The oxidation of the diazo group (-N=N-) leads to the azoxy and azodioxy compounds [1] as shown below:

The functional groups -NN(O)-, -(O)NN- and -N(O)N(O)- differ in their properties as substituents. To quantify the extent of the changes of electronic structure of the diazo group upon oxidation we have applied a method of estimation the resonance substituent constance (σ^- values), which based on the variation of the aromatic character of 8-substituted heptafulvene [2]. While the electronic character of the -NN(O)-, -(O)NN- groups is rather similar to the diazo group (σ^- = 1.08, 1.00 and 1.02, respectively), the azodioxy group is significantly more electron-accepting substituent (with σ^- = 1.20). The analysis is based on the ab initio optimized geometries at DFT B3LYP/6-311+G** level of theory. Since the diazo- and azoxy- groups are isoelectronic with the nitroso and nitro groups, the comparative study of their influence on π-electron delocalization of benzene was performed.

[1] A. F. Wells, Structural Inorganic Chemistry, Oxford University Press, 1984, ch. 18.

[2] a) B. T. Stępień, T. M. Krygowski, M. K. Cyrański, J. Org. Chem. 2002, 67, 5987. b) B. T. Stępień, T. M. Krygowski, M. K. Cyrański, Chem. Phys. Lett. 2001, 350, 537. c) T. M. Krygowski, B. T. Stępień, Chem. Rev. 2005, 105, 3482.

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