| Abstract |
A new hydrate of 4-hydroxynicotinic acid (4HNA), orthorhombic, space group P2(1)2(1)2, was isolated and characterized. The stoichiometry 4HNA center dot 0.5H(2)O was established both from the determination of the crystal structure by single-crystal X-ray diffraction at 296 K and from the results of thermogravimetic experiments. The packing analysis revealed that lattice water molecules are isolated from each other (isolated site hydrate) and form hydrogen bonds with four independent 4HNA units: two of the NH center dot center dot center dot O type, as the acceptor and two others of OH center dot center dot center dot OC type, as donor. A thermodynamic analysis of the driving force for water loss from 4HNA center dot 0.5H(2)O, based on calorimetric results, suggested that the reaction is exergonic (Delta(r)G(m) < 0) at 298.15 K, even for a relative humidity of 100\%. Hence, the experimentally observed robustness Of the hydrate toward spontaneous dehydration under ambient temperature, pressure, and humidity conditions must be due to the existence of a large activation barrier for the removal of the water molecules from the crystal lattice. |
