Theoretical and experimental studies on proton transfer in acetate-based protic ionic liquids

Proton transfer between sulfonic acids and various propylamines by density functional theory calculations

CONTEXT

Proton transfer in acid-base systems is not well understood. Some acid-base reactions do not proceed to the extent that is expected from the difference in the pK_a values between the base and acid in aqueous solutions, yet some do. In that regard, we have computationally studied the process of proton transfer from the acids of varying strength (benzenesulfonic acid (BSu), methansulfonic acid (MsO), and sulfuric acid (SA)) to the amines with different numbers of propyl substituents on the nitrogen atom (propylamine (PrA), dipropylamine (DPrA), and tripropylamine (TPrA)) upon complexation. Density functional theory calculations were used to thoroughly examine the energetic and structural aspects of the molecular complexes and/or ionic pairs resulting from the acid-base interaction. The potential energy curves along the proton transfer coordinate in these acid-amine systems were analyzed. The change in free energies accompanying the molecular complexes and ionic pair formations was calculated, and the relationship between the energy values and the ΔРА parameter (difference in proton affinity of the acid anion and amine) was established. The larger ΔРА values were found to be unfavorable for the formation of ionic pairs. Using structural, energy, QTAIM, and NBO analyses, we determined that the hydrogen bonds in the molecular complexes PrA-MsO and PrA-BSu are stronger than those in their corresponding ionic pairs. The ionic pairs with the TPrA cation possess the strongest hydrogen bonds of all the ionic pairs being studied, regardless of the anion. The results showed that hydrogen bonding interactions in the molecular complexes contribute significantly to the energies of the acid-base interaction, while in the ionic pairs, the most important energy contribution comes from Coulomb interactions, followed by hydrogen bonding and dispersion forces. The ionic pairs with propylammonium, dipropylammonium, and tripropylammonium cations have stronger ion-ion interactions than tetrapropylammonium (TetPrA)-containing ionic pairs with the same anions. This effect rises with the order of the cation: TetPrA → TPrA → DPrA → PrA, and the sequence of anions is SA → BSu → MsO. The results obtained here expand the concept of acid-base interaction and provide an alternative to experimental searches for suitable acids and bases to obtain new types of protic ionic liquids.

METHODS

All quantum-chemical calculations were carried out by using the DFT/B3LYP-GD3/6-31++G(d,p) level as implemented in the Gaussian 09 software package. For the resulting structures, the electron density distribution was analyzed by the "atoms in molecules" (QTAIM) and the natural bond orbital (NBO) methods on the wave functions obtained at the same level of theory by AIMAll Version 10.05.04 and Gaussian NBO Version 3.1 programs, respectively.

Ion Pair Structures and Hydrogen Bonding in RnNH4-n Alkylammonium Ionic Liquids with Hydrogen Sulfate and Mesylate Anions by DFT Computations

Density function theory calculations are employed to study the interaction of amines bearing different numbers of alkyl substituents of different sizes on the nitrogen atom with sulfuric and methanesulfonic acids. The proton affinities of the studied amines are calculated, and it is shown that the higher the value is, the more probable is its protonation. The most stable structures of the ion pairs resulting from the acid-base interaction are obtained and characterized. The geometric parameters of the ion pairs and the characteristics derived from the NBO and QTAIM analysis show that there are hydrogen bonding interactions between the cation and the anion. The hydrogen bonding character of the ion pairs and the strength of the interaction between the ions strongly depend on the nature of the cation itself. The interaction between the ions in the ion pairs weakens with the increase in the cation size. The trend of change in the structural parameters of the H-bonds and energetic characteristics in the cation series for the studied ion pairs is not dependent on the nature of the anion.

How to Harvest Grotthuss Diffusion in Protic Ionic Liquid Electrolyte Systems

Hydrogen is often regarded as fuel of the future, and there is an increasing demand for the development of anhydrous proton-conducting electrolytes to enable fuel-cell operation at elevated temperatures exceeding 120 °C. Much attention has been directed at protic ionic liquids as promising candidates, but in the search for highly conductive systems the possibility of designing Grotthuss diffusion-enabled protic ionic liquids has been widely overlooked. Herein, the mechanics of proton-transfer mechanism in the equimolar mixture of N-methylimidazole and acetic acid was explored using ab initio molecular dynamics simulations. The ionicity of the system is approximated with good agreement to experiments. This system consists mostly of neutral species but exhibits a high ionic conductivity through Grotthuss-like proton conduction. Chains of acetic-acid molecules and other species participating in the proton-transfer mechanisms resembling Grotthuss diffusion could be directly observed. Furthermore, based on these findings, a series of static quantum chemical calculations was conducted to investigate the effect of substituting the anion and cation with different functional groups. We predict whether a given combination of cation and anion will be a true ionic liquid or a molecular mixture and propose some systems as candidates for Grotthuss diffusion-enabled protic ionic liquids.

Changes in microstructure of two ammonium-based protic ionic liquids proved by in situ variable-temperature 1 H NMR spectroscopy: influence of anion

In this work, changes in microstructure of two protic ionic liquids (PILs), namely n-butylammonium acetate (N4Ac) and n-butylammonium nitrate (N4NO₃ ), are proved by in situ variable-temperature ¹ H NMR spectroscopy at the temperature range from 25 to 115 °C, and the influence of the nature of anion is discussed accordingly. The results demonstrate that ¹ H NMR chemical shifts of alkyl protons of both N4Ac and N4NO₃ are almost not changed with the increasing of temperature, due to the absence of hydrogen bond interaction between alkyl protons with anions. Whereas those of ⁺ N-H of cation decrease linearly with the temperature increasing, indicating that the hydrogen bond interaction between ⁺ N-H and anion weakens gradually. In addition, the strength of hydrogen bond interaction between ⁺ N-H and NO₃^- is stronger than that between ⁺ N-H and Ac^- , suggesting that anions have a significant influence on microstructure due to the acidity of a Brønsted acid. Consequently, the proton transfer from cation to anion is much easier in N4Ac compared to N4NO₃ . Further analyses of ¹ H NMR chemical shifts of ⁺ N-H in N4Ac at the temperature range from 100 to 115 °C suggest that the splitting of ⁺ N-H peak may be attributed to obvious evidence of the existence of the proton transfer from ⁺ N-H to Ac^- , which leads to dissociate the contact ion-pair in N4Ac to form the neutral ion-pair 'molecule'. The results will help us to extensively understand the behavior of proton transfer and offer us some valuable information for the design of PILs. Copyright © 2017 John Wiley & Sons, Ltd.

Green synthesis of 1-phenyl-1-ortho-xylene ethane in IL and reaction mechanism

In this study, 1-phenyl-1-ortho-xylene ethane (PXE) is synthesized in IL and the catalysts used were AlCl₃ in 1-butyl-3-methylimidazolium bromide ([BMIM][Br]) or 1-butyl-3-methylimidazolium chloride ([BMIM][Cl]).

Reversibility of imido-based ionic liquids: a theoretical and experimental study

Theoretical and experimental methods were used to study the reversibility of a series of imido-based ionic liquids.