In this article, 20 ns molecular dynamic (MD) simulations and density functional theory (DFT) were used to investigate the absorption of CO ₂molecules by some functionalized 1,8-diazabicyclo[5,4,0]-udec-7-ene (DBU)-based ILs. According to the MD results, the highest coordination number for N⋯C is observed in the case of[DBUH ⁺][Im ⁻], which indicates that the functionalization of the imidazole anion by different alkyl groups decreases the interaction ability of the anion with CO ₂molecules. The addition of water molecules to the ILs decreases the ability of the anion to interact with CO ₂because of the hydrogen bond formation between the imidazole anions and water. Two different paths were proposed for CO ₂absorption by the ILs, and the effect of alkyl groups on the kinetics and thermodynamics of the reaction was analyzed by using the M06-2X functional at the 6-311++G(d,p) level of theory in the gas phase and water. On the basis of the results, CO ₂absorption is more favorable in[DBUH ⁺][Im ⁻], thermodynamically. Kinetic parameters show that the alkylation of the imidazole anion by ethyl, propyl, iso-propyl, and phenyl groups decreases the rate of CO ₂absorption, because of the steric and electron-withdrawing effect of different alkyl groups. In the presence of water molecules, the lowest activation Gibbs energy is related to[DBUH ⁺][Im ⁻], which confirms the greater ability of this IL in CO ₂absorption.