Structures of four o-nitrobenzonitriles

The crystal structures of 6-methyl- (I), 6-chloro- (II) and 5-chloro-3-nirtobenzonitrile (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) Angstrom, T = 180 K; (II), orthorhombic, Pbca, a = 9.469(5), b = 14.752(7), c = 10.859(5) Angstrom, T = 297 K; (III), monoclinic, P2(1)/n, a = 7.889(2), b = 15.064(12), c = 7.311(4) Angstrom, beta = 118.22(3) Angstrom, T = 189 K; (IV), orthorhombic, Pbcn, a = 13.081(6), b = 9.027(4), c = 6.545(3) Angstrom, T = 297 K. In (I)-(III) there is a short intramolecular distance [I 2.552(4), II 2.579(3), III 2.599(2) Angstrom] 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(Ol) --> pi*(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).