Elucidating the proton transport pathways in liquid imidazole with first-principles molecular dynamics

TL;DR

This study uses long ab initio molecular dynamics simulations to reveal that proton transport in liquid imidazole primarily occurs through structural diffusion involving chain hopping and reformation, providing detailed mechanistic insights.

Contribution

It presents the first long-timescale ab initio simulations of proton transport in liquid imidazole, elucidating the multi-scale diffusion mechanism and chain dynamics.

Findings

Proton diffusion constant is ~8 times higher than imidazole self-diffusion.

Proton transport involves three distinct time and length scales.

Chain reformation limits long-time proton diffusion.

Abstract

Imidazole is a promising anhydrous proton conductor with a high conductivity comparable to that of water at a similar temperature relative to its melting point. Previous theoretical studies of the mechanism of proton transport in imidazole have relied either on empirical models or on ab initio trajectories that have been too short to draw significant conclusions. Here, we present the results of ab initio molecular dynamics simulations of an excess proton in liquid imidazole reaching 1 nanosecond in total simulation time. We find that the proton transport is dominated by structural diffusion, and the diffusion constant of the proton defect is ~8 times higher than the self-diffusion of the imidazole molecules. By using correlation function analysis, we decompose the mechanism for proton transport into a series of first-order processes and show that the proton transport mechanism occurs over three distinct time and length scales. Although the mechanism at intermediate times is dominated by hopping along pseudo one-dimensional chains, at longer times, the overall rate of diffusion is limited by the reformation of these chains, thus providing a more complete picture of the traditional, idealized Grotthuss structural diffusion mechanism.