Modeling the Oxidative Degradation of Azo Dyes:  A Density Functional Theory Study

Abstract

In this paper, we show, using DFT methods, reactivity indices, and electron density topology, that oxidative degradation of azo dyes occurs through the cleavage of the N-N bond following hydroxyl radical addition to the chromophore. Structures for both experimentally proposed reaction pathways, involving either cleavage of the C-N or N-N bonds, have been optimized at the B3LYP/6-31G(d) level of theory; the energies were further refined using single point calculations at the B3LYP/6-311+G(d,p)//B3LYP/6-31G(d) level. Potential energy surfaces (PES) have been compared for the two mechanisms to determine which mechanism is energetically more favorable. Reactivity indices and electron density topology calculations confirmed the findings of the PES. Detailed electron density contour mapping allowed accurate visualization of the electron distribution, i.e., its topology, for the transition states. The effect of the medium dielectric constant was allowed for via self-consistent reaction field (SCRF) theory calculations using the IEFPCM method with water as solvent. A "super-molecule" approach involving complex formation between one solvent molecule and the molecules along the reaction pathway, was used to elucidate the mechanism of proton transfer.

Enhanced Photocatalytic Properties of Pure and Cr-Modified ZnS Powders Synthesized by Precipitation Method

“…The more accepted mechanism is outlined in Scheme 1. In outset, azo bond is oxidative cleavage either C-N or N-N cleavage by attaching the hydroxyl radical that generated via photocatalysis process [40] …”

Radiation induced degradation of dyes—An overview

“…Instead of C-N cleavage, N-N cleavage accompanying with the formation of nitroaromatics has also been proposed as the main process (60%) in the ultrasonic degradation of azo dyes initiated by the attack of • OH as well [62]. Based on quantum mechanical molecular orbital calculations using density functional theory, it has been proposed that oxidative degradation of amino-substituted azo dyes occurred through cleavage of N-N bond, following the • OH radical addition to the chromophore [63]. It was suggested that the presence of hydrazone tautomers might be responsible for the involvement of C-N cleavage in the degradation of azo dyes.…”

Tartrazine Solution as Dosimeter for Gamma Radiation Measurement

“…Most of the reported experimental works have been performed in aqueous aerated acidic or alkaline solutions. The decrease of absorbance with irradiation, bleaching, suggests that it can be use as a chemical dosimeter [1][2][3][4].…”