Distance Angle Descriptors of the Interionic and Ion-Solvent Interactions in Imidazolium-Based Ionic Liquid Mixtures with Aprotic Solvents: A Molecular Dynamics Simulation Study

The aim of this paper is to quantify the changes of the interionic and ion-solvent interactions in mixtures of imidazolium-based ionic liquids, having tetrafluoroborate (BmimBF(4)), hexafluorophosphate (BmimPF(6)), trifluoromethylsulfonate (BmimTFO), or bis(trifluoromethanesulfonyl)imide (BmimTFSI), anions, and polar aprotic molecular solvents, such as acetonitrile (AN), gamma-butyrolactone (GBL), and propylene carbonate (PC). For this purpose, we calculate, using the nearest-neighbor approach, the average distance between the imidazolium ring H atom in positions 2, 4, and 5 (H-2,H-4,H-5) and the nearest high-electronegativity atom of the solvent or anion (X) as distance descriptors, and the mean angle formed by the C-2,C-4,C-5-H-2,H-4,H-5 bond and the H-2,H-4,H-5 center dot center dot center dot X axis around the H-2,H-4,H-5 atom as angular descriptors of the cation-anion and cation-solvent interactions around the ring C-H groups. The behavior of these descriptors as a function of the ionic liquid mole fraction is analyzed in detail. The obtained results show that the extent of the change of these descriptors with respect to their values in the neat ionic liquid depends both on the nature of the anion and on the mixture composition. Thus, in the case of the mixtures of the molecular solvents with BmimBF(4) and BmimTFO, a small change of the distance and a drastic increase of the angular descriptor corresponding to the cation-anion interactions are observed with decreasing mole fraction of the ionic liquid, indicating that the anion moves from the above/below position (with respect to the imidazolium ring plane) to a position that is nearly linearly aligned with the C-2-H-2 bond and hinders the possible interaction between the C-2-H-2 group and the solvent molecules. On the other hand, in the case of mixtures of BmimTFSI and BmimPF6 with the molecular solvents, both the observed increase of the distance descriptor and the slight change of the angular descriptor with decreasing ionic liquid mole fraction are compatible with the direct interactions of the solvent with the C-2-H-2 group. The behavior of these descriptors is correlated with the experimentally observed H-1 chemical shift of the C-2-H-2 group and the red shift of the C-2-H-2 vibrational mode, particularly at low ionic liquid mole fractions. The present results are thus of great help in interpreting these experimental observations.