Thermochemical and Theoretical Studies of Dimethylpyridine-2,6-dicarboxylate and Pyridine-2,3-, Pyridine-2,5-, and Pyridine-2,6-dicarboxylic Acids

Abstract

The standard (p° = 0.1 MPa) molar enthalpies of formation for gaseous pyridine-2,5-dicarboxylic acid, pyridine-2,6-dicarboxylic acid, and dimethylpyridine-2,6-dicarboxylate were derived from the standard molar enthalpies of combustion, in oxygen, at T = 298.15 K, measured by static bomb combustion calorimetry, and the standard molar enthalpies of sublimation, at 298.15 K, measured by Calvet microcalorimetry: pyridine-2,5-dicarboxylic acid, −(580.6 ± 5.0) kJ·mol-1; pyridine-2,6-dicarboxylic acid, −(608.0 ± 6.1) kJ·mol-1; dimethylpyridine-2,6-dicarboxylate, −(562.4 ± 4.2) kJ·mol-1. The enthalpy of formation for crystalline pyridine-2,3-dicarboxylic acid was also derived from combustion calorimetry measurements as −(733.1 ± 2.0) kJ·mol-1. In addition, theoretical calculations using the density functional theory and the B3LYP/6-311G** hybrid exchange-correlation energy functional have been performed for these molecules in order to obtain the most stable geometries and to access their relative stabilities. Th...