Heterogeneous Occupancy and Vibrational Dynamics of Spatially Patterned Water Molecules

Abstract

We performed first-principles molecular dynamics simulations of relatively dilute aqueous solutions of sulfate and thiosulfate dianions to analyze the structure, dynamics, and vibrational spectral properties of water molecules around the solute, especially the spatially patterned solvent molecules in the first solvation layer and the extended layers. This study also involves the investigation of dynamics of dangling OH groups in these layers and their role in patterning the water molecules around the dianions. Structural evaluation of the systems is carried out by radial distribution functions, number integrals, and spatial distribution functions. The lifetime of dangling OH groups inside the solvation shell is compared more to that of the bulk. By constructing the O−H groups in three ensembles (S1, S2, and S3) around the anion, we show that the frequency distribution of OH modes in the S1 ensemble show red-shifting for both sulfate and thiosulfate. The O−H groups in the S2 ensemble of the sulfate−water system show red-shifting by 10 cm −1 , while in the case of thiosulfate−water, these O−H groups show blue-shifting by 8 cm −1 . The water molecules in S1 and S2 subensembles have slower dynamics compared to those in the bulk (S3). The dynamics of various kinds of hydrogen bonds were characterized by hydrogen bond population correlation functions. The spectral diffusion of solvation shell O−H modes was performed through a frequency−time correlation function. We find a significant amount of orientational retardation of water molecules in the S1 layer and moderate retardation in the S2 layer as compared to that in the bulk, S3 layer. All these findings, the red shift of the OH stretching frequency in S1 and S2 layers, slowing down of the orientational dynamics of OH vectors in S1 and S2 layers, and less diffusivity of water in S1 and S2 layers, show the long-range kosmotropic effect of multivalent sulfate and thiosulfate oxyanions. Due to the long-range effect, heterogeneous occupancy of water molecules is observed, and the water molecules are found to arrange in a patterned manner in the vicinity of anions with varied local density.

Density functional theory based molecular dynamics study of solution composition effects on the solvation shell of metal ions

“…13,14 An important contribution to this field was the development of ab initio MD (AIMD), where forces are derived from the electronic structure, 15,16 usually in the framework of density functional theory (DFT), 17 which provides the capability of studying non-additivity effects in the dynamics of ions solvation shells. AIMD simulations, using the Car-Parrinello and Born-Oppenheimer schemes, of cations [18][19][20][21][22][23] and anions, 24,25 have been mostly conducted on an isolated ion in pure liquid water, with no counterions in solution. Electrolyte solutions, on the other hand, have been subject to only a few AIMD studies, including the characterization of the dissociation of the NaCl in water, 26 the cooperative ionic effects are of the Na + and Cl À on the hydrogen bonding network, 27 and the influence of a static electric fields.…”

Water dynamics in the hydration shell of hyper-branched poly-ethylenimine

“…For water molecules around hydrophilic groups, it was found that the retardation effect of hydrophilic amino groups on the hydrogen bond accepting water (HBA water) is comparable with that of hydrophobic groups, but is weaker than the effect on the HBD water. 57,58 Given the discussion above, it is reasonable to classify hydrophobic hydration water and HBA water molecules accepting HBs from NH 2 groups into slow water. The time scales of slow water are indicated by red diamond markers in Fig.…”

“…For water molecules around hydrophilic groups, it was found that the retardation effect of hydrophilic amino groups on the hydrogen bond accepting water (HBA water) is comparable with that of hydrophobic groups, but is weaker than the effect on the HBD water. 57,58…”

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

“…Wavelet‐based frequency calculations are reliable and fast and can be applied to interfaces, [108] confined water, [109] and aqueous solutions of peptides and polypeptides [110,111] . Further, details of the wavelet method are available in our previous reports [92,104,106,112–114] …”

“…Time‐series wavelet method has been used to extract the instantaneous frequency of a specific probe through Fourier transform [99] of the time domain data at each timeframe. Earlier wavelet theorem had been applied to analyze the phenomena of vibrational spectral diffusion in water, [95,100,101] ionic solutions, [102,92] and aqueous solutions of neutral molecules [103–107] . In the previously reported studies, the wavelet was the chosen method for frequency calculations, and it could efficiently reproduce the experimental results with a significant amount of accuracy.…”