Gas-Phase Oxidation of the Protonated Uracil-5-yl Radical Cation.

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 O ₂ 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 O ₂ reaction at the 5 position is ortho to an -OH group. Following this addition of O ₂ , 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 cm ³ molecule ^-1 s ^-1 , Φ = 12% (with ±50% total accuracy). For aryl radical reactions with O ₂ , 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.