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 LF-hydroxypyridine and 3-hydroxypyrid ine with H2O is investigated with the use of a combined experimental m atrix-isolation FT-IR and theoretical ab-initio method. Both of these isomeric compounds occur in the hydroxy form in low-temperature Ar mat rices, in accordance with the predicted stability difference from the oxo form of 22 and 55 kT/mol for 4-hydroxypyridine and 3-hydroxypyridi ne, respectively. The most stable II-bonded complex of the hydroxy tau tomers of both compounds with water is the O-H ... OH2 structure, and the stability differences with the nearest lying isomeric N ... HO-H s tructures are 6.74 and 6.41 kT/mol, respectively. Both complexes are i dentified with the use of the predicted frequency perturbations. The c orrelation between the predicted interaction energies and the frequenc y shifts of the modes of bonded water and the scaling factors of these modes is analyzed for the series pyrimidine, pyridine, 3-hydroxypyrid ine, 4-hydroxypyridine, and 4-aminopyridine. The relative basicity and acidity of 4-hydroxypyridine and 3-hydroxypyridine are estimated from this analysis and discussed.