The selective oxidation of CH4 using O2 is one of the most attractive subjects as an elusive target reaction. Ohkubo and Hirose recently reported that chlorine dioxide radical (ClO2•), which is generated by mixing NaClO2 and HCl in an aqueous solution, acts as an efficient oxidant in the oxidation of CH4 to CH3OH and HCOOH under photoirradiation in the two-phase system of perfluorohexane and water (Angew. Chem., Int. Ed. 2018, 57, 2126). The reaction system gives CH3OH and HCOOH without further oxidation products. They proposed that methoxy radical (CH3O•) plays an important role as an intermediate in the oxidation of CH4. In the present work, we focus on the reactivity of CH3O• to CH4 in detail to propose a reasonable radical mechanism for the oxidation of CH4 using DFT calculations at the M06-2X/6-311+G** level of theory and UCCSD(T)/6-311+G** calculations. Our reaction analysis suggests that the reaction of CH3O• with CH4 and the disproportionation of CH3O• take place as CH4 + CH3O• ¼ CH3• + CH3OH and 2CH3O• ¼ CH3OH + HCHO, respectively. In contrast, the isomerization from CH3O• to CH2•(OH), suggested by Ohkubo and Hirose, is unlikely to occur under ambient conditions, due to the high activation barrier for this reaction. A better understanding of the well-controlled radical chain reactions is useful for reaction design of the hydroxylation of methane.
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