| Abstract |
The molar heat capacity of pyridine-3-carboxylic acid (nicotinic acid) for T = (296 to 531) K was investigated by differential scanning calorimetry (DSC) and Calvet-drop microcalorimetry. The measurements extended up to the liquid range and also covered the interval where a reversible and fast solid-solid (cr II -> cr I) phase transition occurs. The molar enthalpies and entropies of that phase transition and of fusion were obtained as T-trs = (455.0 +/- 0.2) K, Delta H-trs degrees(m) = (0.90 +/- 0.10) kJ.mol (1), Delta S-trs degrees(m) = (1.98 +/- 0.22) J.K (1).mol (1), T-fus = (509.91 +/- 0.04) K, Delta H-fus degrees(m) = (28.2 +/- 0.1) kJ.mol (1), and Delta S-fus degrees(m) = (55.30 +/- 0.16) J.K (1).mol (1). By combining these experimental results with the previously reported Delta H-sub(m)(NA; cr II) at T = 366.5 K, the corresponding entropy in the gaseous state calculated at the B3LYP/6-31+G(d,p) level of theory, and Delta H-f degrees(m)(NA); cr (II) at T = 298.15 K, it was possible to estimate the standard molar Gibbs energy of formation functions necessary for the construction of the Delta(f)G degrees(m) vs. T diagram illustrating the enantiotropic nature of this system. (C) 2012 Elsevier Ltd. All rights reserved. |
