S.F. Ghasemi Gildeh et al. / Journal of Molecular Structure 1202 (2020) 127226
3
Fig. 3. The six synthesized DBU-based ILs, (1) [BnꢀDBU][CH3CO2], (2) [BnꢀDBU][PhSO2], (3) [BnꢀDBU][HCO3], (4) [BnꢀDBU][CF3CO2], (5) [BnꢀDBU][BF4] and (6) [BnꢀDBU][SCN].
Functional theory (ADF) (2010.01) software [55e57] to understand
the nature of interaction between cations and anions of the ILs at
the PB86-D3/TZP level of theory.
IR (KBr, cmꢀ1): 3098, 3030, 2934, 2864,1645,1620,1526,1054,1451,
1326, 1054.
Selected data for [Bn-DBU][SCN]. Pale Yellow oil; 1HNMR (D2O,
The Non-Covalent Interactions, NCI, index based on the electron
density and its derivatives was used for characterization of the
intermolecular interactions [58,59]. In this work, we used Multiwfn
software [48] for NCI analysis. Gradient isosurface plots of RDG
400.13 MHz) d (ppm) 7.38e7.28 (3H, m, AreH), 7.17 (2H, d, J 6.4 Hz,
AreH), 4.50 (2H, br s, Ar-CH2), 3.54 (2H, br s, CH2), 3.45 (2H, br s,
CH2), 3.41 (2H, br s, CH2), 2.71 (2H, br s, CH2), 1.99 (2H, br s, CH2),
1.59 (4H, br s, CH2), 1.44 (2H, br s, CH2); FT-IR (KBr, cmꢀ1): 3120,
2934, 2863, 2056, 1645, 1620, 1526, 1450, 1324, 742.
versus the l2
ꢁ
r(r) were viewed by the VMD program [60] based
on the outputs of Multiwfn software.
4. Results and discussion
4.1. Experimental results
4.1.2. Thermogravimetric analysis
Thermal stability is an important property of ILs that largely
depends on their salt structure, i.e., the type of the paired cation
and anion. Fig. 4 exhibits the DSC and TGA curves of the ILs syn-
thesized by the herein presented method. As this Fig. shows, the
decomposition temperatures (Td) of the synthesized ILs fall within
the rage of 160e300 ꢂC. The first step of weight-loss up to the main
degradation step in the TGA curves of these ILs can be attributed to
the evaporation of the water sorbed by the samples upon exposure
to moist air. The first step of weight-loss for [X][SCN], [X][BF4] and
[X][HCO3] ILs (X ¼ [BnꢀDBU]) occurs at wider temperature range,
indicating that weight-loss in these ILs is greater than other syn-
thesized ILs. Noticeably, this step is composed of two smaller steps
in the case of [X][SCN]. These facts can be better interpreted as the
anions in these ILs undergo splitting during the initial step to more
stable anions and the relevant salts. The initial weight-loss step
corresponds to an endothermic trace in the DSC curves of the ILs
peaking at around 90e94 ꢂC is attributed to the evaporation of the
water molecules sorbed by the samples previously. The separate
experiments confirmed the hygroscopic nature of the ILs; the Karl-
Fischer measurement proved that [X][CF3CO2] sorbed about 11%
water from a moist air within 10 h. The main disintegration step of
the ILs starts at the onset points (Td) of 160, 180, 220, 230, 270, and
300 ꢂC for [X][CH3CO2], [X][CF3CO2], [X][PhSO2], [X][HCO3], [X]
[BF4], and [X][SCN] ILs, respectively, displaying a large dependence
to the nature of the constituting anions. Certainly, many factors
including H-bonding, non-bonding (van der Waals) attractions,
proximity and the strength of ionic bonds between the counter ions
affect the thermal stability of the ILs. The nature of anion would
keenly affect the orientation and position of H-bonds as well as the
proximity to the cation (BnꢀDBU). As can be seen, the ILs composed
of weakly coordinating (less basic) anions are more stable than the
others [61e65]. For example, CF3COꢀ2 anion is a weaker base than
CH3COꢀ2 that leads to the greater thermal stability of [X][CH3CO2] IL.
Likewise, the order of thermal stability for the other ILs was found
to be as [X][PhSO2] < [X][HCO3] < [X][BF4] < [X][SCN], which cor-
responds with the decreasing order for basicity of their anions. The
DSC trace of [X][CF3CO2] displays two additional endotherms after
the onset point of the main weight-loss step, peaking at around 204
4.1.1. Spectroscopic characterization of ILs
The FT-IR and 1H NMR spectra of the synthesized ILs were given
as supplementary information in Figs. S1 and S2, respectively.
Selected data for [Bn-DBU][CH3CO2]. Pale yellow oil; 1H NMR
(D2O, 400.13 MHz)
d (ppm) 7.35 (2H, t, J 7.8 Hz, AreH), 7.29 (1H, t, J
7.0 Hz, AreH), 7.17 (2H, d, J 7.2 Hz, AreH), 4.50 (2H, br s, Ar-CH2),
3.55 (2H, br s, CH2), 3.47e3.39 (4H, m, CH2), 2.71e2.69 (2H, m, CH2),
2.01e1.99 (2H, m, CH2), 1.87 (3H, br s, CH2), 1.60 (4H, br s, CH2), 1.44
(2H, br s, CH2); FT-IR (KBr, cmꢀ1): 2935, 2861, 1645, 1620, 1527,
1496, 1325.
Selected data for [Bn-DBU][PhSO2]. Colorless oil; 1H NMR (D2O,
400.13 MHz)
d (ppm) 7.46 (2H, br s, Ar-CH2), 7.33e7.21 (6H, m, Ar-
CH2), 7.06 (2H, d, J 6.4 Hz Ar-CH2), 4.51 (2H, br s, Ar-CH2), 3.40 (2H,
br s, CH2), 3.30 (2H, br s, CH2), 3.23 (2H, br s, CH2), 2.53 (2H, br s,
CH2), 1.89e1.84 (2H, m, CH2), 1.47 (4H, br s, CH2), 1.28 (2H, br s,
CH2); FT-IR (KBr, cmꢀ1): 2932, 2864, 1645, 1620, 1527, 1448, 1274,
1025.
Selected data for [Bn-DBU][HCO3]. Pale yellow oil; 1HNMR (D2O,
400.13 MHz)
d (ppm) 7.33e7.27 (3H, m, AreH), 7.16 (2H, d, J 6.4 Hz,
AreH), 4.49 (2H, br s, Ar-CH2), 3.53 (2H, br s, CH2), 3.44 (2H, br s,
CH2), 3.39 (2H, br s, CH2), 2.69 (2H, br s, CH2), 1.98 (4H, br s, CH2),
1.58 (2H, br s, CH2), 1.42 (2H, br s, CH2); FT-IR (KBr, cmꢀ1): 3422,
3106, 3030, 2934, 2862, 1645, 1620, 1526, 1450, 1326.
Selected data for [Bn-DBU][CF3CO2]. Pale Yellow oil; 1HNMR
(D2O, 500 MHz)
d (ppm) 7.45 (2H, t, J 7.3 Hz, AreH), 7.39 (1H, t, J
7.1 Hz, AreH), 7.27 (2H, d, J 7.4 Hz, AreH), 4.45 (2H, br s, Ar-CH2),
3.57e3.48 (6H, m, CH2), 2.82e2.80 (2H, m, CH2), 2.11e2.08 (2H, m,
CH2), 1.70 (4H, br s, CH2), 1.53 (2H, br s, CH2); FT-IR (KBr, cmꢀ1):
3126, 3058, 2936, 1710, 1647, 1598, 1447, 1205, 1122.
Selected data for [Bn-DBU][BF4]. Colorless oil; 1HNMR (D2O,
400.13 MHz) d (ppm) 7.32 (2H, t, J 6.4 Hz, AreH), 7.27 (1H, t, J 6.4 Hz,
AreH), 7.15 (2H, d, J 6.4 Hz, AreH), 4.49 (2H, br s, Ar-CH2), 3.52 (2H,
br s, CH2), 3.43 (2H, br s, CH2), 3.38 (2H, br s, CH2), 2.69 (2H, br s,
CH2), 1.97 (2H, br s, CH2), 1.57 (4H, br s, CH2), 1.42 (2H, br s, CH2); FT-