Ionic Dynamics and Vibrational Spectral Diffusion of a Protic Alkylammonium Ionic Salt through Intrinsic Cationic N–H Vibrational Probe from FPMD Simulations,The Journal of Physical Chemistry A

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Ionic Dynamics and Vibrational Spectral Diffusion of a Protic Alkylammonium Ionic Salt through Intrinsic Cationic N–H Vibrational Probe from FPMD Simulations
The Journal of Physical Chemistry A ( IF 2.7 ) Pub Date : 2022-07-28 , DOI: 10.1021/acs.jpca.2c03387
Aritri Biswas ₁ , Bhabani S Mallik ₁

Affiliation

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.

中文翻译：

通过 FPMD 模拟的本征阳离子 N-H 振动探针对质子烷基铵离子盐的离子动力学和振动光谱扩散

我们采用基于密度泛函理论 (DFT) 的分子动力学模拟来探索质子离子实体三甲基氯化铵 (TMACl) 的结构、动力学和光谱特性。结构研究包括计算位点径向分布函数（RDF），组成阳离子和阴离子在三维空间中的分布，以及氢键对RDF与角度的组合分布函数，揭示离子溶剂化的结构特征以及离子内的分子间相互作用。此外，我们通过应用时间序列小波方法确定了固有 N-H 拉伸探针模式的瞬时振动拉伸频率。通过计算波动频率的时间演化和频率-时间相关函数 (FFCF)，研究了质子离子化合物内的相关离子动力学。研究了与局部结构弛豫过程和平均氢键寿命、离子笼动力学和均方位移相关的时间尺度。FFCF 的更快衰减分量（描述 TMACl 中完整氢键的分子间运动）对于基于 Perdew-Burke-Ernzerhof (PBE) 的模拟为 0.07 ps，对于 PBE-D2 表示为 0.06 ps。PBE 和 PBE-D2 表示的较长皮秒衰减时间分量的较慢时间尺度分别为 3.13 和 2.87 ps。这些皮秒时间尺度代表了离子实体中氢键伙伴的更显着波动以及伴随大角度跳跃的氢键跳跃事件。最长的皮秒时间尺度代表结构松弛，包括大角度跳跃和离子对动力学。此外，尽管 DFT 泛函的类型，离子笼寿命与振动光谱扩散的相关动力学的最慢时间尺度相关。本研究对使用和不使用范德华 (vdW) 色散校正方案的交换相关泛函的 DFT 处理进行基准测试。将 vdW 相互作用纳入 PBE 泛函代表了离子实体的结构较少的状态和相对于 PBE 系统预测的分子运动的更快动力学。

更新日期：2022-07-28

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