Ion Effects on Terahertz Spectra of Microsolvated Clusters

Water clusters containing Na⁺ and Cl^- ions play a key role in the atmospheric chemistry of sea salt aerosols. While Na⁺ is clearly buried deep inside, Cl^- appears to be a chameleon since evidence for both surface-localized and interior solvation states are reported. Thus, disclosing the preferred location of Cl^- within clusters remains challenging. Here, we investigate whether THz spectroscopy, a powerful tool for directly probing hydrogen bonds in water, provides insights into the location of Cl^- ions in water clusters. We performed ab initio molecular dynamics simulations on water clusters containing a single Cl^- ion and up to 64 water molecules to compute the THz spectra with reference to Na⁺ and bulk. The THz spectrum of the 64-water Cl^- cluster closely agrees with that of the bulk solution. Surprisingly, this match is not caused by bulk-like solvation of Cl^- as suggested by phenomenological line shape analyses. Instead, the similarity stems from Cl^- being mostly located at the cluster surface, thus leaving the water-water interactions largely unperturbed.