| Abstract |
The standard molar enthalpies of formation and sublimation of crystalline (monoclinic, space group P2(1)/c) nicotinic acid (NA), at 298.15 K, were determined as Delta H-f(m)o(NA, cr) = -344.7 +/- 1.2 kJ . mol(-1) and Delta H-sub(m)o(NA) = 112.1 +/- 0.5 kJ.mol(-1) by using combustion calorimetry, drop-sublimation Calvet microcalorimetry, and the Knudsen effusion method. The experimental determinations were all based on a sample of NIST Standard Reference Material 2151, which was characterized in terms of chemical purity, phase purity, and morphology. From the above results, Delta H-f(m)o(NA, g) = -232.6 +/- 1.3 kJ.mol(-1) could be derived. On the basis of this value and on published experimental data, the enthalpy of the isodesmic reaction nicotinic acid(g) + benzene(g) -> benzoic acid(g) + pyridine(g) was calculated as -3.6 +/- 2.7 kJ.mol(-1) and compared with the corresponding predictions by the B3LYP/cc-pVTZ (-3.6 kJ.mol(-1)), B3LYP/(aug-cc-pVTZ) (-3.7 kJ.mol(-1)), B3LYP/6-311++G(d,p) (-4.2 kJ.mol(-1)), G3MP2 (-4.3 kJ.mol(-1)), and CBS-QB3 (-4.0 kJ.mol(-1)) quantum chemistry models. The excellent agreement between the experimental and theoretical results supports the reliability of the Delta H-f(m)o(NA, cr), Delta H-sub(m)o(NA), and Delta H-f(m)o(NA, g) recommended in this work. These data can therefore be used as benchmarks for discussing the energetics of nicotinic acid in the gaseous and crystalline states and, in particular, to evaluate differences imparted to solid forms by the production and processing methods. Such differences are perhaps at the root of the significant inconsistencies found between the published enthalpies of sublimation of this important active pharmaceutical ingredient and thermochemical standard. The molecular packing in the crystalline phase studied in this work was also discussed and its influence on the molecular structure of nicotinic acid was analyzed by comparing bond distances and angles published for the solid state with those predicted by the B3LYP/cc-pVTZ method. No advantage in terms of accuracy of the structural predictions was found by the use of the larger aug-cc-pVTZ or 6-311++G(d,p) basis sets. |
