Direct observation of a hydrogen atom adduct to C-5 in uracil. A neutralization-reionization mass spectrometric and ab initio study

The elusive hydrogen atom adduct to the C-5 position in uracil was generate d specifically in the gas phase, and its unimolecular dissociations were el ucidated by neutralization - reionization mass spectrometry. Collisional el ectron transfer to the 5,6-dihydrouracine-6-yl cation generated the 5,6-dih ydropytimidine- 2,4(1H,3H)-dion6-yl radical (1), which was the most stable hydrogen atom adduct to C-5 in uracil. A substantial fraction of I was stab le on the 5.1 mus time scale. The main unimolecular dissociations of 1 were specific losses of hydrogen atoms from C-5 and ring cleavages, as determin ed by deuterium labeling. Radical I did not isomerize unimolecularly to 5,6 -dihydropyrimidine-2,4(1H,3H)-dion-5-yl (2). Ab initio calculations up to e ffective QCISD(T)/6-311 + G(3df,2p) and combined density functional theory and perturbational calculations up to B3-MP2/6-311 + G(3df,2p) provided bon d dissociation and transition state energies for several radical and ion di ssociations that were used for assessing rate constants by RRKM theory. The observed dissociations occur to a large part from excited electronic state s of uracil radicals that are formed by vertical electron capture. The adia batic ionization energies of uracil and 1 were calculated as 9.24 and 6.70 eV, respectively.