Quantum-chemical calculations were performed using the Gaussian 16 program suite. . The M06-2X functional was used with the TZVP basis set for the initial optimization, then def2TZVP 31 was applied. M06-2X is specially developed to describe main-group thermochemistry and non-covalent interactions. It shows best performance, among the existing density functionals, implemented in Gaussian 16, in predicting the relative stability and specific structural parameters (i.e. non planarity and bond lengths) in tautomeric azo naphthols. All structures were optimized without restrictions, using tight optimization criteria and an ultrafine grid in the computation of two-electron integrals and their derivatives, and the true minima were verified by performing frequency calculations in the corresponding environment.