How Ions Reshape the Infrared Spectra of Aqueous NaCl.

Infrared (IR) spectroscopy probes the hydrogen-bond structure of electrolyte solutions, yet relating spectra to molecular mechanisms is challenging. We combine machine-learning molecular dynamics with a deep Wannier model to compute concentration-dependent IR spectra of aqueous NaCl with strongly constrained and appropriately normed (SCAN)-level accuracy. The spectra semiquantitatively reproduce the experimental blue shift, intensity enhancement, band narrowing, and isosbestic point of the O-H stretching band. Dipole-current decomposition reveals two ionic effects. First, ions reshape the water IR response: by disrupting the hydrogen-bond network of first-shell water, they blue-shift the stretching band and suppress its intensity, with Na+ more disruptive than Cl-. Second, ions themselves carry an IR response: the electronic polarization of Cl-, driven by O-H stretching of its hydrogen-bonded waters, adds high-frequency intensity essential for the isosbestic point. Because both effects are local and additive, spectra at any concentration can be reconstructed from hydration-shell and ionic contributions, a transferable framework for electrolyte IR spectra.