Exploring the Terahertz Absorption Spectra of Aqueous Guanidinium Chloride Solutions using Polarizable Models for Water and the Salt

Abstract

We have presented calculations of the terahertz absorption spectra of aqueous guanidinium chloride (GdmCl) solutions through molecular dynamics simulations using polarizable models for both water and the ions. It is found that the guanidinium cations cause disruption to the hydrogen bonded network of water, which leads to a red shift in the relevant features of the THz spectrum with increase of salt concentration. The calculated spectra are further deconvoluted into different parts such as the ion-ion, water-water and ion-water cross correlation contributions. Our results show that the effects of guanidinium cations extend beyond the first hydration shell. We also dissected the total spectra into contributions from permanent dipoles, induced dipoles, and their cross-interactions to gain deeper insights into the origin of various spectral features. The diffusion coefficients of ions and water are also calculated, which show a decreasing trend with increasing. Furthermore, the orientational dynamics of water molecules are slowed down in the presence of guanidinium and chloride ions, with the extent of slowing down becoming more pronounced at higher concentrations. The red shifts in the hydrogen bond bending and stretching modes of the aqueous GdmCl solution relative to pure water in the total absorption spectra, combined with the negative intensity observed in the hydrogen-bond (HB) stretching region of the water-water component of the difference absorption spectra, capture the THz spectral signatures of the structure-disrupting effect of GdmCl on the hydrogen bonded network of water.