POLYMORPHISM OF 1-CHLORO-2,4-DINITROBENZENE - STRUCTURES OF ALPHA-FORMS AND -BETA-FORMS

C6H3ClN2O4, M(r) = 202.55, lambda-(Mo K-alpha) = 0.7107-angstrom. alpha-form: orthorhombic, Pccn, a = 8.937 (5), b = 11.035 (5), c = 15.844 (5) angstrom, V = 1562.5-angstrom-3, F(000) = 816, Z = 8, D(x) = 1.721 Mg m-3, mu = 0.407 mm-1, T = 290 K. beta form: monoclinic, P2(1)/c, a = 8.91 (1), b = 6.870 (5), c = 12.98 (1) angstrom, beta = 96.15 (5)-degrees, V = 790.0-angstrom-3, F(000) = 408, Z = 4, D(x) = 1.701 Mg m-3, mu = 0.403 mm-1, T = 290 K. For the alpha-form, R = 0.045 for 1474 observed reflexions; for the beta-form, R = 0.051 for 976 observed reflexions. Different molecular conformations are found: in the alpha-form the p-nitro group is rotated 41.8 (4)-degrees relative to the benzene ring, in the beta-form the rotation is 68.8 (6)-degrees. The p-nitro groups are rotated 11.1 (4) and 13.1 (6)-degrees, in opposite directions. Molecular-energy calculations reveal negligible differences between the two conformations.