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Ionic liquids (ILs) provide a promising medium for CO capture. Recently, the family of ILs comprising imidazolium-based cations and acetate anions, such as 1-ethyl-3-methylimidazolium acetate (EMIOAc), has been found to react with CO and form carboxylate compounds. N-Heterocyclic carbene (NHC) is widely assumed to be responsible by directly reacting with CO though NHC has not been detected in these ILs. Herein, a computational analysis of CO capture in EMIOAc is presented. Quantum chemistry calculations predict that NHC is unstable in a polar environment, suggesting that NHC is not formed in EMIOAc. Ab initio molecular dynamics simulations indicate that an EMI ion "activated" by the approach of a CO molecule can donate its acidic proton to a neighboring OAc anion and form a carboxylate compound with the CO molecule. Analysis of this termolecular process indicates that the EMI-to-OAc proton transfer and the formation of 1-ethyl-3-methylimidazolium-2-carboxylate occur essentially concurrently. Based on these findings, a novel concerted mechanism that does not involve NHC is proposed for CO capture.