The monoclinic crystals (space group P2 1 /a, Z = 8) of 2-phenyl-4-hydroxyphthalazin-1-one, C 14 H 10 N 2 O 2 , have two independent molecules (A and B) in the asymmetric unit. Both occur as the lactim±lactam (hydroxy±one) structure, which is also found in the parent molecule maleic hydrazide (both triclinic and monoclinic polymorphs), dichloromaleic hydrazide and luminol (3-aminophthalhydrazide). The molecular arrangement is based on strings of alternating A and B molecules linked by hydroxylÁ Á Ácarbonyl hydrogen bonds, with only van der Waals interactions between adjacent strings. Comparison is made of the measured bond lengths for (monoclinic) maleic hydrazide and values from highlevel ab initio calculations, and reasonably good agreement is obtained, with indications of improvements when allowance is made for electron correlation and hydrogen bonding.

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--H-..O 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 (To = 283 K; AHtransf --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 Antransf = 1.62 kJ mol -a and AHfus = 26.9 kJ mo1-1. This phase transformation is an example of the common type (occurrence ~,95%) where A Vtransf = (VmonoclinicVorthorhombic ) 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 m Vtransf = (Vtriclini c -Vmonoclinic ) positive. The solubility (and other) measurements of Chattaway & Lambert (1915) gave T~ = 328.43 K and AHtransf = 4.17 kJ mo1-1. 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; AHt .... f = 3.6 kJ mo1-1 and AHfus = 21.7 kJ mo1-1. These two compounds provide further examples of molecular crystals with a large hysteresis in their firstorder enantiotropic solid-state phase transformations, the transformation to the high-temperature phase © 1998 International Union of Crystallography Printed in Great Britain -all rights reserved occurring well above Tc and the low-temperature phase not being recovered on cooling below T~. Presumably the hysteresis must be ascribed to as-yet unknown features of the nucleation processes.

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