On Tautomerism and Self-Association of 2-Pyridinol/2-Pyridone System

The semiempirical “Effective Pair Correlation Energy” method of Sinanoglu et al. has been employed to investigate the role of the electron correlation energy in the cis-trans, protomeric and dimerization equilibria in the 2-pyridinol (HP2)/2-pyridone (P2) system. CNDO/2 wave functions were used throughly. Preliminary calculations on test cases formic acid, water dimer, and ammonia-water dimer gave acceptable figures in comparison with more accurate literature data. The results show that in HP2 the N–C–O–H cis isomer is more stable than the trans isomer by 2.7 kJ mol−1. Correlation energy favors the trans isomer by 2.9 kJ mol−1 and the keto P2 form over the HP2 form by 11.3 kJ mol−1, in qualitative agreement with results of ab initio calculations. The correlation energy term is stabilizing for the associative process of both tautomers. The numerical data are greatly dependent on the intermolecular equilibrium distance, the best results being obtained at experimental equilibrium distances. (P2)2 dimer is correctly predict to be more stable than (HP2)2 dimer. The correlation energy contribution amounts to 22 and 12.4 kJ mol−1 per hydrogen bond in (HP2)2 and (P2)2, respectively.