Disorder in the crystals of trans-4-fluoroazoxybenzene. Synthesis, spectral properties, crystal structures and DFT calculations

Two crystals of trans-4-fluoroazoxybenzene were obtained using two different methods. Oxidation of 4-fluoroazobenzene provided crystals of trans-4-fluoroazoxybenzene (I) consisting of two isomers; 51% a (ONN) and 49% b (NNO) isomer. From trans-aminoazoxybenzenes in Schiemann reaction it were obtained crystals of trans-4-fluoro-NNO-azoxybenzenes (II, b isomer) containing 4.7% of the a-isomer according to the HPLC analysis. The crystal structures of I and II were determined by the X-ray diffraction method. In the crystal I two nitrogen atoms of azoxy bridge and fluorine atom are disordered. In the crystal II, there are two independent molecules of trans-4- fluoroazoxybenzene; the molecule of b isomer and the molecule with disordered fluorine atom. The molecule of the b isomer is not planar. The torsion angles along the Ar–N(O) and the Ar–N bonds are 21.3(3) and 30.3(3)8 respectively. The weak C–H/O and C–H/F hydrogen bonds were observed in both crystal networks. To aid structural data analysis, the molecular geometry of both isomers of the trans-4- fluoroazoxybenzene were optimized at the DFT B3LYP/6-311CG** level of theory, with no imaginary frequencies. The resultant equilibrium structure is planar in each calculation (CS point group symmetry). The descriptor of p-electron delocalization HOMA was calculated to estimate aromaticity of benzene rings in the trans-4-fluoroazoxybenznes. q 2005 Published by Elsevier B.V.