An ab initio Study of the Structure and Energetics of Hydrogen Bonding in Ionic Liquids

Abstract

Unlike typical hydrogen-bonded networks such as water, hydrogen bonded ionic liquids display some unusual characteristics due to the complex interplay of electrostatics, polarization, and dispersion forces in the bulk. Protic ionic liquids in particular contain close-to traditional linear hydrogen bonds that define their physicochemical properties. This work investigates whether hydrogen bonded ionic liquids (HBILs) can be differentiated from aprotic ionic liquids with no linear hydrogen bonds using state-of-the-art ab initio calculations. This is achieved through geometry optimizations of a series of single ion pairs of HBILs in the gas phase and an implicit solvent. Using benchmark CCSD(T)/CBS calculations, the electrostatic and dispersion components of the interaction energy of these systems are compared with those of aprotic ionic liquids. The inclusion of the implicit solvent significantly influenced geometries of single ion pairs, with the gas phase shortening the hydrogen bond to reduce electrostatic interactions. HBILs were found to have stronger interactions by at least 10EtMeNH0 kJ mol −1 over aprotic ILs, clearly highlighting the electrostatic nature of hydrogen bonding. Geometric and energetic parameters were found to complement each other in determining the extent of hydrogen bonding present in these ionic liquids.

The explicit role of electron exchange in the hydrogen bonded molecular complexes

“…Intermolecular hydrogen bonds (H‐bonds) are of paramount importance in supramolecular chemistry, 1,2 crystal engineering 3 and biochemistry 4 . These noncovalent interactions have been intensively studied by various theoretical and experimental techniques 5–8 : ab initio calculations, 9,10 molecular dynamic modeling, 11 quantum mechanics/molecular mechanics calculations, 12 NMR and IR spectroscopy, 13–15 X‐Ray and neutron diffraction 16–18 and thermochemical experiments 19 . Among theoretical approaches, the electron density 7,20–23 and energy decomposition analyses 7,24 are gaining considerable popularity in the studies of H‐bonds in the isolated state, 25,26 solution 27 and solid state 28,29 .…”

Ab Initio Molecular Dynamics Study of Phospho-Amino Acid-Based Ionic Liquids: Formation of Zwitterionic Anions in the Presence of Acidic Side Chains

“…At the same time, gas-phase isolated pair computations are difficult to converge to the correct ionic (charge-separated) structure because of the natural tendency for isolated molecules to mutually neutralize. 10 Implicit solvent models provide a partial solution to this problem. 11 In order to perform MD, either a polarizable force field has to be employed, or electronic density has to be taken into account to calculate fluctuating atomic charges.…”

Multi-Interactions in Ionic Liquids for Natural Product Extraction

“…Theoretical studies based on simulation, including ab initio calculations, Car-Parrinello molecular dynamics simulations as well as classical molecular dynamics simulations, have also been employed to reveal the interactions between ions in ILs. Combining these simulations with various spectroscopic characterizations could further prove the existence of hydrogen bonding interactions in ILs [ 33 , 34 ]. Zentel et al [ 35 ] utilized density functional-based tight-binding (DFTB) and DFT to study the infrared spectroscopic and dynamics signals of hydrogen bonding in ethylammonium nitrate salt.…”