Heterogeneous Occupancy and Vibrational Dynamics of Spatially Patterned Water Molecules

Biswas, Sohag and Mallik, Bhabani Shankar (2019) Heterogeneous Occupancy and Vibrational Dynamics of Spatially Patterned Water Molecules. The Journal of Physical Chemistry B. ISSN 1520-6106

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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 (RDFs), number integrals (NIs) and spatial distribution functions (SDFs). The lifetime of dangling OH groups inside the solvation shell is more compared to that of the bulk. By constructing the O–H groups in three ensembles (S1, S2, and S3) around the anion, we show that frequency distribution of OH modes in the S1 ensemble show red-shifting for both sulfate and thiosulfate. The O–H groups in S2 ensemble of sulfate-water system show red-shifting by 10 cm-1 while in case of thiosulfate-water, these O–H groups show blue-shifting by 8 cm-1. The water molecules in S1 and S2 sub ensembles have slower dynamics compared to 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 the bulk, S3 layer. All these findings, the redshift of OH stretching frequency in S1 and S2 layer, slowing down the orientational dynamics of OH vectors in S1 and S2 layers and less diffusivity of water in S1 and S2 layer, show the long-range kosmotropic effect of multivalent sulfate and thiosulfate oxyanions. Due to long-range effect, heterogeneous occupancy of water molecules is observed, and the water molecules are found to be in a patterned manner in the vicinity of anions with varied local density

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