Details of the crystal structure and quantum chemistry calculations of the title mol­ecule, C13H9BrO, illustrate the effects of intermolecular interactions and the substitution of one of the two aromatic rings on the molecular conformation. The asymmetry of the mol­ecule is documented by the two Caryl—Caryl—C=O torsion angles of −68.3 (5) and −17.6 (6)°. A C—H·O hydrogen bond [H·O = 2.5 Å, C·O = 3.412 (5) Å and C—H·O = 174°] and a C—H·π contact involving the H atom at position 4 of the substituted ring and the π‐system of the unsubstituted ring of an adjacent mol­ecule [H·Cg = 2.96 Å, C·Cg = 3.806 (5) Å and C—H·Cg = 153°; where Cg is the centroid of the unsubstituted ring] are observed in the crystal structure.

A combination of single-crystal and powder X-ray diffractometry was used to study the structure of two polymorphs of 4-bromobenzophenone over the temperature range from 100 to 300 K. One of the polymorphs of the title compound was known previously and its structure has been determined at room temperature [Ebbinghaus et al. (1997). Z. Kristallogr. 212, 339-340]. Two crystal growth methods were employed, one of which (a modification of the Bridgman-Stockbarger technique) resulted in single crystals of a previously unknown structure. The basic physical properties of the stable polymorph are: growth method, from 2-propanol solutions or gradient sublimation; space group, monoclinic P2(1)/c; melting point, T(m) = 355.2 K; X-ray density (at 100 K), D(x) = 1.646 g cm(-3). The same properties of the metastable polymorph (triclinic P\overline 1 ) are: growth method, modified Bridgman-Stockbarger method; X-ray density (at 100 K), D(x) = 1.645 g cm(-3); T(m) = 354 K. Thermograms suggest that the melting of the metastable form is accompanied by at least a partial crystallization presumably into the monoclinic form; the transformation is therefore monotropic. Analysis of short distances in both polymorphs shows that numerous weak hydrogen bonds of the C-H...pi type ensure additional stabilization within the respective planes normal to the longest dimension of the molecules. The strong temperature dependence of the lattice constants and of the weak bond distances in the monoclinic form suggest that the weak bond interactions might be responsible for both the large thermal expansion within plane bc and the considerable thermal expansion anisotropy.

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