Dynamics and Spectral Response of Water Molecules around Tetramethylammonium Cation

Abstract

Till now, there has been ambiguity about the structural heterogeneity inside a solute solvation shell and the dynamical response of the surrounding solvent molecules. To address the dynamics and spectral response of solvent molecules, we performed first-principles molecular dynamics simulations for the comprehensive study of water’s hydroxyl stretch frequency evolution due to environmental variations (also called “spectral diffusion”) in the vicinity of a hydrophobe, tetramethylammonium (TMA) cation. The N–Ow radial distribution function (RDF), spatial distribution function (SDF), and combined distribution function (CDF) were calculated to provide information about the arrangement of water molecules around TMA. In the probability distribution plot of the cosine of the angle (θ) between Ow–Hw and NTMA–Ow bond vectors, the hydrogen atoms are observed being oriented toward TMA and the oxygen atoms aligned away. The decaying dynamics of the orientation autocorrelation function (OACF) reveals the reorientation time is more inside the solvation shell (4.1 ps) as compared to bulk (2.8 ps), matching with the trend obtained from water’s orientational dynamics in tetraalkylammonium salts. Wavelet transform of the obtained trajectory was used to calculate the time-dependent vibrational stretching frequencies of the OH modes of water molecules. The normalized frequency distribution in the aqueous solvation shell of TMA, tagging a particular water molecule within the N–Ow cutoff distance 5.5 Å, displays an intense peak at 3661 cm–1 representing non-hydrogen bonded or dangling or free OH modes. Simulations around aromatic solutes and Raman-MCR studies in the hydration shell of hydrophobic TBA reported a distinctive dangling OH band at 3660 cm–1 (range: 3661 ± 2 cm–1). Besides dangling water molecules in the first hydration layer of ammonium nitrogen, few OH modes are strongly hydrogen-bonded having an average frequency at 3300 cm–1. The predominance of dangling hydroxyl modes around apolar hydrophobic TMA was further explored by comparing the dangling lifetime (∼0.68 ps) with the lifespan of hydrogen bonded OH modes (∼0.48 ps). At our simulation temperature 330 K, a significant fraction of the water molecules in the vicinity of TMA ion are free or dangling, and a few of them form an ordered structure with enhanced hydrogen bonding. Structural analysis, orientation correlation, frequency fluctuations, dangling, and hole dynamics calculations provide the evidence of the existence of dangling OH modes dominating over highly ordered strong hydrogen bonded structure in the cationic TMA solvation shell at an elevated temperature.

Amphiphilicity of Intricate Layered Graphene/g-C3N4 Nanosheets

“…As depicted in Figure 3c,d, the P(r OH 1 ) certainly overlaps for the three layers, so is the case with P(r OH 2 ). Previously, we have reported the same distance distribution for water molecules in the vicinity of hydrophobic solute 73 as well as close to the interface of hydrophobic hetero-surface, 50 and in both cases, we find a slight broadening of P(r OH 1 ) and an increased probability of r OH 2 for L1 toward the higher distance as compared to L2 and L3. However, that fact is missing in Gr/g-C 3 N 4 as shown in Figure 3e,f for SPC and SPC/E, respectively.…”

“…are the velocity vector of the H w atom at the time t 0 and t 0 + t. 87 A complete spectral analysis of the water molecules would provide the three primary peaks corresponding to the liberation motion caused by orientational degrees of freedom, an intramolecular property, H−O−H bending, and O−H stretching motion. Hydro-affinity of small solutes, 73 biomolecules, and surfaces 88,89 affect the intermolecular and intramolecular stretching frequency of water molecules in the closest layer and interface. Because we are using two rigid models, our analysis cannot capture the bending and stretching modes.…”

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

“…In the above expression, δω(t) refers to the shift in frequency from the average value at time t. The time− frequency correlation measures the extent of vibrational spectral diffusion. 74,78,84,105 We fitted the decaying curves of the frequency correlation with a triexponential function mentioned below to obtain the time scales of spectral diffusion dynamics. 106,107 and the exchange of its hydrogen-bond partners 108 accompanied by large angular jumps.…”

“…Fourier transform of the time domain data generates these frequencies. The probe frequency randomization with respect to time in response to the surrounding environmental alterations is referred to as the phenomena of vibrational spectral diffusion. ,− Earlier frequency calculations based on wavelet transform examined spectral diffusion dynamics in water, , aqueous ionic mixtures, − pure ionic liquids, and aqueous solutions of neutral entities. ,, Wavelet calculations accurately predicted the experimental results. It turned out to be a fast, efficient, and reliable technique that could be readily applied to interfaces, confined water, and aqueous solutions of peptides and proteins. , The mathematical formulation of the wavelet theorem begins with the construction of a complex function f ( t ), expressed as a combination of the time-dependent real and imaginary terms …”

Solvation Structures of Tetraethylammonium Bromide and Tetrafluoroborate in Aqueous Binary Solvents with Ethanol, Trifluoroethanol, and Acetonitrile

“…Tetraalkylammonium tetrafluoroborate (TAABF 4 ) salts with symmetric and asymmetric alkyl chain lengths have been studied for the application of them as electric carriers in an electrochemical double-layer capacitor . In particular, computer calculations, such as density functional theory (DFT) and molecular dynamics (MD) simulations, have often been applied to elucidate the solvation structure of TAA + because of their visualities. − The thermodynamic investigation has been made on TAABr in several single solvents of water, formamide, and ethylene glycol . According to the temperature and alkyl chain length dependence of the heat capacities and the standard enthalpies, the solvation structure of TAA + , particularly solvophobic solvation, was discussed.…”