Radicals generated by the photocatalytic reaction of carboxylic acids with a platinum-loaded TiO₂ photocatalyst were trapped by a DBNBS (3,5-dibromo-4-nitrosobenzenesulfonate sodium salt) spin trapping agent, and the radical adducts were observed using electron spin resonance spectroscopy. In the case of ethanoic acid and propanoic acid in mixed solvents of water and acetonitrile, only adducts of methyl and ethyl radicals were observed as a result of decarboxylation reactions. The longer the alkyl chain of the linear alkyl carboxylic acid, the greater the percentage of adducts of secondary radicals and the higher the concentration of the total adduct. The results show that as the alkyl chain length increases, the diffusion rate of the generated radicals slows down. Additionally, during diffusion, intramolecular hydrogen transfer reactions take place. These reactions occur at a rate higher than that of the reaction with the spin-trapping reagent. However, not only the adducts of undecyl radicals but also those of dodecanoic acid radicals were detected by electrospray ionization mass spectrometry (ESI-MS), indicating that competitive reactions of radical cations ([HRCOOH]^•+) of longer alkyl carboxylic acids to decarboxylation to produce an alkyl radical (HR^•) or proton elimination to produce an alkyl carboxylic acid radical (^•RCOOH) occur due to stabilization of radical cations by hyperconjugation with longer alkyl chain and the presence of water in the solvent as a proton acceptor. The same is true for branched alkylcarboxylic acids with tertiary carbon, which stabilizes the radical cations. Although this result also implies an increase in product diversity due to coupling between product radicals and a lack of product selectivity, we found that only the decarboxylation reaction proceeds when the percentage of water in the solvent is reduced to less than 5% to reach an increase in the product selectivity. Although OH radicals, which can produce multiple alkyl carboxylic acid radicals, were produced to a small extent in our systems, they made a very limited contribution to the overall product by increasing the ratio of acetonitrile in the solvent.

中文翻译：

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羧酸光催化活化中的自由基中间体：自旋捕获研究


羧酸与载铂的 TiO₂ 光催化剂的光催化反应产生的自由基被 DBNBS（3,5-二溴-4-亚硝基苯磺酸钠盐）自旋捕获剂捕获，并使用电子自旋共振光谱观察自由基加合物。在乙酸和丙酸在水和乙腈的混合溶剂中，由于脱羧反应，仅观察到甲基和乙基自由基的加合物。直链烷基羧酸的烷基链越长，次级自由基加合物的百分比就越大，总加合物的浓度就越高。结果表明，随着烷基链长度的增加，生成的自由基的扩散速率减慢。此外，在扩散过程中，会发生分子内氢转移反应。这些反应的发生速率高于使用自旋捕获试剂的反应的速率。然而，通过电喷雾电离质谱 （ESI-MS） 不仅检测到十一烷基自由基的加合物，而且十二烷基酸自由基的加合物也检测到十二烷基自由基的加合物，这表明较长烷基羧酸的自由基阳离子 （[HRCOOH]^•+） 与脱羧生成烷基自由基 （HR^•） 或质子消除产生烷基羧酸自由基 （^•RCOOH） 的发生是由于与较长烷基链的超共轭使自由基阳离子稳定，并且溶剂中存在水作为质子受体。具有叔碳的支链烷基羧酸也是如此，它稳定了自由基阳离子。 虽然这一结果也意味着由于产品自由基之间的耦合和缺乏产品选择性而增加了产品多样性，但我们发现，当溶剂中水的百分比降低到5%以下时，只有脱羧反应才会进行，从而达到产品选择性的增加。尽管在我们的系统中产生可以产生多个烷基羧酸自由基的 OH 自由基的比例很小，但它们通过增加溶剂中乙腈的比例，对整个产品的贡献非常有限。