Influence of Cation Size on the Structural Features and Interactions in Tertiary Alkylammonium Trifluoroacetates: A Density Functional Theory Investigation

Abstract

We present the results of electronic structure calculations based on density functional theory (DFT) in order to investigate the reactions of the interaction of tertiary alkylamines with alkyl groups of different sizes (triethyl, tributyl, dimethylethyl, and diisopropylethyl) with trifluoroacetic acid. We have obtained data on the affinity of the studied amines with a proton. It has been shown that amine interaction with the acid leads to proton transfer from the acid to the amine and formation of ions held together in the ion pair by electrostatic interaction and a very strong hydrogen bond. We have also investigated the energy profiles of the proton transfer from the tertiary alkylammonium cation to the trifluoroacetate anion within the ion pair and found different correlations between the geometric characteristics of the H-bond and the parameters obtained by the natural bond orbitals and quantum theory of atoms in molecules analyses. It has been established that the interionic interactions in these systems weaken as the length and the degree of cation alkyl chain branching increase. A good qualitative agreement between the theoretical results and the experimental data on physicochemical properties has been obtained.

Ion Pair Structures and Hydrogen Bonding in RnNH4–nAlkylammonium Ionic Liquids with Hydrogen Sulfate and Mesylate Anions by DFT Computations

“…At the same time, for secondary amines, the PA values correlate with the p K a ones and increase as the alkyl chain becomes longer in the series DMA < DEA < DBA. A similar situation is observed for a series of tertiary (triethyl, tributyl, dimethylethyl, and diisopropylethyl) amines, suggesting that both the PA and the p K a values of these amines increase as the alkyl groups become larger . In other words, the higher the PA is, the higher is the probability of its protonation.…”

“…From Figure 1f,g,f′,g′ it can be seen that the structures of the ion pairs of TMA/A (A = CH 3 SO 3 and HSO 4 ) are largely similar to the structures of the ion pairs with a TEA cation 35 hydrogen bonds (r HO = 1.421−1.589 Å and ∠NHO = 176.2− 170.2°) were also found in the ion pairs of the TEA cation with different anions of sulfonic, carboxylic, and inorganic acids. 33,35 And as the hydrogen bond becomes shorter, straighter, and stronger, the directional character of the bond increases. The N−H•••O bonds, however, in the ion pairs with a triethyl, 93 diethylmethyl, and dimethylpropylammonium 23 cation and a chloride anion fall into the moderate hydrogen bond category (r HO = 1.773, 1.897, and 1.891 Å, respectively).…”

“…Furthermore, all these studies indicated that the magnitude of the interaction energies of the ion pairs of the triethylammonium class of PILs was significantly lower than that for the PILs with a triethanolammonium cation, which in turn led to the increase in the viscosity and decrease in the electric conductivity of the latter. , The H-bonding interaction in the ion pairs of triethylammonium-based PILs with different anions tended to become stronger with the increase in the anion proton affinity . An increase in the alkyl chain length of the cation and the degree of branching weakened the interaction between the ions in the trifluoroacetate-containing PILs with dimethylethyl, triethyl, tributyl, and diisopropylethylammonium cations . Depending on the cation and anion, the hydrogen bonds in primary alkylammonium-based PILs differ from each other covering the full range from strong, short, and linear to weak, long, and bent types. ,, An extensive study of ethylammonium nitrate (one of the earliest PILs) was performed to understand its fundamental properties. ,− It was shown that hydrogen bonding between the ethylammonium cation and the nitrate anion leads to the formation of a three-dimensional network structure, akin to water. , The same holds for propyl- and butylammonium nitrate and their solutions in acetonitrile, isopropanol, and methanol .…”

“…A number of works now aim at studying the structure and interionic interactions in tertiary alkylammonium PILs. ,,, The tertiary cation in these cases demonstrates a marked ability to act as a proton donor (due to a single N–H site) and to form one strong and directional hydrogen bond with the acid anion. In our earlier DFT studies, − we obtained numerous correlations between the quantum chemical calculations of the ion pairs with a tertiary cation and experimental data on the physicochemical properties of the corresponding PILs. Furthermore, all these studies indicated that the magnitude of the interaction energies of the ion pairs of the triethylammonium class of PILs was significantly lower than that for the PILs with a triethanolammonium cation, which in turn led to the increase in the viscosity and decrease in the electric conductivity of the latter. , The H-bonding interaction in the ion pairs of triethylammonium-based PILs with different anions tended to become stronger with the increase in the anion proton affinity .…”

Modelling biocompatible ionic liquids based on organic acids and amino acids: challenges for computational models and future perspectives

“…In other words, if the position of equilibrium ( 1) is not known or uncertain, the possibilities of obtaining a clear-cut answer from simulations are rather limited. Useful indications can certainly be had from the accurate computations of proton transfer energy profiles in isolated pairs, 126,127 and from AIMD simulations, [128][129][130] but the reliable prediction of equilibrium (1) in condensed system still defies a straightforward computational approach.…”