Direct Observation of a Hydrogen Atom Adduct to C-5 in Uracil. A Neutralization-Reionization Mass Spectrometric and ab Initio Study

Abstract

The elusive hydrogen atom adduct to the C-5 position in uracil was generated specifically in the gas phase, and its unimolecular dissociations were elucidated by neutralization−reionization mass spectrometry. Collisional electron transfer to the 5,6-dihydrouracil-6-yl cation generated the 5,6-dihydropyrimidine-2,4(1H,3H)-dion-6-yl radical (1), which was the most stable hydrogen atom adduct to C-5 in uracil. A substantial fraction of 1 was stable on the 5.1 μs time scale. The main unimolecular dissociations of 1 were specific losses of hydrogen atoms from C-5 and ring cleavages, as determined by deuterium labeling. Radical 1 did not isomerize unimolecularly to 5,6-dihydropyrimidine-2,4(1H,3H)-dion-5-yl (2). Ab initio calculations up to effective 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 bond dissociation and transition state energies for several radical and ion dissociations that were used for assessing rate constants by RRKM theory. The observed dissociations occur to a large part from excited electronic states of uracil radicals that are formed by vertical electron capture. The adiabatic ionization energies of uracil and 1 were calculated as 9.24 and 6.70 eV, respectively.