Ion Effects on Terahertz Spectra of Microsolvated Clusters

Area of Science:

• Atmospheric Chemistry
• Physical Chemistry
• Computational Chemistry

Background:

• Sea salt aerosols are crucial in atmospheric chemistry, with water clusters containing sodium (Na+) and chloride (Cl-) ions playing a significant role.
• The precise location of Cl- ions within these water clusters (surface vs. interior) is debated, impacting our understanding of their behavior.
• Terahertz (THz) spectroscopy is a technique capable of directly probing hydrogen bonds in water, offering a potential method to resolve Cl- ion location.

Purpose of the Study:

• To investigate the preferred location of Cl- ions in water clusters using THz spectroscopy.
• To determine if THz spectroscopy can provide insights into the solvation state of Cl- ions in aqueous environments.
• To compare the behavior of Cl- ions in small water clusters to their behavior in bulk solutions.

Main Methods:

• Performed ab initio molecular dynamics simulations on water clusters containing a single Cl- ion and up to 64 water molecules.
• Computed the THz spectra of these simulated clusters.
• Compared the computed THz spectra with those of Na+ containing clusters and bulk water solutions.

Main Results:

• The THz spectrum of a 64-water cluster with a Cl- ion closely matched the spectrum of bulk water.
• This spectral similarity was not due to bulk-like interior solvation of Cl- as previously hypothesized.
• The primary reason for the spectral match was the predominant surface localization of the Cl- ion within the water cluster.

Conclusions:

• Cl- ions in water clusters up to 64 molecules are predominantly located at the cluster surface.
• Surface localization of Cl- leaves water-water interactions largely unperturbed, explaining the bulk-like THz spectra.
• THz spectroscopy, combined with molecular dynamics, is a valuable tool for elucidating ion solvation states in water clusters.