Thermodynamic and Structural Aspects of Hydrated and Unhydrated Phases of 4-Hydroxybenzamide

Single crystals of the anhydrate and hydrate [4-OH-BZA + H 2 O] with 1:1 stoichiometry of 4-hydroxybenzamide (4-OH-BZA) were grown, and their structures were solved by X-ray diffraction methods. Furthermore, the temperature dependence of the vapor pressure of the anhydrate phase was obtained, and the thermodynamic parameters of sublimation were calculated (ΔG° sub =58.9 kJ.mol -1 ; ΔH 298 sub = 117.8 ± 0.6 kJ·mol -1 ; ΔS 298 sub = 198 ± 2 J·mol -1 ·K -1 ). The differences in crystal lattice energies between the 2:1 and 1:1 hydrates and the anhydrous phase were found to be 2.2 and 6.2 kJ·mol -1 , respectively. The absolute values of the crystal lattice energies of the outlined hydrates (ΔH 298 sub (2:1) = 120 ± 1 kJ.mol -1 ; ΔH 298 sub (1:1) = 124 ± 1 kJ.mol -1 ) were obtained on the basis of sublimation and solution calorimetric experiments. Finally, the thermodynamic stability of the three crystalline phases of 4-hydroxybenzamide was estimated. In conclusion, the comparison of the crystal lattice energy of an unsolvated phase with the stabilization energy (an energy gain introduced by guest molecules) of the crystal lattice of an solvated phase enables a practical classification of host-guest molecular crystals when the sizes of the host and guest are comparable, thus making it difficult to find out an appropriate qualitative criterion to distinguish them.