This study targets the kinetics and product detection of the gas-phase oxidation reaction of the protonated 5-dehydrouracil (uracil-5-yl) distonic radical cation using ion-trap mass spectrometry. Protonated 5-dehydrouracil radical ions (5-dehydrouracilH⁺ radical ion, m/z 112) are produced within an ion trap by laser photolysis of protonated 5-iodouracil. Storage of the 5-dehydrouracilH⁺ radical ion in the presence of controlled concentration of O₂ reveals two main products. The major reaction product pathway is assigned as the formation of protonated 2-hydroxypyrimidine-4,5-dione (m/z 127) + ^•OH. A second product ion (m/z 99), putatively assigned as a five-member-ring ketone structure, is tentatively explained as arising from the decarbonylation (−CO) of protonated 2-hydroxypyrimidine-4,5-dione. Because protonation of the 5-dehydrouracil radical likely forms a dienol structure, the O₂ reaction at the 5 position is ortho to an −OH group. Following this addition of O₂, the peroxyl-radical intermediate isomerizes by H atom transfer from the −OH group. The ensuing hydroperoxide then decomposes to eliminate ^•OH radical. It is shown that this elimination of ^•OH radical (−17 Da) is evidence for the presence of an −OH group ortho to the initial phenyl radical site, in good accord with calculations. The subsequent CO loss mechanism, to form the aforementioned five-member-ring structure, is unclear, but some pathways are discussed. By following the kinetics of the reaction, the room temperature second-order rate coefficient of the 5-dehydrouracilH⁺ distonic radical cation with molecular oxygen is measured at 7.2 × 10^–11 cm³ molecule^–1 s^–1, Φ = 12% (with ±50% total accuracy). For aryl radical reactions with O₂, the presence of the ^•OH elimination product pathway, following the peroxyl-radical formation, is an indicator of an −OH group ortho to the radical site.

中文翻译：

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质子化的尿嘧啶-5-基自由基阳离子的气相氧化

这项研究的目标是使用离子阱质谱法对质子化的5-脱氢尿嘧啶（尿嘧啶-5-基）二氢自由基阳离子的气相氧化反应进行动力学和产物检测。通过激光光解质子化的5-碘尿嘧啶，在离子阱内产生质子化的5-脱氢尿嘧啶自由基离子（5-脱氢尿嘧啶H ⁺自由基离子，m / z 112）。在控制浓度的O ₂存在下，5-dehydrouracilH ⁺自由基离子的存储显示出两种主要产物。主要反应产物途径为形成质子化的2-羟基嘧啶-4,5-二酮（m / z 127）+ ^• OH。第二产物离子（m / z 99），被推定为五元环酮结构，初步解释为质子化的2-羟基嘧啶-4,5-二酮的脱羰作用（-CO）。因为5-脱氢尿嘧啶基的质子化可能形成二烯醇结构，所以在5位的O ₂反应与-OH基邻位。在添加O _2之后，过氧自由基基中间体通过从-OH基团转移H原子而异构化。随后产生的氢过氧化物分解以消除^• OH自由基。结果表明，^• OH自由基（−17 Da）的消除是-OH邻位基团存在的证据。与最初的苯基自由基位点相符，与计算结果非常吻合。形成上述五元环结构的随后的CO损失机理尚不清楚，但是讨论了一些途径。通过反应动力学，测定了5-脱氢尿嘧啶H ⁺异狄氏自由基阳离子与分子氧的室温二阶速率系数，测量值为7.2×10 ^–11 cm ³分子^–1 s ^–1，Φ= 12％（总精度为±50％）。对于与O _2的芳基自由基反应，过氧自由基形成后^• OH消除产物途径的存在是-OH基邻位的指示 到激进的网站。