TD-DFT calculations and thermal effects on conformers of calmagite in protic solvents varying the degree of protonation

The main absorption peaks were obtained for 1-(1-hydroxy-4-methyl-2-phenylazo)-2-naphthol-4-sulfonic acid. Generalized gradient approximation, hybrid, semi-empirical, and Coulomb attenuating methods were utilized to compare theoretical electronic transitions and experimental absorption spectra at different pH. The main peaks and shoulders observed in experimental spectra were assigned to its correct conformer. In order to find the most populated conformer, thermal effects on stability calculations were investigated to obtain molar fractions of possible isomers present at room and higher temperature. Theoretical electronic transitions at distinct pH could be obtained varying the protonation a deprotonation degree. It was found that generalized gradient approximation performs very well the first transition peak at neutral pH. For higher pH, all methodologies got a bathochromic shift in agreement with experiment and finally, from these theoretical results, it was obtained that this azo dye is hardly protonated in experiments since results presented here, predict a variation of absorption spectra for all proposed methodologies when the molecule is protonated, which is different to experimental results. Graphical Abstract Calculated electronic transitions of azo and hydrazone tautomers in water implicit solvent (BLYP/6-311G(2d,p) methodology)