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

Abstract

We employed density functional theory (DFT)-based molecular dynamics simulations to explore the structure, dynamics, and spectral properties of the protic ionic entity trimethylammonium chloride (TMACl). Structural investigations include calculating the site–site radial distribution functions (RDFs), the distribution of constituent cations and anions in three-dimensional space, and combined distribution functions of the hydrogen-bonded pair RDF versus angle, revealing the structural characteristics of the ionic solvation and the intermolecular interactions within ions. Further, we determined the instantaneous vibrational stretching frequencies of the intrinsic N–H stretch probe modes by applying the time-series wavelet method. The associated ionic dynamics within the protic ionic compound were investigated by calculating the time-evolution of the fluctuating frequencies and the frequency-time correlation functions (FFCFs). The time scale related to the local structural relaxation process and the average hydrogen bond lifetime, ion cage dynamics, and mean squared displacement were investigated. The faster decay component of the FFCFs, depicting the intermolecular motion of intact hydrogen bonds in TMACl, is 0.07 ps for the Perdew-Burke-Ernzerhof (PBE)-based simulation and 0.06 ps for the PBE-D2 representation. The slower time scale of the longer picosecond decay time component of PBE and PBE-D2 representations are 3.13 and 2.87 ps, respectively. These picosecond time scales represent more significant fluctuations of the hydrogen-bonding partners in the ionic entity and hydrogen-bond jump events accompanied by large angular jumps. The longest picosecond time scales represent structural relaxation, including large angular jumps and ion-pair dynamics. Also, ion cage lifetimes correlate with the slowest time scale of the associated dynamics of vibrational spectral diffusion despite the type of DFT functional. This study benchmarks DFT treatments of the exchange-correlation functional with and without the van der Waals (vdW) dispersion correction scheme. The inclusion of vdW interactions to the PBE functional represents a less structured state of the ionic entity and faster dynamics of the molecular motions relative to the one predicted by the PBE system. All the results illustrate the necessity of accurately describing the Coulomb interactions, vdW dispersive interactive forces, and localized hydrogen bonds required to sustain the energetic balance in this ionic salt.

Direct Correlation between Short-Range Vibrational Spectral Diffusion and Localized Ion-Cage Dynamics of Water-in-Salt Electrolytes

“…Previously, the wavelet theorem had already been applied to determine the frequency distribution and examine the process of spectral diffusion in neat systems, 62,70 aqueous solvation shells, 64,71 solutions of neutral moieties, 63,72 and ionic liquid solutions. 67,73 We have presented a detailed discussion on the WTCT method employing classical MD simulations in our recent reports. 67,74−76…”

“…Thus, WT runs repeatedly through the entire trajectory timeframe to calculate the vibrational stretching frequencies of the probe modes and determine the frequency distribution directly comparable to the experimental absorbance spectrum. Previously, the wavelet theorem had already been applied to determine the frequency distribution and examine the process of spectral diffusion in neat systems, , aqueous solvation shells, , solutions of neutral moieties, , and ionic liquid solutions. , We have presented a detailed discussion on the WTCT method employing classical MD simulations in our recent reports. ,− …”

Multiple Ensembles of the Hydrogen‐bonded Network in Ethylammonium Nitrate versus Water from Vibrational Spectral Dynamics of SCN− Probe

“…Time-frequency correlation determines the extent of vibrational spectral diffusion. [92,95,104,106,113,114,[127][128][129][130][131][132] Finally, the probe acquires a frequency w t ð Þ at time t uncorrelated to the initial frequency value. In the ultrafast vibrational echo spectroscopic experiments [53,133] and polarization-resolved pump-probe (PP) techniques, [134,135] the dynamic linewidth, [59] the eccentricity, [136][137][138][139] the slope of the nodal plane, [140,141] as well as the center-line slope (CLS) [134,135,142] methods determined the frequency fluctuation correlation functions measuring the dynamics of spectral diffusion.…”