The crystal and molecular structures of two para-substituted azobenzenes with π-electron-donating -NEt2 and π-electron-withdrawing -COOEt groups are reported, along with the effects of the substituents on the aromaticity of the benzene ring. The deformation of the aromatic ring around the -NEt2 group in N,N,N',N'-tetraethyl-4,4'-(diazenediyl)dianiline, C20H28N4, (I), may be caused by steric hindrance and the π-electron-donating effects of the amine group. In this structure, one of the amine N atoms demonstrates clear sp(2)-hybridization and the other is slightly shifted from the plane of the surrounding atoms. The molecule of the second azobenzene, diethyl 4,4'-(diazenediyl)dibenzoate, C18H18N2O4, (II), lies on a crystallographic inversion centre. Its geometry is normal and comparable with homologous compounds. Density functional theory (DFT) calculations were performed to analyse the changes in the geometry of the studied compounds in the crystalline state and for the isolated molecules. The most significant changes are observed in the values of the N=N-C-C torsion angles, which for the isolated molecules are close to 0.0°. The HOMA (harmonic oscillator model of aromaticity) index, calculated for the benzene ring, demonstrates a slight decrease of the aromaticity in (I) and no substantial changes in (II).

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The two isomeric compounds 4-amino-ONN-azoxybenzene [or 1-(4-aminophenyl)-2-phenyldiazene 2-oxide], i.e. the alpha isomer, and 4-amino-NNO-azoxybenzene [or 2-(4-aminophenyl)-1-phenyldiazene 2-oxide], i.e. the beta isomer, both C(12)H(11)N(3)O, crystallized from a polar solvent in orthorhombic space groups, and their crystal and molecular structures have been determined using X-ray diffraction. There are no significant differences in the bond lengths and valence angles in the two isomers, in comparison with their monoclinic polymorphs. However, the conformations of the molecules are different due to rotation along the Ar-N bonds. In the alpha isomer, the benzene rings are twisted by 31.5 (2) and 14.4 (2) degrees towards the plane of the azoxy group; the torsion angles along the Ar-N bond in the beta isomer are 24.3 (3) and 23.5 (3) degrees. Quantum-mechanical calculations indicate that planar conformations are energetically favourable for both isomers. The N-H.O hydrogen bonds observed in both networks may be responsible for the deformation of these flexible molecules.