Structural Features of the [C4mim][Cl] Ionic Liquid and Its Mixtures with Water: Insight from a 1H NMR Experimental and QM/MD Study

The 1H NMR chemical shift of water exhibits non-monotonic dependence on the composition of an aqueous mixture of 1-butyl-3-methylimidazolium chloride, [C4mim][Cl], ionic liquid (IL). A clear minimum is observed for the 1H NMR chemical shift at a molar fraction of the IL of 0.34. To scrutinize the molecular mechanism behind this phenomenon, extensive classical molecular dynamics simulations of [C4mim][Cl] IL and its mixtures with water were carried out. A combined quantum mechanics/molecular mechanics approach based on the density functional theory was applied to predict the NMR chemical shifts. The proliferation of strongly hydrogen-bonded complexes between chloride anions and water molecules is found to be the reason behind the increasing 1H NMR chemical shift of water when its molar fraction in the mixture is low and decreasing. The model shows that the chemical shift of water molecules that are trapped in the IL matrix without direct hydrogen bonding to the anions is considerably smaller than the 1H NMR chemical shift predicted for the neat water. The structural features of neat IL and its mixtures with water have also been analyzed in relation to their NMR properties. The 1H NMR spectrum of neat [C4mim][Cl] was predicted and found to be in very reasonable agreement with the experimental data. Finally, the experimentally observed strong dependence of the chemical shift of the proton at position 2 in the imidazolium ring on the composition of the mixture was rationalized.


Structural analysis
The H4-Cl − and H5-Cl − RDFs shown in Figures S1 and S2, respectively, exhibit similar structure as that of H2-Cl − RDF in Figure 2a of the article. The most probable H4/5· · · Cl − hydrogen bond lengths are also equal to 2.67 Å, and spherical integration of H4-Cl − and H5-Cl − RDFs up to 4.0 Å gives the coordination numbers of 1.28 and 1.22, respectively, thus somewhat lower than the coordination number of chloride anions around the C2-H2 moiety.
The distribution of the C4-H4· · · Cl − and C5-H5· · · Cl − angles in neat [C4mim][Cl] IL shown in Figures S3 and S4, respectively, exhibit similar shapes as that seen for the distribution of the C2-H2· · · Cl − angle shown in Figure 3a of the article. However, the most probable C4-H4· · · Cl − and C5-H5· · · Cl − angles are found to be around 140 deg in both cases, thus somewhat larger as compared to the most probable angle of the C2-H2· · · Cl − hydrogen bond. Part of the anions are found to be situated on top of the C4-H4 and C5-H5 bonds, and linear hydrogen bonds are rare. The distribution of the C5-C4-H4· · · Cl − and C4-C5-H5· · · Cl − dihedral angles illustrated in Figures S5 and S6, respectively, show pronounced maxima at ±120 deg just as seen for the distribution of the N1-C2-H2· · · Cl − dihedral angle in Figure 3b of the article, thus the preference for the chlorides to be located either on the butyl or the methyl side with equal probabilities. In addition, the probability for the anions to be located in the plane of the imidazolium ring are higher in these cases as compared to the corresponding case where anions are found in the vicinity of the C2-H2 bond. The distributions seen in Figures S5 and S6 are rather different in the range of -90 to +90 deg as compared to that shown in Figure 3b. They suggest that chloride anions are also often found in the area in-between C4-H4 and C5-H5 bonds, and they prefer to be located roughly in the plane of the imidazolium ring in this case.
Unlike for the case of distribution of chloride around the C2-H2 moiety, the angular distributions of chloride anions around the C4-H4 and C5-H5 bonds shown in Figures S3  and S4, respectively, are less sensitive to the amount of water in the mixture. However, the dihedral angle distributions shown in Figures S5 and S6 indicate the increasing preference for the chloride anions to stay in the region in-between the C4-H4 and C5-H5 moieties with the rising content of water in the mixture. The populations of anions on the methyl and butyl sides are thus seen to diminish, especially in the system with χ IL = 0.25. Figure S1: Radial distribution functions between H4 atom and Cl − ion in neat [C4mim] [Cl] IL and its mixtures with water, scaled by the number density of the chloride anions, n Cl , for each specific system.    [Cl] IL and its mixtures with water, scaled by the number density of the imidazolium cations, n BMI , for each specific system. Figure S11: Visualization of a QM region in the QM/MM calculations for systems A to F, see main text of the article for details. Central water molecule is represented by the ball-and-stick model. Additional species included to the QM region around the central water molecule are represented by the stick model, except for the chloride anions which are represented as green balls. Ions and water molecules described by the point charges in the QM/MM calculations are not shown. Note that the geometry of each system is taken from a single randomly selected configuration; the structure of the quantum mechanically treated region changes from configuration to configuration in each specific system. Figure S12: Visualization of a QM region in the QM/MM calculations for systems G to M, see main text of the article for details. Central C4mim + cation is represented by the ball-and-stick model. Additional species included to the QM region around the central cation are represented by the stick model, except for chloride anions which are represented as green balls. Ions and water molecules described by the point charges in the QM/MM calculations are not shown. Note that the geometry of each system is taken from a single randomly selected configuration; the structure and composition of the quantum mechanically treated region changes from configuration to configuration in each specific system. See Table 2 in the article for statistical information concerning the nature and amount of different species included to the QM region in these systems.  [Cl] IL simulated at the temperature of 298 K. The KT3 functional and def2-TZVP basis set were used in the QM/MM calculation unless stated otherwise. All entries for shielding constants are arithemtic averages over 100 molecular configurations with central cation selected randomly in each configuration.
Statistical errors calculated as sample standard deviations are given in paranthesis. The first column indicates the number of ions included to the QM region along with the central cation. Here, the three integers given in parentheses indicate the numbers of ions promoted to the QM region closest to H2, H4, and H5 atoms, respectively. Second column indicates point-charge potential used for classical C4mim + cations.
Refer to Figure 3 in the main text of the article for atom labeling in C4mim +