Semi-empirical ZINDO/S description of the electronic structure and the spectral features of methyl orange and its products of oxidation. A study of relationship between molecular geometry and spectroscopic properties

•Theoretical description of the spectral properties of dyes helps assign the product of oxidation.•Dye decolorization reactions using manganese complexes as catalysts.•Comprehensive electronic structure analysis of methyl orange and its products of oxidation.

Molecular modelling and a detailed spectroscopic characterization have been made of 4-(4-sulfophenylazo)-N-N-dimethylaniline (methyl orange) and its products of oxidation at the azo group and at the dimethylamino group. Correlations between structural and spectroscopic features are discussed and assignments were proposed on the basis of semi-empirical (molecular mechanics MMFF and ZINDO/S (Zerner method of intermediate neglect of differential overlap for spectroscopy) calculations. Our findings are consistent with the H2O2/manganese complex catalyzed oxidation of methyl orange with the formation of the azo bond oxidized derivative.