To illustrate the formation mechanism of normal and abnormal N-heterocyclic carbene-carbon dioxide adducts (NHC-CO(2)and aNHC-CO2), we implement density functional theory calculations on the reactions of two imidazolium-based ionic liquids ([C(2)C(1)Im]-[OAc] and [C2C (1)Im][CH3SO3]) with CO2. The reaction of [C(2)C(1)Im]-[OAc] with CO2 is mimicked using the gas phase model, implicit solvent model, and combined explicit-implicit solvent model. In the gas phase, the calculated barriers at 125 degrees C and 10 MPa are 12.1 kcal/mol for the formation of NHC-CO2 and 22.5 kcal/mol for the formation of aNHC-CO2, and the difference is significant (10.4 kcal/mol). However, the difference becomes less important (1.5 kcal/mol) as the solvation effect is considered more realistically using the combined explicit-implicit solvent model, rationalizing the experimental observation of aNHC-CO2 adduct in the [C2C (1)Im][OAc]-CO(2)system. The anion of the ionic liquid is shown to play a substantial role, which can adjust the reactivity of imidazolium cation toward CO2: upon replacement of the basic [OAc](-)anion with a less basic [CH3SO3] (-)anion, the reaction becomes very difficult, as indicated by high free energy barriers involved (41.4 kcal/mol for the formation of NHC-CO2 and 39.2 kcal/mol for the formation of aNHC-CO2). This is in agreement with the fact that neither NHC-CO2 or aNHC-CO2 is formed in the [C C-2 (1)Im][CH3SO3]-CO(2)system, emphasizing the important dependence of the reactivity on the basicity of the anion of imidazolium-based ionic liquids for the formation of NHC- and aNHC-CO2 adducts.