Ion Effects\non Terahertz Spectra of Microsolvated\nClusters

Water clusters containing Na<sup>+</sup> and Cl<sup>–</sup> ions play a key role in the atmospheric chemistry of sea salt aerosols.\nWhile Na<sup>+</sup> is clearly buried deep inside, Cl<sup>–</sup> appears to be a chameleon since evidence for both surface-localized\nand interior solvation states are reported. Thus, disclosing the preferred\nlocation of Cl<sup>–</sup> within clusters remains challenging.\nHere, we investigate whether THz spectroscopy, a powerful tool for\ndirectly probing hydrogen bonds in water, provides insights into the\nlocation of Cl<sup>–</sup> ions in water clusters. We performed\nab initio molecular dynamics simulations on water clusters containing\na single Cl<sup>–</sup> ion and up to 64 water molecules to\ncompute the THz spectra with reference to Na<sup>+</sup> and bulk.\nThe THz spectrum of the 64-water Cl<sup>–</sup> cluster closely\nagrees with that of the bulk solution. Surprisingly, this match is\nnot caused by bulk-like solvation of Cl<sup>–</sup> as suggested\nby phenomenological line shape analyses. Instead, the similarity stems\nfrom Cl<sup>–</sup> being mostly located at the cluster surface,\nthus leaving the water–water interactions largely unperturbed.