Matrix isolation IR spectroscopy of tautomeric systems and its theoretical interpretation. 2-Hydroxypyridine/2(1H )-pyridinone

Prototropic tautomerism of a model system of 2-hydroxypyridine/2(1H)-pyridinone has been investigated in low-temperature inert matrices and in the gas phase. The ratio of tautomer concentrations, [hydroxy]:[oxo], for the 2-pyridinone sample deposited in Ar or N₂ matrices was estimated as 2.80:1 and 2.99:1, respectively. A similar ratio of tautomers was observed in the gas phase. The matrix and gas-phase infrared spectra of 2-hydroxypyridine and 2(1H)-pyridinone are reported. The identification of the bands corresponding to the two tautomers was accomplished by performing the UV-induced transformation of the oxo tautomer into the hydroxy tautomer in matrices, and with the use of theoretically predicted vibrational spectra. Ab initio theoretical calculations of the infrared spectra were performed within the harmonic approximation at the SCF/6-31G** level. A good agreement between experimental and theoretically predicted frequencies and intensities enabled a reliable assignment of IR bands in the spectra of both tautomers.