Heterocyclic Radicals in the Gas Phase. An Experimental and Computational Study of 3-Hydroxypyridinium Radicals and Cations

Abstract

Radicals 3-hydroxy-(1 H )-pyridinium ( 1H ) through 3-hydroxy-(6 H )-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-(1 H )-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 C 3 H x and C 2 H x N 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-pyridylhydroxonium 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 1H 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.