The type of radical detected by esr when first-formed short-lived radicals are generated by photolytic, radiolytic, or flow techniques in the presence of nitromethane is shown to depend upon the nature and structure of the transient radical and on the pH. Thus electron-donating radicals may react with CH3NO2 itself under certain conditions to give rise to CH3NO2−• and the alkoxy nitroxides CH3N(OR)O•. At higher pH, a wide variety of radicals (X) react very readily with the aci-anion CH2:NO2− to give adducts XCH2NO2−• (and, from radicals capable of one-electron transfer, O2NCH2CH2NO2−•). Structural characteristics of adducts XCH2NO2−• are described and it is shown how radicals which are normally undetectable in fluid solution can be trapped and recognized in this way. Mechanistic pathways diagnosed with this trap (involving, e.g. alkoxyl radicals, aroyloxyl radicals, aromatic carboxylate cation-radicals) are exemplified.
通过光解、辐解或流动技术在硝基甲烷存在的情况下生成的初生短寿命自由基,通过电子自旋共振(ESR)检测到的自由基类型显示取决于瞬态自由基的性质和结构以及pH值。因此,在某些条件下,电子给体自由基可能与CH3NO2本身发生反应,生成CH3NO2−•和烷氧基亚硝基 CH3N(OR)O•。在较高的pH下,各种自由基(X)可以与酸性负离子CH2:NO2−非常容易地发生加成反应,形成加合物 XCH2NO2−•(以及,从能进行单电子转移的自由基形成的O2NCH2CH2NO2−•)。描述了加合物 XCH2NO2−•的结构特征,并展示了通常在液体溶液中无法检测到的自由基如何被捕获和识别。利用这种捕获方式诊断的机理途径(例如烷氧基自由基、芳香族羧酸酯基自由基、芳香族羧酸酯阳离子自由基)进行了举例说明。