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.