Controlling the Subtle Energy Balance in Protic Ionic Liquids: Dispersion Forces Compete with Hydrogen Bonds

Abstract

The properties of ionic liquids are determined by the energy-balance between Coulomb-interaction, hydrogen-bonding, and dispersion forces. Out of a set of protic ionic liquids (PILs), including trialkylammonium cations and methylsulfonate and triflate anions we could detect the transfer from hydrogen-bonding to dispersion-dominated interaction between cation and anion in the PIL [(C6 H13 )3 NH][CF3 SO3 ]. The characteristic vibrational features for both ion-pair species can be detected and assigned in the far-infrared spectra. Our approach gives direct access to the relative strength of hydrogen-bonding and dispersion forces in a Coulomb-dominated system. Dispersion-corrected density functional theory (DFT) calculations support the experimental findings. The dispersion forces could be quantified to contribute about 2.3 kJ mol(-1) per additional methylene group in the alkyl chains of the ammonium cation.

Non-covalent interactions in ionic liquid ion pairs and ion pair dimers: a quantum chemical calculation analysis

“…2,3 A significant contribution is also supposed to arise from a three-dimensional network of hydrogen bonds between the counterions 4,5 as well as from dispersion interactions. 8,9 Apart from the binding energy perspective, a comprehensive understanding of the complete pattern of non-covalent interactions between the constituting ions and the related electron density distribution is a key prerequisite for consistent IL characterization. These weak non-covalent interactions can contribute up to 20 percent of the overall interaction energy in ILs.…”

Density Functional Theory Analysis of the Impact of Steric Interaction on the Function of Switchable Polarity Solvents

“…A significant portion (if not all) of the 43−50% ion-pairing character is expected to involve the bicarbonate anion associated with the ammonium cation through a hydrogen bond, which is consistent with other protic ionic liquids (PIL). 32 Previously published data from our lab identified that, when the carbonate anion is not associated, there is likely a tightly bonded hydration environment. 18,33 It was suggested that this hydration sphere required a steric environment which is supported by significant steric impacts on entropic stability of BH + ·H 2 O complexes.…”

Theoretical study on the structure and cation–anion interaction of triethylammonium chloroaluminate ionic liquid

“…Therefore, the interaction energy is not determined by H-bond interaction or Van der Waals interaction individually but by the combined effect of those two interactions. As a rule, the considerable effect of Van der Waals interaction results from the ethyl chains in cation [47].…”