This study targets the kinetics and product detection of the gas-phase oxidation reaction of protonated 5-dehydrouracil (uracil-5-yl) distonic radical cation using iontrap mass spectrometry. Protonated 5-dehydrouracil radical ion (5-dehydrouracilH+ radical ion, m/z 112) is 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 5-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 di-enol structure, the O2 reaction at the 5 position is ortho to an-OH group. Following this addition of O2, the peroxyl radical intermediate isomerises 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 5-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 molecul-1s-1, Φ = 12% (with ±50% total accuracy). For aryl radical reactions with O2, the presence of OH elimination product pathway, following the peroxyl radical formation, is a indicator of an -OH group ortho to the radical site.
CORE (COnnecting REpositories)
CORE (COnnecting REpositories)