Abstarct: In this thesis, the structural, electronic, thermochemical and UV-Vis spectrum properties of the Cyanorona-20 (Cy) molecule as an anti-virus drug for COVID-19 was investigated using the density functional theory (DFT) at the theoretical level B3LYP/6-311++G(d,p). Examining the structural properties of three Cy isomers (Cy-A, Cy-B, Cy-C) in water and ethanol solvents showed that all three isomers have a planar structure, and among them, the Cy-A molecule has less compactness and more expansion. The structural properties of the tautomeric forms of Cy isomers in both gas phase and water solvent showed that the enol forms have a planar structure and keto forms deviates from planarity. Examining the frontier orbitals in the enolic forms showed that the energy gap of Cy-A is lower than the other two isomers in both gas phase and water solvent. In the gas phase, the value of the Cy-AT energy gap is lower than the other two keto forms. In all three isomers, a lower energy gap value was obtained in the water solvent have with respect to the gas phase, indicating the greater reactivity in the water solvent. The value of the dipole moments showed a larger value in the water solvent with respect to the gas phase, in which the keto forms have large values the enol forms. Thermochemical results showed that tautomeric processes are non-spontaneous at ambient temperature and pressure (298.15 K and 1 atm). All three tautomeric processes of enol to keto isomers are associated with an increase in enthalpy as the endothermic. The reaction results in the gas phase obtained showed that Cy-AT is the most stable keto form compared to its enol form in water solvent. The results of the simulated UV-Vis spectrum show that Cy-A has the highest _max and the lowest E_g among the Cy isomers in both gas phase and water solvent. In this regard, the HOMO-LUMO charge transfer is easier for Cy-A with respect to Cy-B and Cy-C. The evaluated Parameters including absorption, distribution, mextabolism and excretion (ADME) of the Cy molecule in all three isomers were in a suitable functional range. All three isomers have a low amount of lipophilicity corresponding to tendency to be hydrophilic, and so toxic effects, as a good medicinal property. consequently, these molecules have the necessary medicinal and therapeutic capabilities.