A newly synthesized Schiff base, 3,3’-Dihydroxy-4,4’-[5-methyl-1,3-phenylenebis(nitrilomethylidyne)]-bis-phenol, was characterized experimentally. Its geometries optimization, tautomerization, assignment of the IR bands and NMR chemical shifts were calculated by using density functional theory (DFT) method. In addition, the Atoms in molecules (AIM) analysis was employed for investigation of its tautomerization. Four possible tautomers of the investigated Schiff base were optimized in both of the gas and solution phases. The Schiff base has no planar structure, but each of the benzene rings is in a separate plane. In the most stable tutomer, the phenolic protons of the two –OH groups are engaged in the intramolecular-hydrogen bond with the azomethine nitrogens. Good consistency between the theoretical and experimental results confirms validity of the optimized geomrtry. Also, kinetics and mechanism of the intramolecular-proton transfer of the studied Schiff base was demonstrated theoretically.

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