Hydrated Excess Protons in Acetonitrile/Water Mixtures - Solvation Species and Ultrafast Proton Motions

TL;DR

This study uses 2D-IR spectroscopy and simulations to investigate ultrafast proton motions and solvation structures of excess protons in acetonitrile/water mixtures, revealing stochastic proton transfer dynamics and dominant H5O2+ species.

Contribution

It combines experimental 2D-IR spectroscopy with quantum mechanics/molecular mechanics simulations to elucidate proton solvation and dynamics in mixed solvents, highlighting the transient nature of proton localization.

Findings

Excess protons predominantly form H5O2+ complexes in water dimers.

Proton transfer exhibits stochastic large-amplitude motions.

Proton dynamics are driven by fluctuating electric fields.

Abstract

The solvation structure of protons in aqueous media is highly relevant to electric properties and to proton transport in liquids and membranes. At ambient temperature, polar liquids display structural fluctuations on femto- to picosecond time scales with a direct impact on proton solvation. We apply two-dimensional infrared (2D-IR) spectroscopy for following proton dynamics in acetonitrile/water mixtures with the Zundel cation H$_5$O$_2^+$ prepared in neat acetonitrile as a benchmark. The 2D-IR spectra of the proton transfer mode of H$_5$O$_2^+$ demonstrate stochastic large-amplitude motions in the double-minimum proton potential, driven by fluctuating electric fields. In all cases the excess proton is embedded in a water dimer, forming an H$_5$O$_2^+$ complex as major solvation species. This observation is rationalized by quantum mechanics/molecular mechanics molecular dynamics simulations including up to 4 water molecules embedded in acetonitrile. The Zundel motif interacts with its closest water neighbor in an H$_7$O$_3^+$ unit without persistent proton localization.