Theoretical calculations and matrix-isolation FT-IR studies of hydrogen-bonded complexes of molecules modeling cytosine or isocytosine tautomers. 7. 2-Hydroxypyridine/2-oxopyridine complexes with H₂O

The H-bond interaction between the tautomeric cytosine-modeling system, 2-hydroxypyridine (2HP)/2-oxopyridine (2OP), with water is investigated using matrix-isolation FT-IR spectroscopy and ab initio calculations performed with the RHF and MP2 methods and the DFT method with the B3-LYP hybrid functional. Equilibrium structures of six different complexes have been found in the calculations. The theoretical results indicate that the closed N⋯H-O⋯H-O and C=O⋯H-O⋯H-N H-bonded water complexes are the most stable systems for the 2HP and the 2OP tautomers, respectively. Both closed complexes as well as the three open complexes O-H⋯OH₂, N⋯H-OH, and C=O⋯H-OH have been identified in the experimental spectrum. A detailed analysis of the structural, energetic and vibrational parameters for the closed complexes has been performed. Both in the matrix (Ar) and in the gas phase, the hydroxy tautomer, 2HP, is more stable than the oxo form, 2OP. An important observation is that addition of a single water molecule shifts the tautomeric equilibrium toward the oxo 2OP form, which is in agreement with the theoretical calculations.