How Ions Reshape the Infrared Spectra of Aqueous NaCl

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.