Crystal structure of 8-hydroxyquinoline: a new monoclinic polymorph

In an attempt to grow 8-hy­droxy­quinoline–acetamino­phen co-crystals from equimolar amounts of conformers in a chloro­form–ethanol solvent mixture at room temperature, the title compound, C9H7NO, was obtained. The mol­ecule is planar, with the hy­droxy H atom forming an intra­molecular O—H(...)N hydrogen bond. In the crystal, mol­ecules form centrosymmetric dimers via two O—H(...)N hydrogen bonds. Thus, the hy­droxy H atoms are involved in bifurcated O—H(...)N hydrogen bonds, leading to the formation of a central planar four-membered N2H2 ring. The dimers are bound by inter­molecular π–π stacking [the shortest C(...)C distance is 3.2997 (17) Å] and C—H(...)π inter­actions into a three-dimensional framework. The crystal grown represents a new monoclinic polymorph in the space group P21/n. The mol­ecular structure of the present monoclinic polymorph is very similar to that of the ortho­rhom­bic polymorph (space group Fdd2) studied previously [Roychowdhury et al. (1978 [triangle]). Acta Cryst. B34, 1047–1048; Banerjee & Saha (1986 [triangle]). Acta Cryst. C42, 1408–1411]. The structures of the two polymorphs are distinguished by the different geometries of the hydrogen-bonded dimers, which in the crystal of the ortho­rhom­bic polymorph possess twofold axis symmetry, with the central N2H2 ring adopting a butterfly conformation.