Hydroxyl radical adducts to pyridine. The generation and properties of theelusive N-hydroxypyridyl radical

The elusive N-hydroxypyridyl radical 1 has been generated transiently in th e gas phase by collisional neutralization of the stable cation 1(+). The ra dical underwent extensive dissociation by specific losses of H, OH, and rin g-cleavage reactions, as elucidated by neutralization-reionization mass spe ctrometry aided by deuterium labeling. Effective QCISD(T)/6-311+G(3df,2p) a nd combined Moller-Plesset and density functional theory calculations indic ated that loss of OH from 1 was 49 U mol(-1) exothermic and proceeded on th e potential energy surface of the ground doublet electronic state of the ra dical. The loss of H and ring-cleavage dissociations were initiated by the formation of excited electronic states in 1 that provided the internal ener gy for these endothermic reactions. OH radical addition to pyridine was pre dicted by transition-state theory calculations to occur mainly (82%) in the C-3 and C-5 positions. Hydrogen atom addition to pyridine-N-oxide (2) was predicted to occur selectively at the oxygen atom and trigger a reaction se quence that can result in a highly exothermic catalytic isomerization of 2 to hydroxypyridines.