Structures 3d-f,h can be considered as models of 3a and 3c, where the individual effects of the substituents on the tautomerism of the core 3 are shown. As seen from Table each of substituents individually leads to stabilization of the H-tautomer, i.e. there is no cancelation of the effects, except in the case of 3h. Although it is believed that electron donating substituents in the phenyl ring shift the tautomeric equilibrium in 1 towards A-tautomer, exactly the opposite is seen comparing 3 and 3d. Very recently Neuerová and Lyčka have shown that the effect is exactly opposite in the case of nitro substituted in the naphthyl ring derivatives of 3, which validate the current theoretical prediction. The same hydrazone stabilization can be stated for 3d,e,g, where the substitution in 3b is modeled. It can be mentioned here that substitution with NH 2 group on position 7 (3g) does not influence the stabilization of the hydrazo tautomer compared to 3. This could suggest that the H-tautomer in 3b should be less stable compared to 3a and 3c, as it is actually observed in Table . In general, considering 3 and 3d-h it can be stated that there is no reason the H-form to be less stable in the discussed 3a-c. The relative energies shown in Table very clearly indicate that in these three compounds (and, logically, also in the rest of compounds discussed by Zhu at al., which share the same naphthyl core) the content of the azo tautomer is negligible and the optical spectra are defined by the absorption of the H-form.