Structure and energetics correlations in some chlorohydroxypyridines

Abstract

We have performed a study of the structure and energetics of some chlorohydroxypyridines based on experimental calorimetry techniques and high level ab initio computational calculations. The standard (p°=0.1MPa) molar enthalpies of formation of 2-chloro-3-hydroxypyridine (2-Cl-3-OHPy), 2-chloro-6-hydroxypyridine (2-Cl-6-OHPy) and 3-chloro-5-hydroxypyridine (3-Cl-5-OHPy) in the crystalline phase, at T=298.15K, were derived from the respective standard massic energies of combustion measured by rotating-bomb combustion calorimetry, in oxygen, at T=298.15K. The standard molar enthalpies of sublimation, at T=298.15K, were measured by Calvet microcalorimetry. From these experimentally determined enthalpic parameters we have derived the standard molar enthalpies of formation of the three compounds in the gaseous phase, at T=298.15K: 2-Cl–3-OHPy, −(76.8±2.0) kJ·mol−1; 2-Cl-6-OHPy, −(105.0±1.7)kJ·mol−1, 3-Cl-5-OHPy −(61.2±2.4) kJ·mol−1. These values were compared with estimates obtained from very accurate computational calculations using the G3(MP2)//B3LYP composite method and appropriately chosen reactions. These calculations have also been extended to the remaining chlorohydroxypyridine isomers that were not studied experimentally. Based on B3LYP/6-31G∗ optimized geometries and calculated G3(MP2)//B3LYP absolute enthalpies some structure–energy correlations were discussed.