Structures of four o-nitrobenzonitriles

The crystal structures of 6-methyl- (I), 6-chloro- (II) and 5-chloro-2-nitrobenzonitrile (III), as well as 2,6-dinitrobenzonitrile (IV), have been determined. (I), orthorhombic, Pbca, a = 9.969 (2), b = 14.728 (4), c = 10.179 (3) Å, T = 180 K; (II), orthorhombic, Pbca, a = 9.469 (5), b = 14.752 (7), c = 10.859 (5) Å, T = 297 K; (III), monoclinic, P2₁/n, a = 7.889 (2), b = 15.064 (12), c = 7.311 (4) Å, β = 118.22 (3)°, T = 189 K; (IV), orthorhombic, Pbcn, a = 13.081 (6), b = 9.027 (4), c = 6.545 (3) Å, T = 297 K. In (I)–(III) there is a short intramolecular distance [I 2.552 (4), II 2.579 (3), III 2.599 (2) Å] between one of the nitro O atoms and the adjacent nitrile C atom. These short distances plus the accompanying molecular distortions are taken as indications of incipient nucleophilic attack of the O atoms on the electrophilic nitrile C atom. Molecular orbital calculations at the Hartree–Fock level using the 6-31G* basis set support this interpretation; natural bond-orbital analysis indicates an n_O1 → π*_CN delocalization energy of 10–15 kJ mol⁻¹ for (I), (II) and (III). In (III) and (IV) the molecules pack in sheets, apparently driven by two C—H...O hydrogen bonds and a CN...Cl interaction in (III) and two C—H...O and one C—H...N hydrogen bonds in (IV).

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