Several solvents are applied in removal of harmful gases. In petroleum industry, to remove gases such as SO2, amine solvents are used. As in flue gas both SO2 and CO2 gases are present in high concentration, it is important to study the reactivity and stability of each gas with diamines. As a result, in the present study, quantum density functional theory method at gas phase was applied to study the absorption characteristics of SO2 and CO2. First, the optimum structure and energy of all three diamine compounds, SO2 and CO2 were explored by the computational level of theory B3LYP/6-31G(d,p). Then, the frequency analysis of the combinations was performed to guarantee the structures are at the ground state; notice that frequency must be positive. In order to investigate the reactivity [1] of SO2 and CO2 in the amines, the natural charge and LUMO and HOMO energies were obtained. and Egap are given through the following relations:

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