Orientation and intramolecular hydrogen bonding of nitro groups in the crystal structure of picric acid, C6H3N3O7*

Abstract

Abstract Crystals of Picric acid, C6H3N3O7 are orthorhombic, space group Pca21, and cell dimensions a = 9.262(1), b = 19.137(1), c = 9.714(1) Å with Dm = 1.78 g· cm−3 and Dx = 1.768 g · cm−3. There are eight molecules in the unit cell. The crystal structure was solved by a combination of multisolution technique and “dummy atom” method with three-dimensional data collected (to the limit of 2θ ∼ 165° for CuKα) by the stationary crystal-stationary counter technique on a GE XRD-6 diffractometer. The crystal structure was refined by least-squares method with block diagonal approximation to a final R value of 0.058. The two molecules in the asymmetric unit have different orientations of the nitro groups. In molecule I, the nitro groups are inclined by 17.0, 0.4, 7.7° whereas the corresponding values in molecule II are 2.7, 5.2 and 20.3° respectively. In both the molecules, there is an internal hydrogen bond from the OH group to the proximal oxygen of an adjacent nitro group (O–H… O 2.572, 2.619 Å). There appears to be no correlation between the C —N bond distance and the twist of the nitro groups from the mean phenyl plane. There is a wealth of internal short contacts between the various substituents of the phenyl ring.