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We report the crystal structure and crystallization conditions of a first hydrated form of metacetamol (a hemihydrate), C 8 H 9 NO 2 ·0.5H 2 O. It crystallizes from metacetamol-saturated 1:1 (v/v) water-ethanol solutions in a monoclinic structure (space group P2 1 /n) and contains eight metacetamol and four water molecules per unit cell. The conformations of the molecules are the same as in polymorph II of metacetamol, which ensures the formation of hydrogen-bonded dimers and R 2 2 (16) ring motifs in its crystal structure similar to those in polymorph II. Unlike in form II, however, these dimers in the hemihydrate are connected through water molecules into infinite hydrogen-bonded molecular chains. Different chains are linked to each other by metacetamol-water and metacetamol-metacetamol hydrogen bonds, the latter type being also present in polymorph I. The overall noncovalent network of the hemihydrate is well developed and several types of hydrogen bonds are responsible for its formation.

We present the results of an experimental and computational study of structural changes in two polymorphs of tolazamide {systematic name: 1-[(azepan-1-ylamino)carbonyl]-4-methylbenzenesulfonamide}, C 14 H 21 N 3 O 3 S, on cooling to 100 K and reverse heating. No phase transitions occurred in this temperature range. The anisotropy of the thermal expansion was different for the two polymorphs and differed from that reported previously for the hydrostatic compression. The changes in different intermolecular contacts responsible for the strain anisotropy were analysed. Relative shortening of the contacts was related directly to their initial length and reversely to the steric density around them. Increasing steric density is likely to be the driving force for the conformational ordering of the azepane ring under compression.