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

Abstract

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.

Microheterogeneity-Induced Vibrational Spectral Dynamics of Aqueous 1-Alkyl-3-methylimidazolium Tetrafluoroborate Ionic Liquids of Different Cationic Chain Lengths

“…With time evolution, as the local environment surrounding the probe fluctuates and the frequency of the vibrational chromophore changes. Due to their varied structural configurations and differential interactions within the IL ions, the probe gives rise to the vibrational frequency change with time, referred to as spectral diffusion. ,,,,,− We determine the frequency–frequency autocorrelation function (FFCF), C ω ( t ), using the following relation In the above expression, δω­( t ) yields the shift in instantaneous frequency from the average frequency value. At an initial instant of time t = 0, the probe possesses an initial frequency ω(0).…”

“…We applied the Grossman–Morlet wavelet , analysis for the calculation of instantaneous vibrational frequency. The application of this method can be found in earlier literature − employing first-principle molecular dynamics simulations. Our recent reports also present a detailed discussion on the WTCT method employing classical molecular dynamics simulations. − …”

Ionic Dynamics and Vibrational Spectral Diffusion of a Protic Alkylammonium Ionic Salt through Intrinsic Cationic N–H Vibrational Probe from FPMD Simulations

“…The structural and spectral results computed in this study are not expected to be affected by the finite-size effects and systematic errors. Also, on the basis of previous proceedings, – we assume the simulation size adequate to satisfactorily determine the local response of the intrinsic vibrational N–H probes.…”

“…The time-frequency representations impart spectral information, automatically adjusting the time window size to the frequency of oscillations, providing a better time localization. , Fourier transform of the time domain data generates the instantaneous vibrational frequencies extracted at each time frame. Differential site-specific interactions of the probe with the surrounding local environment result in the probe frequency randomization with time, referred to as vibrational spectral diffusion. ,,− , Previously, the wavelet-based frequency calculations were successfully applied to examine spectral diffusion dynamics in pure water, , ionic mixtures, ,, aqueous electrolytic solutions, solutions of neutral molecular entities, ,− interfaces, confined water, and aqueous solutions of peptides and proteins. , The time-series wavelet method , generates the fluctuating frequencies based on the variable functions of the associated N–H bond vector derived from the FPMD simulation trajectory. Wavelet analysis , computes the probe stretching frequencies utilizing the instantaneous distance fluctuations and relevant momentum of the corresponding N–H bond vector.…”

“…We performed the simulations with the Perdew–Burke–Ernzerhof (PBE) , functional and the corresponding D2-corrected ,− (PBE-D2) representation to check the sensitivity of different density functional treatments. Our past studies − employed DFT-MD simulations and dispersion corrections to probe the structural dynamics of liquid methylammonium formate (MAF). Herein, we investigated the effects of different DFT-GGA functionals BLYP and PBE, in conjunction with the types of vdW dispersive interactions like the D2, D3, and DCACP methods.…”