Heterocyclic radicals in the gas phase. An experimental and computational study of 3-hydroxypyridinium radicals and cations

Radicals 3-hydroxy-(1H)-pyridinium (1H) through 3-hydroxy-(6H)-pyridinium (6H) and 3-pyridylhydroxonium (7H) were studied as models of hydrogen atom adducts to nitrogen heterocycles. Radical 1H was generated in the gas phase by femtosecond collisional electron transfer to stable 3-hydroxy-(1H)-pyridinium cations (1H(+)). The fractions of nondissociating 1H decreased with increasing internal energies of the precursor cations as determined by the gas-phase protonation energetics. Radical 1H dissociated unimolecularly by loss of the N-bound hydrogen atom to produce 3-hydroxypyridine (1). The dissociation showed large isotope effects that depended on the radical's internal energy. Other dissociations of 1H were loss of OH., ring contraction forming C-OH and pyrrole, and ring cleavages leading to C3Hx and C2HxN fragments, Combined MP2 and B3LYP/6-311G(2d,p) calculations yielded topical proton affinities in 1 as 938 (N-1), 757 (C-2), 649 (C-3), 721 (OH), 727 (C-4), 714 (C-5), and 763 (C-6) kJ mol(-1). 1H+ was the most stable ion isomer formed by protonation of 1. Radical 1H was the most stable isomer whereas 2H, 3H, 4H, 5H, and 6H were calculated to be 9, 48, 16, 18, and 22 kJ mol(-1) less stable than 1H, respectively. The 3-pyridylhydrosonium radical 7H dissociated without barrier by cleavage of the O-H bond. N-H bond dissociation in 1H was 102 kJ mol(-1) endothermic at 298 K and required an activation energy of 126 kJ mol(-1). Deuterium isotope effects on the N-(H,D) bond dissociations were modeled by RRKM calculations and used to estimate the internal energy distribution in 1H. Isomerizations of 1H to 2H and 2H to 3H required activation energies of 174 and 130 kJ mol(-1), respectively. Ring-cleavage dissociations of 18 were >220 kJ mol(-1) endothermic. The occurrence of competitive ring cleavage dissociations pointed to a bimodal internal energy distribution in 1H due to the formation of excited electronic states upon electron transfer. The electronic properties and excited states of heterocyclic radicals are discussed.