Quadrupolar NMR relaxation as a local probe of collective dynamics in aqueous alkaline and alkaline-earth chloride solutions

Abstract

While nuclear magnetic resonance (NMR) provides valuable insights into the local environment of many nuclei, the unambiguous interpretation of the signal in terms of microscopic dynamics is often difficult, particularly when the quadrupolar relaxation mechanism comes into play. Here, we investigate the quadrupolar NMR relaxation of cations and anions in aqueous alkaline and alkaline-earth chloride solutions across a broad range of salt concentrations. Using a combination of density functional theory calculations and classical molecular dynamics simulations, we compute the electric field gradient (EFG) fluctuations over the relevant time scales. Predicted NMR relaxation rates are in good agreement with experiments from the literature. As previously reported for NaCl, we find that the increase in relaxation rate with salt concentration is primarily driven by the slowing of EFG fluctuations, while changes in the static variance of the EFG play a minor role. We highlight some specific features for smaller and divalent cations compared to the other monovalent ones. In addition, we assess the relevance of the Stokes–Einstein–Debye model, frequently used to analyze NMR relaxation experiments, for these aqueous electrolytes and highlight the link between the collective dynamics of the liquid underlying the EFG fluctuations at the ion positions and the stress fluctuations. Our results generalize observations for Na+ in aqueous NaCl solutions, showing that models assuming a viscous model of the solvent dynamics are insufficient to describe EFG fluctuations in these systems and illustrate the relevance of molecular simulations to interpret NMR relaxation experiments in terms of microscopic dynamics.