Vibrational Probe of Aqueous Electrolytes: The Field Is Not Enough

In this work, we study the vibrational solvatochromism and dynamics of dilute acetone as a carbonyl probe in simple aqueous electrolytes as a function of salt composition and concentration. We observe a linear dependence of the redshift of the CO stretch mode as a function of concentration for each salt, with the magnitude of the effect scaling with the charge densities of the cations. Using molecular dynamics (MD) simulations, we compare the observed spectral shifts with the electrostatic field distributions imparted on the acetone O, comparing a fixed-charge model and a polarizable model, and find that the experimentally observed frequencies scale linearly with the electric field for a given salt, but there remains a substantial component of the solvatochromism that depends on the identity of the cation and apparently cannot be explained by the electrostatic fields alone. Finally, we use ultrafast 2D IR spectroscopy to study the salt dependence of the solvation dynamics. We observe an anomalous nonmonotonic dependence of the time scale of the dynamics on the salt concentration, which cannot be reproduced by the fluctuations in the electrostatic field determined from MD simulations. These results point to the importance of both electrostatic and nonelectrostatic effects in the vibrational solvatochromism and dynamics in this apparently simple model system.