Structures of ionic liquid–water mixtures investigated by IR and NMR spectroscopy

Abstract

Imidazolium-based ionic liquids having different anions 1-butyl-3-methylimidazolium ([BMIM]X: X = Cl(-), Br(-), I(-), and BF4(-)) and their aqueous mixtures were investigated by IR absorption and proton NMR spectroscopy. The IR spectra of these ionic liquids in the CHx stretching region differed substantially, especially for C-H bonds in the imidazolium ring, and the NMR chemical shifts of protons in the imidazolium ring also varied markedly for ILs having different anions. Upon the introduction of water to screen the electrostatic forces and separate the ions, both IR and NMR spectra of [BMIM]X (X = Cl(-), Br(-), I(-)) showed significant changes, while those of [BMIM]BF4 did not change appreciably. H-D isotopic exchange rates of C(2)-H in [BMIM]X-D2O mixtures exhibited an order: C(2)-HCl > C(2)-HBr > C(2)-HI, while the C(2)-H of [BMIM]BF4 was not deuterated at all. These experimental findings, supported by DFT calculations, lead to the microscopic bulk configurations in which the anions and the protons of the cations in the halide ionic liquids have specific, hydrogen-bond type of interaction, while the BF4(-) anion does not participate in the specific interaction, but interacts less specifically by positioning itself more above the ring plane of the imidazolium cation. This structural change dictated by the anion type will work as a key element to build the structure-property relationship of ionic liquids.

Intermolecular interactions in mixtures of 1-n-butyl-3-methylimidazolium acetate and water: Insights from IR, Raman, NMR spectroscopy and quantum chemistry calculations

“…When water is added to BmimAc, they decrease with a larger extent for the C (2,4,5) -H, indicating an increase of the electron density. This behavior of the 1 H and 13 C chemical shifts of the ring atoms correlates well with the blue shift of the corresponding vibrational modes [41,80,81,[83][84][85][86][87][88][89][90]. The blue shift of the ν(C (2,4,5) -H) as a function of decreasing X BmimAc as well as the decrease of the corresponding 1 H and 13 C chemical shift is compatible with the physical picture that the interactions between the acetate anion and the Bmim + are gradually weakened because of the concomitant increasing predominance of the interactions between water molecules and the COO − moiety of the Ac − over both those between water molecules and between water and the Bmim + .…”

“…The position of these IR vibrational modes blue shifts slightly upon dilution with water particularly at low BmimAc content. The study of the literature data shows that the region above 3000 cm −1 is very sensitive to the interactions involving the C (n) -H m of the imidazolium ring of Bmim + [6,8,39,69,70,80,81]. Indeed, Fig.…”

A FT-IR spectroscopic study of ultrasound effect on aqueous imidazole based ionic liquids having different counter ions

“…The anions of PF 6 À , BF 4 À , and Cl À and oxygen atom in water could be acted as acceptor, while the hydrogen atoms in water and imidazole ring could play as donor to form hydrogen bond [5,13]. However, the O-H region in the FT-IR spectra of the OH stretching band were overlapped in different water OH band, especially for the BMI-Cl ionic liquids as indicated by previous works [30]. Therefore, deconvolution for the OH region was used to tell the change after US exposure.…”

“…These results presented that the aqueous BMI-Cl contained hydrogen bonds where water and Cl À was interacted as the form of influenced. The peak of 4 was assigned to the C-H stretching of CH 3 group which connected with N 3 atom [30] and the peaks of 1, 2 and 3 were assigned to the stretching vibration of C 4(5) -H. . .water, C 2 -H. .…”

Effects of C(2) Methylation on Thermal Behavior and Interionic Interactions in Imidazolium-Based Ionic Liquids with Highly Symmetric Anions

“…Table 3 summarizes the measured peaks and their assignments based on [4,40,41]. Signatures of the C(2) and C(10) protons can only be found in the nonmethylated and methylated ILs, respectively.…”