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.