Acta Cryst. (1998). B54, 277-290 [ doi:10.1107/S0108768197012135 ]
Abstract: The crystal structures of the orthorhombic and monoclinic polymorphs of N-anilinophthalimide (m.p. of monoclinic polymorph 457 K) have been determined by X-ray diffraction at 293 K and were found to have only small differences between the molecular conformations in the two phases, but quite different molecular arrangements. There is very weak N-H
O hydrogen bonding in the orthorhombic phase and weak N-HO hydrogen bonding in the monoclinic phase. The thermal motion in the crystals of both phases has been analyzed and their thermal expansion determined. The enthalpies of solution in a number of solvents have been calculated from the solubility measurements of Chattaway & Lambert [(1915), J. Chem. Soc. 107, 1773-1781], which also give the temperature and enthalpy of the enantiotropic `orthorhombic to monoclinic' phase transformation (Tc = 283 K; ![[Delta]](/logos/entities/Delta_rmgif.gif)
Htransf = 1.54 kJ mol-1). The phase-transformation endotherm in a DSC (differential scanning calorimetry) trace from the orthorhombic polymorph occurs only at 
310 K on heating and there is no corresponding exotherm on cooling; the DSC trace gives Htransf = 1.62 kJ mol-1 and Hfus = 26.9 kJ mol-1. This phase transformation is an example of the common type (occurrence 95%) where Vtransf = (Vmonoclinic - Vorthorhombic) is positive. Analogous (but less complete) results have been obtained for the monoclinic and triclinic polymorphs of N-(N'-methylanilino)phthalimide (m.p. of triclinic polymorph 398 K). There were only minor differences between the molecular conformations in the two phases, but the molecular arrangements were quite different. This `monoclinic to triclinic' phase transformation also has Vtransf = (Vtriclinic - Vmonoclinic) positive. The solubility (and other) measurements of Chattaway & Lambert (1915) gave Tc = 328.43 K and Htransf = 4.17 kJ mol-1. A DSC trace for the monoclinic crystals shows a broad endotherm at 372-376 K on heating, but there is no corresponding exotherm on cooling; Htransf = 3.6 kJ mol-1 and Hfus = 21.7 kJ mol-1. These two compounds provide further examples of molecular crystals with a large hysteresis in their first-order enantiotropic solid-state phase transformations, the transformation to the high-temperature phase occurring well above Tc and the low-temperature phase not being recovered on cooling below Tc. Presumably the hysteresis must be ascribed to as-yet unknown features of the nucleation processes.
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