To look into the different underlying mechanism of CO2absorption in
[N2222][CH(CN)2]-Eim from that
in the pure IL, NMR and FTIR analyses are done and given inFigures 2, 3 and S3-S5 . It can be seen from Figure 2Athat a new signal appears at 173.9 ppm (peak 10*) after
CO2 chemisorption, and the signals of two carbons in the
anion [CH(CN)2]- shift downfield
from -0.0002 to 31.5 ppm (peak 4*) and upfield from 133.3 to 123.6 ppm
(peak 3*), respectively. Such signal changes in the DES are nearly the
same as those in the pure IL (Figures S3 and S4 ), both
indicating the formation of a new species,
[C(CN)2]-COOH, from the adduction of
CO2 with the anion,[14] However,
the carbon signals in Eim all shift upfield after CO2absorption, especially the three aromatic carbons of Eim (peaks 7* to
9*). In addition, it is shown in 1H NMR
(Figure 2B ) that hydrogens 5* to 9* are also all shift upfield.
In particular, hydrogen 7* has the tendency of forming two adjacent
signal peaks. All the observations above implies the active
participation of Eim in the reaction of CO2 with
[N2222][CH(CN)2]. It is deduced
that Eim participates the reaction probably via forming hydrogen bonds
with [CH(CN)2]-CO2 adduct, since the
acidic hydrogen 7 and its adjacent alkaline nitrogen of Eim are hydrogen
bond donor and acceptor, respectively. It is interestingly noted that a
weak peak appears at 3.3 ppm in the 1H-NMR spectra of
samples after CO2 absorption. It should be assigned to
the hydrogen in [CH(CN)2]-, due to
the fact that the signal peak 2 masks peak 3 in the fresh
[N2222][CH(CN)2]-Eim and the
hydrogen in the samples in use or after use is detectable again as a
weak signal when peak 2 moves slightly its location. The conclusion is
also supported indirectly from the 13C NMR of
[N2222][CH(CN)2]-Eim, where no
impurity carbon signals are visible even
after
5 cycles of use.
FIGURE 2. (A) 13C NMR spectra
(CDCl3 in capillary tubes,
101 MHz) of
[N2222][CH(CN)2] before and
after CO2 absorption; (B) 1H NMR
spectra (CDCl3 in capillary tubes, 400 MHz) of
[N2222][CH(CN)2]-Eim before and
after CO2 absorption.
In FTIR spectra (Figure 3 ), two characteristic peaks at 1621
cm-1 and 1286 cm-1 are dramatically
strengthened after the saturation of CO2, evidenced as
the stretching vibrations of C=O and C-O groups in the species
[C(CN)2]-COOH. The two peaks exist also in the FTIR
of fresh sample as very weak signals, primarily due to the slight uptake
of CO2 from the air during the offline FTIR analysis.
The existence of COOH group can also be evidenced from the two broad and
weak peaks at 2700 to 2500 cm-1 that originate from
the frequency combination of C=O stretching with O-H bending and the
frequency doubling of C-O stretching. Another very important information
is the two peaks of -C≡N that shift from 2104 to 2160
cm-1 and from 2156 to 2195 cm-1after the absorption of CO2. There is also a peak at
2077 cm-1 disappearing after CO2absorption, probably due to the fade of C=C=N stretching when
CO2 is inserted into the anion
[CH(CN)2]-. The FTIR analysis
above is in consistence with the NMR results, confirming the formation
of [C(CN)2]-COOH.