Quadrupolar $^{23}$Na$^{+}$ NMR Relaxation as a Probe of Subpicosecond Collective Dynamics in Aqueous Electrolyte Solutions

TL;DR

This study combines ab initio calculations and molecular dynamics to link quadrupolar NMR relaxation of sodium ions with subpicosecond collective water dynamics, providing a microscopic understanding of ion relaxation mechanisms.

Contribution

It introduces a combined computational approach to connect quadrupolar NMR relaxation rates with subpicosecond molecular dynamics in electrolyte solutions, a novel insight.

Findings

Quadrupolar relaxation is sensitive to subpicosecond water dynamics.

Relaxation rates are mainly influenced by the first two solvation shells.

The model agrees well with experimental data across various conditions.

Abstract

Nuclear magnetic resonance relaxometry represents a powerful tool for extracting dynamic information. Yet, obtaining links to molecular motion is challenging for many ions that relax through the quadrupolar mechanism, which is mediated by electric field gradient fluctuations and lacks a detailed microscopic description. For sodium ions in aqueous electrolytes, we combine ab initio calculations to account for electron cloud effects with classical molecular dynamics to sample long-time fluctuations, and obtain relaxation rates in good agreement with experiments over broad concentration and temperature ranges. We demonstrate that quadrupolar nuclear relaxation is sensitive to subpicosecond dynamics not captured by previous models based on water reorientation or cluster rotation. While ions affect the overall water retardation, experimental trends are mainly explained by dynamics in the first two solvation shells of sodium, which contain mostly water. This work thus paves the way to the quantitative understanding of quadrupolar relaxation in electrolyte and bioelectrolyte systems.