Spectral signatures of excess-proton waiting and transfer-path dynamics in aqueous hydrochloric acid solutions

TL;DR

This study links infrared spectral features of solvated excess protons in hydrochloric acid solutions to their microscopic transfer mechanisms using ab initio simulations, revealing characteristic time scales and vibrational couplings.

Contribution

It provides a detailed microscopic analysis of proton transfer dynamics and their spectral signatures, extending understanding beyond normal-mode analysis to include barrier crossing events.

Findings

Continuum band due to H3O+ oscillations

Coupling of proton motion to water vibrations at 400 cm$^{-1}$

Proton transfer involves waiting (~200-300 fs) and transfer (~14 fs) time scales

Abstract

Signatures of solvated excess protons in infrared difference absorption spectra, such as the continuum band between the water bend and stretch bands, have been experimentally known for a long time, but the theoretical basis for linking spectral signatures with the microscopic proton-transfer mechanism so far relied on normal-mode analysis. We analyze the excess-proton dynamics in ab initio molecular-dynamics simulations of aqueous hydrochloric acid solutions by trajectory-decomposition techniques. The continuum band in the 2000 - 3000 cm$^{-1 }$ range is shown to be due to normal-mode oscillations of temporary H$_3$O$^+$ complexes. An additional prominent peak at 400 cm$^{-1}$ reports on the coupling of excess-proton motion to the relative vibrations of the two flanking water molecules. The actual proton transfer between two water molecules, which for large water separations involves crossing of a barrier and thus is not a normal mode, is characterized by two characteristic time scales: Firstly, the waiting time for transfer to occur in the range of 200 - 300 fs, which leads to a broad weak shoulder around ~100 cm$^{-1}$, consistent with our experimental THz spectra. Secondly, the mean duration of a transfer event of about 14 fs, which produces a rather well-defined spectral contribution around 1200 cm$^{-1}$ and agrees in location and width with previous experimental mid-infrared spectra.