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.

Azo-hydrazone tautomerism in azo dyes has been modeled by using density functional theory (DFT) at the B3LYP/6-31+G(d,p) level of theory. The most stable tautomer was determined both for model compounds and for azo dyes Acid Orange 7 and Solvent Yellow 14. The effects of the sulfonate group substitution and the replacement of the phenyl group with naphthyl on the tautomer stability and on the behavior in solvent have been discussed. Intramolecular hydrogen bond energies have been estimated for the azo and hydrazone tautomers to derive a relationship between the tautomer stability and the hydrogen bond strength. The transition structures for proton transfer displayed resonance assisted strong hydrogen bonding properties within the framework of the electrostatic-covalent hydrogen bond model (ECHBM). Evolution of the intramolecular hydrogen bond with changing structural and environmental factors during the tautomeric conversion process has been studied extensively by means of the atoms-in-molecules (AIM) analysis of the electron density. The bulk solvent effect was examined using the self-consistent reaction field model. Special solute-solvent interactions were further investigated by means of quantum mechanical calculations after defining the first-solvation shell by molecular dynamics simulations. The effect of cooperative hydrogen bonding with solvent molecules on the tautomer stability has been discussed.

Intramolecular interactions of noncovalent nature in the conjugated oligomers of co-thiophene-thieno [3,4b]pyrazine-thiophene and co-thiophene-benzo[c]1,2,5-thiadiazole-thiophene are studied quantum mechanically. S‚‚‚N contacts are identified by the existence of topological bond critical points (bcp's) along the bond path by means of atoms-in-molecules (AIM) theory. The planarity of the oligomers is attributed to the observed ring critical points among the thiophene and pyrazine or thiadiazole rings. Similar systems, obtained by replacing the thiophene by pyrrole and pyrazine by ethylenedioxy, are further studied with AIM analysis. The extent of the contribution of intramolecular interactions to the intrinsic conductivity of the conjugated polymers is investigated by extrapolating the oligomer band gap values from the time-dependent density functional theory excitation energies and density functional theory HOMO-LUMO gaps and is found to be in excellent agreement with measured experimental values, where values are present. Pyrrole substitution leads to the lowest band gap value (<1 eV) among the oligomers studied. A rule of thumb for designing and engineering new low band gap polymers, which focuses on the interplay between the HOMO of the donor and LUMO of the acceptor moieties, is posed.

Sonochemical bleaching of monoazo dyes C.I. Acid Orange 7 and C.I Acid Orange 8, which exist in their hydrazone forms in dye solutions, was investigated by irradiating 40 microM dye solutions using a 300 kHz emitter. It was found that the rate of bleaching was first-order with respect to the maximum absorption of the dye in the visible band and accelerated with increased acidity. Decolorization of Acid Orange 7 was slightly faster than that of Acid Orange 8 at equivalent test conditions. The oxidative degradation of Acid Orange 7 and Acid Orange 8 were modeled by means of density functional theory calculations. The adduct formation by hydroxyl radical attack to the carbon atom bearing the azo linkage was more preferred over the attack on the nitrogen atom. A competing reaction of hydrogen abstraction from the CH3 group in C.I Acid Orange 8 was found responsible for the difference in color removal rates.