Direct observation of a hydrogen atom adduct to 0-4 in uracil energetics and kinetics of uracil radicals

The 4-hydroxy-3,4-dihydropyrimidine-2(1H)-on-4-y1 radical (1), an elusive h ydrogen atom adduct to the 0-4 position in uracil, was generated in the gas phase by femtosecond collisional electron transfer to 0-4 protonated uraci l and investigated by neutralization -reionization mass spectrometry (NRMS) . A fraction of radicals 1 was stable on the 5.1 mus time scale. The main u nimolecular dissociations of 1 were ring cleavages and a specific loss of t he hydrogen atom from 0-4, as determined by deuterium labeling. Ab initio c alculations up to effective QCISD(T)/6-311+G(3df,2p) and combined density f unctional theory and Perturbational calculations up to B3-MP2/6-311+G(3df,2 p) were used to obtain bond dissociation and transition state energies for several radical and ion dissociations. The energies were used for calculati ons of rate constants by RRKM and transition state theory. The dissociation s observed by NRMS could not be explained by competitive reactions occurrin g on the potential energy surface of the ground doublet state of I and must to a large part originate from excited electronic states of I that are for med by vertical electron capture. The adiabatic ionization energy of 1 was calculated as 5.55 eV. Activation energies for hydrogen atom additions to u racil were found to inversely correlate with the addition exothermicities. The most reactive position in uracil was C-5 which was calculated to accoun t for > 95%, hydrogen atom additions, followed by C-6 (<5% additions), whil e additions to O-2 and 0-4 were kinetically negligible.