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

Abstract

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 COH 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 mo...