Hydrogen-shift isomers of ionic and neutral hydroxypyridines: a combined experimental and computational investigation

Apart from pyridine N-oxide (1a), the C5H5NO family of stable molecules comprises, 2-, 3- and 4-hydroxypyridine (2a, 3a and 4a) as well as their keto counterparts 2-, 3- and 4(1H)-pyridone (2b, 3b and 4b). This study focuses on the characterisation of their radical cations and a number of stable H-shift isomers, which are α- or β-distonic ions. This was done by using a combination of mass spectrometric experiments and computational chemistry, at the B3LYP/CBSB7 level of theory. The ionic species were identified on the basis of both their collision-induced dissociation (CID) characteristics and specific associative ion–molecule reactions with dimethyl disulfide and tert-butyl isocyanide as substrates.The distonic ions (1b+, 2c+, 2d+ and 3c+) were obtained by dissociative electron impact ionisation and subjected to neutralisation–reionisation mass spectrometry (NRMS). From CID spectra of the intense survivor ions, it follows that the neutral counterparts of the α-distonic ions 2c+ and 3c+ are viable chemical species in the rarefied gas phase. The energy-rich ylide type neutrals 1b, on the other hand, readily isomerise into pyridine N-oxide, 1a, or else dissociate.The neutral counterpart of the β-distonic ion 2d+ only has a marginal stability and part of these neutrals are proposed to isomerise into energy-rich 2-pyridone molecules 2b. This is in agreement with the computational results. However, ionised 2-pyridone cannot readily be differentiated from its enol isomer 2-hydroxypyridine. In contrast, the keto isomers of ionised 3- and 4-hydroxypyridine display characteristically different CID spectra.