Generally, two aims are followed in this thesis. The first aim is the theoretical study of the behavior of the stable tautomers of 5-aminotetrazole in adsorption on the (001) and (101) surfaces of the anatase (TiO 2 ). For this purpose, three tautomers (1HAT, 2HAT and HIMN) which are stable in the gas phase and solution were placed on the selected surfaces and the structures were optimized at the density functional theory (DFT) using DMol3 software. It was observed that the adsorption of the tautomers on the surfaces could be occurred chemically which means that the interaction between the tautomers and surface is stable. The adsorption energies were calculated by subtracting the energy of the total system from the energy of the isolated fragments in the gas phase. In addition, the effect of interaction on the energy band gap was also studied. The results showed that the adsorption of 1HAT and 2HAT on the (001) surface and HIMN on (101) surface reduces the band gap of TiO 2 . This was caused that the conductivity of TiO 2 surface and its photocatalytic activities increased. Similar studies were performed in the solvent (acetonitrile) to show the effect of solvent on the adsorption energies of tautomers. It was abserved that the adsorption energy and the energy band gap decreases in the presence of solvent. The second aim of this work was the study of the effect of the surfaces on the electronic structure of tautomers. For this purpose, a two layer ONIOM system considering tautomer as the high layer and the surface as low layer was selected. Time-dependent density functional theory (TD-DFT) was used for calculating the absorption spectra of tautomers. In addition, the IR spectra of the tautomers interacting with the surfaces were calculated.