Modeling nucleobase radicals in the gas phase. Experimental and computational study of 2-hydroxypyridinium and 2-(1H)pyridone radicals

Isomeric radicals corresponding to hydrogen atom adducts to 2-hydroxypyridi ne (1) and 2-(1H)pyridone (2) were investigated by neutralization-reionizat ion mass spectrometry and combined ab initio and density functional theory calculations. Gas-phase protonation of 1 and 2 occurred preferentially at t he nitrogen and oxygen atoms, respectively, to give a single 2-hydroxypyrid inium ion 3(+). The calculated topical proton affinities in 1 were 922, 602 , 777, 649, 786, 694, and 746 kJ mol(-1) for the N-1, C-2, C-3, C-4, C-5, C -6, and OH positions, respectively. The topical proton affinities in 2 were 756, 824, 815, 692, and 930 kJ mol(-1) for the N-1, C-3, C-5, C-6, and car bonyl oxygen positions, respectively. The 2-hydroxy-(1H)pyridinium radical (3(.)) was generated by collisional neutralization of ion 3(+) and found to be stable on a 4.67 mu s time scale. Radical 3(.) dissociated by losses of the hydroxyl and amine hydrogen atoms and by ring cleavages. MP2 and B3LYP calculations with the 6-311G(2d,p) basis set established the 298 K relativ e energies of hydrogen atom adducts derived from 1 and 2 as 3-H-2 (11(.), m ost stable, 0) < 6-H-2 (14(.), +20) < 3H-1 (5(.),+37) < 4H-2 (12(.),+59) < 5H-2 (13(.),+60) < 5H-1 (7(.),+62) < 3(.) (+67) < 6H-1 (8(.),+76) < 4H-1 (6 (.),+86) < 2H-1 (4(.),+107) < 1H-2 (10(.),+139 kJ mol(-1)). Hydrogen atom a dducts to C-2 in 2 and to the hydroxyl group in 1 were found to be unstable and dissociated by ring opening and loss of H, respectively. RRKM calculat ions on the effective QCISD(T)/6-311+G(2d,p) potential energy surface showe d cleavages of the O-H and N-H bonds in 3(.) to be the lowest energy dissoc iations occurring in a 10:1 ratio. 1 was calculated to be 4.7 kJ mol(-1) mo re stable than 2 in the gas phase at 0 K. Fitting the experimental and calc ulated isotope effects on dissociations of deuterium-labeled radicals yield ed a distribution function for the internal energy in the ground electronic state of 3(.) formed by collisional electron transfer. The maximum of the internal energy distribution in ground-state 3(.) (129 kJ mol(-1)) was foun d to be expressed accurately by a combination of the internal energy of the precursor ion and the Franck-Condon energy gained on vertical electron tra nsfer. The three lowest excited electronic states in 3(.) were found by CIS /6-311G(2d,p) calculations to be outer states resulting from excitation of the unpaired electron in 3(.) or electron capture by 3(+). The energetics a nd radiative lifetimes of the outer excited states of 3(.) allowed interpre tation of the highly endothermic ring-cleavage dissociations. The unimolecu lar chemistry of 3 can be explained by a bimodal energy distribution due to the formation of the ground and excited electronic states upon femtosecond collisional electron transfer.