FIGURE 4. Absorption of CO2 in (a) [N2222][CH(CN)2] and (b) [N2222][CH(CN)2]-Eim at different temperatures.
Combining all the results or information obtained, a plausible mechanism of CO2 absorption in [N2222][CH(CN)2]-Eim is proposed and schematically given in Figure 5 (the detailed data are given in Table S8 ). The mechanism is expressed as the synergy of carbanion siting for the insertion of CO2 and hydrogen bonding for the stabilization of [CH(CN)2]-CO2-Eim complex. To further verify the mechanism, theoretical calculations are performed using the density functional theory (DFT) at the B3LYP/6-311++G(d, p) level.45-47 It is indicated from the optimized configurations (Figure 5b ) that [C(CN)2]-COOH is produced via the insertion of CO2 into the carbanion site followed by the hydrogen transfer from the carbanion site to the oxygen site of CO2 in the first stage. The central carbon of carbanion undergoes structural changes from sp2 to sp3 and finally back to sp2hybridization, and the carbon of CO2 is also hybridized from sp to sp2 in this stage, same as the finding by Dai and his coworker24. However, due to the existence of Eim in the DES, [C(CN)2]-COOH can be further stabilized through the formation of hydrogen bonding with Eim in the second stage (Figure 5c ). A quasi-six-membered ring in the same plane is generated with the assistance of two intramolecular hydrogen bonds (bond lengths of HEim−OCO2 and H−NEim are 2.55 and 1.66 Å). The enthalpy change for the carbanion siting of CO2 in the first stage is −29.6 kJ/mol, while the value under the synergy of carbanion siting and hydrogen binding is as negative as −68.3 kJ/mol. The energy change due to the formation of two intramolecular hydrogen bonds in the second stage is thus calculated to be −38.7 kJ/mol from the two data above, close to the value of -39.6 kJ/mol calculated from the experimental solubility of CO2. The synergistic mechanism of carbanion siting and hydrogen bonding reveals the feasibility of efficient CO2 capture using carbanion-based DESs.