The influence of like-charge attraction on the structure and dynamics of ionic liquids: NMR chemical shifts, quadrupole coupling constants, rotational correlation times and failure of Stokes–Einstein–Debye

Abstract

Ion pairing is one of the most fundamental atomic interactions in chemistry and biology. In contrast, pairing between like-charged ions remains an elusive concept. So far, this phenomenon was observed only for large-scaled structures, assemblies, stabilizing frameworks, or in aqueous solution wherein like-charge attraction is supported by mediating water molecules. Recently, we reported the formation of cationic clusters in pure ionic liquids (ILs) which all include hydroxyl groups (OH) for possible hydrogen bonding. In such structures like-charge repulsion is overcome by cooperative hydrogen bonds. The vibrational bands in the OH-stretch region of the infrared spectra can be clearly assigned to H-bonded ion pairs (c-a) or to H-bonded cationic clusters (c-c). The equilibrium between both types of ionic clusters can be controlled by using the same cation but differently strong interacting anions. In the present work, we study the influence of the cationic cluster formation on structural and dynamical NMR properties of ionic liquids, where we know that they form cationic clusters to different extent. First, we measure proton chemical shifts, δH, and determine deuteron quadrupole coupling constants, χ, from a calculated relation between both NMR properties. Reliable χ values for the liquid phase are a prerequisite for calculating reorientational correlation times, τ, from measured deuteron relaxation times, T. It is shown that the correlation times are significantly influenced by the amount of cationic clusters present in the IL. The Stokes-Einstein-Debye (SED) relation is valid for the ILs wherein H-bonded ion pairs (c-a) are the dominant species. With increasing cationic cluster (c-c) formation of e.g. cyclic tetramers, SED breaks down because of the structural heterogeneities.

Hydrogen Bonding Between Ions of Like Charge in Ionic Liquids Characterized by NMR Deuteron Quadrupole Coupling Constants—Comparison with Salt Bridges and Molecular Systems

“…However, H‐bonds in ionic liquids are manifold. They can also be present between ions of like charge . This has recently been shown for cation–cation interaction by means of vibrational spectroscopy and neutron diffraction (ND) .…”

Spectroscopic Evidence for an Attractive Cation–Cation Interaction in Hydroxy‐Functionalized Ionic Liquids: A Hydrogen‐Bonded Chain‐like Trimer

“…[23][24][25] It includes polarization as well as diffuse functions and has been shown to be suitable for calculating hydrogen bonded clusters of like-charged ions. [26,27] Although we were not able to isolate the (HPPy + ) 2 dications experimentally,we note that diatomic dications (e.g., N 2 2+ )h ave been directly observed and spectroscopically characterized in other experiments. [28] Interestingly,t hese curves often feature al ocal minimum at small internuclear distances that enable metastable bound states far into the continuum arising from separation of the two positively charged atoms.…”

“…These were calculated using unrestricted second‐order Møller‐Plesset perturbation theory (UMP2) with the well‐balanced, but small 6‐31+G* basis set . It includes polarization as well as diffuse functions and has been shown to be suitable for calculating hydrogen bonded clusters of like‐charged ions . Although we were not able to isolate the (HPPy + ) 2 dications experimentally, we note that diatomic dications (e.g., N 2 2+ ) have been directly observed and spectroscopically characterized in other experiments .…”

Curled cation structures accelerate the dynamics of ionic liquids

“…Another rationalisation is that the ammonium systems behave in a more ideal way compared to other ionic liquids cations, such as the imidazolium, where slopes of the Stokes-Einstein plots smaller than unity were found. 94 This could be explained by the absence of directed intermolecular interactions such as hydrogen bonding, 103 and the closer to spherical shape of the ammonium ions. Further experiments, which are beyond the scope of this work, could clarify whether the Stokes-Einstein relation remains valid for the ammonium ionic liquids in the low -region and on short timescales, in which the breakdown of the Stokes-Einstein behaviour is reported and dynamic heterogeneities should occur.…”