Matrix-isolation FT-IR studies and theoretical calculations of different types of hydrogen-bonding: 2-hydroxypyridine/2-oxopyridine complexed with HCl

The H-bond interaction of the cytosine model compound 2-hydroxypyridine and its tautomer 2-oxopyridine with HCl is investigated using the combined matrix-isolation FT-IR and theoretical density functional and ab initio methods. The theoretical calculations have been carried out at the B3-LYP/6-31++G** and RHF/631++G** levels of theory. Different types of hydrogen-bonding have been found: two closed complexes of the proton transfer type, each containing two hydrogen bends, i.e., N+-H ... Cl- H-O and C=O+-H ... CL- ... H-N; two open complexes of intermediate strength, N ... H- -Cl and C=O H- -Cl; and one weak complex, H--O ... H-Cl. The theoretical results indicate that the closed H-bonded complexes are the most stable systems for both the hydroxy and the oxo tautomers. The increased stability of these complexes is due to a cooperative H-bonding effect. The experimental spectra are consistent with this prediction, but the weaker complexes are also identified. A comparison of the experimental and calculated IR frequencies demonstrates that the frequency shifts of the vibrational modes directly involved in the H-bond interactions, especially the X-H stretching modes, are better predicted by the DFT method than by the RHF method. For the other vibrational modes not directly involved in the H-bonds, the RHF methodology has a similar accuracy compared to the DFT method.