.
Phys Chem Chem Phys. 2019 Aug 28;21(32):17939-17949. doi: 10.1039/c9cp01935a. Epub 2019 Aug 6.
Product detection studies of the gas-phase oxidation of o-methylphenyl radicals and m-methylphenyl radicals are reported at ambient temperature (ca. 298 K) and 4 Torr (533.3 Pa) using VUV synchrotron photoionisation mass spectrometry. It is shown that cyclopentadienone (c-CH[double bond, length as m-dash]O) + CHCO and o-quinone methide (o-CH[double bond, length as m-dash]CH[double bond, length as m-dash]O) + OH are unique product pathways to the o-methylphenyl + O reaction due to mechanisms requiring the CH group to be adjacent to the phenyl radical site. Common product pathways include methylphenoxy radical + O(P) and isomers of methylcyclopentadienone (CHCH[double bond, length as m-dash]O) + HCO. G3X-K quantum chemical calculations are deployed to rationalise experimental results for o-methylphenyl and m-methylphenyl radical oxidation. The o-quinone methide formation mechanism from o-methylphenyl + O is analogous to the formation of o-benzoquinone from o-hydroxyphenyl + O where, after O addition, the ortho-substituent in the phenylperoxyl intermediate undergoes a 1,5-H shift and eliminates OH. Other reaction products, including methylcyclopentadienone species and methylphenyoxy radicals, are rationalised by applying known phenyl oxidation mechanisms. Transition state bifurcations are present in both radical systems and have exclusive end products (with different molecular mass). Compared to previous o-hydroxyphenyl and charged-tagged methylphenyl radical oxidation studies, there are significantly more products owing to the activation in this radical system and the competitiveness of rate limiting pathways.
利用真空紫外同步辐射光电离质谱，在环境温度（约298K）和4托（533.3帕）条件下，报道了邻甲基苯基自由基和间甲基苯基自由基气相氧化的产物检测研究。结果表明，环戊二烯酮（c-CH[双键，长度为m短划线]O）+CHCO和邻苯醌甲基化物（o-CH[双键，长度为m短划线]CH[双键，长度为m短划线]O）+OH是邻甲基苯基+O反应独特的产物途径，这是由于反应机理要求CH基团与苯基自由基位点相邻。常见的产物途径包括甲基苯氧基自由基+O(P)和甲基环戊二烯酮（CHCH[双键，长度为m短划线]O）+HCO的异构体。采用G3X-K量子化学计算来合理解释邻甲基苯基和间甲基苯基自由基氧化的实验结果。邻甲基苯基+O生成邻苯醌甲基化物的机理类似于邻羟基苯基+O生成邻苯醌的机理，即O加成后，苯基过氧中间体中的邻位取代基发生1,5-H迁移并消除OH。通过应用已知的苯基氧化机理，对包括甲基环戊二烯酮物种和甲基苯氧基自由基在内的其他反应产物进行了解释。两个自由基体系中均存在过渡态分支，且有排他性的终产物（具有不同的分子量）。与先前邻羟基苯基和带电标记甲基苯基自由基氧化研究相比，由于该自由基体系中的活化作用以及限速途径的竞争性，产物明显更多。
