Matrix-isolation FT-IR studies and ab-initio calculations of hydrogen-bonded complexes of molecules modeling cytosine or isocytosine tautomers. 3. Complexes of 4-hydroxypyridine and 3-hydroxypyridine with H₂O in Ar matrices

The hydrogen-bond interaction of 4-hydroxypyridine and 3-hydroxypyridine with H₂O is investigated with the use of a combined experimental matrix-isolation FT-IR and theoretical ab-initio method. Both of these isomeric compounds occur in the hydroxy form in low-temperature Ar matrices, in accordance with the predicted stability difference from the oxo form of 22 and 55 kJ/mol for 4-hydroxypyridine and 3-hydroxypyridine, respectively. The most stable H-bonded complex of the hydroxy tautomers of both compounds with water is the O-H⋯OH₂ structure, and the stability differences with the nearest lying isomeric N⋯HO-H structures are 6.74 and 6.41 kJ/mol, respectively. Both complexes are identified with the use of the predicted frequency perturbations. The correlation between the predicted interaction energies and the frequency shifts of the modes of bonded water and the scaling factors of these modes is analyzed for the series pyrimidine, pyridine, 3-hydroxypyridine, 4-hydroxypyridine, and 4-aminopyridine. The relative basicity and acidity of 4-hydroxypyridine and 3-hydroxypyridine are estimated from this analysis and discussed.