High-Z' and twinning behavior in 3,4-dinitrobenzoic acid.

The title compound, C(7)H(4)N(2)O(6), (I), crystallizes in P1 with Z' = 4, but with no approximate or pseudosymmetry. The crystals were nonmerohedrally twinned, with at least four twin components related to the major moiety by 180° rotations around the real b and c axes, and by 180° rotation around the a* axis. The excessive twinning is not caused by a phase change from an untwinned room-temperature higher-symmetry polymorph. The molecules are planar chiral and, owing to the tilt angle of the nitro groups and the position of protonation, there are altogether eight conformers possible. Six of these theoretically possible eight conformers are realized in the solid-state structure of (I). Packing analysis and force-field calculations indicate that the largest part of the packing interactions does not originate from the hydrogen-bonding interactions, as one might initially be tempted to assume; π-π stacking and O(nitro)···π interactions between neighboring molecules of (I) seem to be the dominant factor that determines the packing observed in the structure of this nitro-substituted benzoic acid derivative.