On the dissociation of methyl orange:: Spectrophotometric investigation in aqueous solutions from 10 to 90°C and theoretical evidence for intramolecular dihydrogen bonding

The dissociation of methyl orange was investigated by spectrophotometry in aqueous solutions from 10 to 90 degrees C and by quantum chemical calculations. Combined chemometric and thermodynamic analyses of the spectrophotometric data were used to simultaneously extract the thermodynamic stabilities and the spectrophotometric attributes of the dominant methyl orange species in solutions containing less than 20.00 mmol-kg(-1) perchloric acid and submillimolal concentrations of methyl orange. The analyses revealed the presence of only one monomeric deprotonated and one monomeric protonated species. The spectra did not reveal any evidence for the presence of tautomeric equilibria between the protonated azo and ammonium species in the experimental range studied. Thermodynamic analyses of the temperature-dependent dissociation constants showed the reactions to be endothermic and enthalpy driven with increasing acidity and increasing temperature. All molar absorption coefficients in the 275-375 nm range can be adequately reproduced in the 10-90 degrees C range with a set of Gauss-Lorentz parameters and used to predict the absorption spectra for any desired condition. The dominant features of the spectrophotometric attributes of the methyl orange species could also be retrieved in Time Dependent-Density Functional Theory (TD-DFT) calculations. Topological analyses of the electron density also revealed the formation of a dihydrogen bond between the azo proton and an adjacent phenyl ring hydridic hydrogen which increases the stability of the azo molecules relative to the ammonium molecule.