2 POLYMORPHS OF 3,5-DINITROBENZOIC ACID

C7H4N2O6, M(r) = 212.1, monoclinic, P2(1)/c, a = 10.0290 (6), b = 8.8711 (7), c = 9.514 (2) angstrom, beta = 95.639 (12)degrees, V = 842.4 (3) angstrom 3, Z = 4, D(x) = 1.672 g cm-3., lambda(Cu K-alpha) = 1.54184 angstrom, mu = 12.74 cm-1, F(000) = 432, T = 297 K, R = 0.042 for 1610 observations (of 1721 unique data); monoclinic, C2/c, a = 21.036 (2), b = 8.7331 (6), c = 9.7659 (8) angstrom, beta = 111.051 (7)degrees, V = 1674.3 (5) angstrom 3, Z = 8, D(x) = 1.683 g cm-3., lambda(Cu K-alpha) = 1.54184 angstrom, mu = 12.82 cm-1, F(000) = 864, T = 296 K, R = 0.043 for 1583 observations (of 1725 unique data). The two polymorphs are both monoclinic, albeit with different space groups. Distances and angles are quite similar in the two polymorphs. There is no melting point difference. There is a strong intermolecular O-H...O hydrogen bond between two carboxylic groups; the donor is the acidic hydrogen on one molecule, and the acceptor is the carbonyl-type oxygen of the carboxylic group on the other molecule. There is no important difference in the strength of those interactions between the two polymorphs. The HO2...O1 distances are 1.51 (4) and 1.55 (3) angstrom, the O1-HO2-O2 angles are 165.5 (30) and 161.9 (30)degrees in the P2(1)/c and C2/c polymorphs respectively. In both polymorphs, one nitro group is out of the plane formed by the aromatic ring by 24.0 (1) for P21/c and 20.0 (1)degrees for C2/c. The hydroxy moiety of the carboxyl group lies on the same side of the molecule as the out-of-plane nitro group.