Computational NMR Spectroscopy of Ionic Liquids: [C4C1im]Cl/Water Mixtures

Abstract

In this work, I have analyzed the structure of binary mixtures of 1-butyl-3-methylimidazolium chloride ionic liquid, [C4C1im]Cl, and water, using computational NMR spectroscopy. The structure of the complex fluid phase, where the ionic and hydrophobic nature of ionic liquids is further complicated by the addition of water, is first generated by classical Molecular Dynamics (MD) and then validated by calculating the NMR properties with DFT at the ONIOM(B3LYP/cc-pVTZ//B3LYP/3-21G) on clusters extracted during the MD trajectories. Three ionic liquid/water mixtures have been considered with the [C4C1im]Cl mole fraction of 1.00, 0.50, and 0.01, that is the pure ionic liquid [C4C1im]Cl, the equimolar [C4C1im]Cl/water mixture, and a diluted solution of [C4C1im]Cl in water. A good agreement is obtained with published experimental data that, at the same time, validates the structural features obtained from the MD and the force field used, and provides an example of the power of NMR spectroscopy applied to complex fluid phases.

Revisiting Ionic Liquid Structure-Property Relationship: A Critical Analysis

“…It can be concluded that the main 129 Xe chemical shift differences observed between the described ILs are due to the nature of the anion. The ILs can be grouped according to the type of anion and it is possible to observe five separate trends (dashed lines) between free volume and 129 distribution around Xe is more positive [110]. Accordingly, the increase in the alkyl chain of [Cnmim][NTF₂] and [Cnmim][PF₆], moves the 129 Xe chemical shift to the low field (deshielding).…”

“…Another technique used to study the local structure of ILs was 129 Xe NMR spectroscopy. The Xenon atom can occupy cavities in liquids with enough space to accommodate it, [84,[107][108][109][110] and the chemical shift (δ) of free 129 Xe gas, which is close to 0 ppm, can increase up to several hundred ppm for strongly confined environments.…”

“…Another technique used to study the local structure of ILs was 129 Xe NMR spectroscopy. The Xenon atom can occupy cavities in liquids with enough space to accommodate it, [84,[107][108][109][110] and the chemical shift (δ) of free 129 Xe gas, which is close to 0 ppm, can increase up to several hundred ppm for strongly confined environments. Brooks and co-authors [84] have shown a strong correlation between δ and V h (V h obtained from PALS, 10 −9 s timescale), proving the possibility to use 129 Xe NMR spectroscopy (10 −3 s timescale) to study free volume in ILs despite the vastly different timescales involved in the two techniques [84,111].…”

“…In order to check for a possible correlation between free volume and 129 Xe chemical shift, we compiled the data of 129 Xe chemical shift for several ImILs, reported by Morgado et al [108], and related them with the free volume obtained using the equation presented before, the results are presented in Figure 17. It can be concluded that the main 129 Xe chemical shift differences observed between the described ILs are due to the nature of the anion. The ILs can be grouped according to the type of anion and it is possible to observe five separate trends (dashed lines) between free volume and 129 distribution around Xe is more positive [110].…”

“…The ILs can be grouped according to the type of anion and it is possible to observe five separate trends (dashed lines) between free volume and 129 distribution around Xe is more positive [110]. Accordingly, the increase in the alkyl chain of [Cnmim][NTF₂] and [Cnmim][PF₆], moves the 129 Xe chemical shift to the low field (deshielding). The reverse effect is observed, the 129 Xe δ moving to a high field (shielding) with increasing alkyl chain, for the [Cnmim][Cl] and [Cnmim][I] series.…”