Ab initio/DFT and AIM studies on dual hydrogen-bonded complexes of 2-hydroxypyridine/2-pyridone tautomerism.

The second-order Møller-Plesset (MP2) and density functional theory (DFT) calculations have been carried out to investigate the structures and stabilities of hydrogen (H-) bonded 2-hydroxypyridine (2HP)/2-pyridone (2PY) dimeric forms as well as 2HP-2PY complexes. The results on single-point counterpoise (CP) correction of these complexes were compared against CP-optimized correction. The nature of the intermolecular contacts in the sense of normal H-bond or blue-shifting H-bond was determined on the basis of harmonic vibrational, atom-in-molecule (AIM), and natural bond orbital (NBO) analysis. A blue-shifting C-H...N H-bond was found and NBO analysis revealed a slight decrease in the population of the contacting sigmaC-H* antibonding orbital as the primary reason of the C-H contraction. Good correlations have been established between the interaction energies and the H-bond distances versus other characteristic H-bond parameters.