Molecular structure effects on the kinetics of hydroxyl radical addition to azo dyes

Abstract

The effect of the molecular structure of azobenzene and related azo dyes on their reactivity towards .OH radicals in water was investigated by performing ultrasonic irradiation experiments on their aqueous solutions and density functional theory (DFT) calculations. Sonolysis of azobenzene, methyl orange, o-methyl red and p-methyl red was performed at a frequency of 500 kHz and 50 W applied power under air saturation. Under such irradiation conditions, these molecules were shown to decompose through .OH radical addition reactions taking place in the bulk liquid. The ortho isomer of methyl red reacted at significantly higher rates (nearly 30% higher) than the other three studied compounds in non-buffered aqueous solutions. In contrast, measurements performed at lower pH (10 mM HNO3), at which the carboxylic group vicinal to the azo group is protonated, yielded a similar reaction rate for all four substrates, i.e. the specific acceleration observed in the ortho-substituted dye disappeared with protonation. These results were rationalized by the computation of formation energies of the adduct originated in the .OH addition to the azo group, performing DFT calculations combined with the polarized continuum model (PCM) of solvation. The calculations suggest that intramolecular H-bonding in the o-methyl red–OH adduct provides extra stabilization in that particular case, which correlates with the observed higher addition rates of .OH radical to the anionic form of that isomer in non-buffered solutions. On the other hand, the energy changes calculated for the .OH addition to an o-methyl red molecule which is protonated in the carboxylic group (representative of the situation at pH 2) do not differ significantly from those computed for the other three molecules studied.