Hydration behavior of protic ionic pair of methyl ammonium formate: A comparative molecular dynamics simulation study with their conjugate neutral forms

Molecular Insights into Binary Ionic Melts of Protic Ionic Liquid 1,2,4-Triazolium Methanesulfonate and Methanesulfonic Acid Electrolytes

Binary ionic melts formed by a protic ionic liquid (PIL) 1,2,4-triazolium methanesulfonate ([TAZ][MS]) dissolved in methanesulfonic acid are studied as non-stoichiometric electrolytes. The composition-driven structure-property relationship of methanesulfonic acid is explored at varying molar fraction ratios from 0/100 to 10/90, 20/80, and 30/70 by the addition of 1,2,4-triazolium methanesulfonate [TAZ][MS] IL. To unveil molecular characteristics of these mixtures of [TAZ][MS] PIL and CH3SO3H, we performed classical molecular dynamics simulations at varying temperatures from 293 to 303, 363, and 423 K. Ion-pair interactions and ion-solvent interactions are captured through various atom-atom radial distribution functions, and hydrogen bonding is presented through N-H hydrogen atom interactions with the [MS]- anion or CH3SO3H oxygen atoms. Apart from the temperature, the PIL ratio itself plays a critical role in hydrogen bonding interactions, and spatial density distribution maps show strengthening of ion-pair interactions with an increase in PIL concentration. The mobility of ions and CH3SO3H is also explored through mean square displacement plots and by calculating the diffusion coefficient. The diffusion coefficient is used to calculate Nernst-Einstein conductivity, which shows an excellent match with experimental conductivity at 363 and 423 K.

Dynamics of Ionic Liquid through Intrinsic Vibrational Probes Using the Dispersion-Corrected DFT Functionals

First principles molecular dynamics simulations have been utilized to study the spectral properties of the protic ionic liquid, methylammonium formate (MAF). All simulations were performed using density functional theory (DFT) and various van der Waals-corrected exchange-correlation functionals. We calculated the vibrational stretch frequency distributions, determined the time-frequency correlations of the intrinsic vibrational probes, the N-H and C-O modes in MAF, and the frequency-structure correlations. We also estimated the average hydrogen-bond lifetimes and orientation dynamics to capture the ultrafast spectral response. The spectroscopic signature of the N-H stretching vibrations using the Becke-Lee-Yang-Parr (BLYP) and Perdew-Burke-Ernzerhof (PBE) functionals displays a spectral shift in the lower frequency side, suggesting stronger hydrogen-bonding interactions represented by the gradient approximation functionals than the van der Waals (vdW)-corrected simulations. The carboxylate frequency profiles with the dispersion-corrected representations are almost similar without a significant difference in the normalized distributions. Besides, the COO stretching frequencies at the peak maxima positions of the PBE functionals exhibit a lesser deviation from the experimental data. Spectral diffusion dynamics of the intrinsic vibrational probes on the cationic and anionic sites of the ionic liquid proceed through a short time relaxation of the intact hydrogen bonds followed by an intermediate time constant and a longer time decay indicating the switchover of hydrogen bonds. Dispersion-corrected atom-centered one-electron potential (DCACP) correction added to the BLYP system slows down the picosecond time scales of frequency correlation and the time constants of rotational motion, lengthening the overall system dynamics. The observed trends in the time-dependent decays of frequency fluctuations and the orientation autocorrelation functions correlate with the structural interactions in liquid MAF and hydrogen-bond dynamics. In this study, we examine the predictions made by different density functional treatments comparing the results of the uncorrected BLYP and PBE representations with the semiempirical vdW methods of Grimme and matching our calculated data with the experimental observations.