Density Functional Computations and Molecular Dynamics Simulations of the Triethylammonium Triflate Protic Ionic Liquid

Abstract

Systematic molecular dynamics simulations based on an empirical force field have been carried out for samples of triethylammonium trifluoromethanesulfonate (triethylammonium triflate, [TEA][Tf]), covering a wide temperature range 200 K ≤ T ≤ 400 K and analyzing a broad set of properties, from self-diffusion and electrical conductivity to rotational relaxation and hydrogen-bond dynamics. The study is motivated by recent quasi-elastic neutron scattering and differential scanning calorimetry measurements on the same system, revealing two successive first order transitions at T ≈ 230 and 310 K (on heating), as well as an intriguing and partly unexplained variety of subdiffusive motions of the acidic proton. Simulations show a weakly discontinuous transition at T = 310 K and highlight an anomaly at T = 260 K in the rotational relaxation of ions that we identify with the simulation analogue of the experimental transition at T = 230 K. Thus, simulations help identifying the nature of the experimental transitions, confirming that the highest temperature one corresponds to melting, while the one taking place at lower T is a transition from the crystal, stable at T ≤ 260 K, to a plastic phase (260 ≤ T ≤ 310 K), in which molecules are able to rotate without diffusing. Rotations, in particular, account for the subdiffusive motion seen at intermediate T both in the experiments and in the simulation. The structure, distribution, and strength of hydrogen bonds are investigated by molecular dynamics and by density functional computations. Clustering of ions of the same sign and the effect of contamination by water at 1% wgt concentration are discussed as well.

Linking Structure to Dynamics in Protic Ionic Liquids: A Neutron Scattering Study of Correlated and Single-Particle Motions

“… 15 analyzed the temperature dependent efficiency of proton transfer events. In our recent work 14 we investigated triethylammonium triflate (TEA-TF, or [NH(C 2 H 5 ) 3 ][SO 3 CF 3 ]) with the prevailing vehicular mechanism of the acidic proton transport as it has been shown by pulsed field gradient NMR (PFG-NMR) 26 , 27 and also proved by our computer simulations 28 . Using D/H-isotope substitution in the ethyl chains of the cation we left the proton attached to the triethylamine unchanged to be used as a tagged particle.…”

“…The absolute values for D H and R H have yet changed significantly after the subtraction of the coherent component. To explain the nature of this process we addressed the DFT computations and MD simulations 28 . A close contact between the triethylammonium cation and the triflate anion in the gas-phase equilibrium corresponds to a strong hydrogen bond, the potential energy surface for the N-H proton exhibiting a single minimum.…”

“…It should be mentioned, however, that the MD relaxation processes appear to be significantly slower as compared to the experiment. This a major drawback of non-polarizable force fields 28 , 38 , 39 . Moreover, while a sum of several exponents is a good approximation for the QENS intermediate scattering functions, the corresponding MD curves are decidedly “stretched”, requiring, for example, Kohlrausch-Williams-Watts (KWW) functions for modeling.…”

“…The details of the MD simulation are presented in our previous publication 28 . The comparison of the MD trajectories with respect to the neutron scattering experiment was carried out using the nMoldyn/MDANSE software 31 .…”

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

“…In this work, we have studied protic ILs based on alkylammonium cations. 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.…”