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

Florian N. Brünig, Manuel Rammler+3 more|arXiv (Cornell University)|Apr 8, 2022

Spectroscopy and Quantum Chemical Studies92 references36 citations

TL;DR

This study uses ab initio molecular dynamics simulations and trajectory decomposition to link infrared difference spectra of aqueous HCl to distinct proton-transfer dynamics. It identifies the 2000–3000 cm⁻¹ continuum band as vibrational modes of transient H₃O⁺ complexes, the 400 cm⁻¹ peak as coupling to flanking water vibrations, and assigns a 14 fs transfer duration and 200–300 fs waiting time to spectral features matching experimental THz and mid-IR data.

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.

Motivation & Objective

• To resolve the microscopic origin of spectral features in excess-proton dynamics in aqueous HCl solutions.
• To address the limitation of normal-mode analysis in capturing thermally activated proton transfer over free-energy barriers.
• To link experimentally observed IR spectra to distinct time scales of proton transfer, waiting, and vibrational dynamics.
• To provide a theoretical framework beyond normal modes that accounts for unstable modes associated with barrier-crossing events.

Proposed method

• Performed ab initio molecular dynamics (AIMD) simulations of aqueous HCl at 2–6 M concentrations under ambient conditions.
• Projected proton trajectories onto a two-dimensional reaction coordinate space defined by proton position (d) and oxygen–oxygen distance (ROO).
• Used trajectory decomposition to separate dynamics into three time-scale contributions: normal-mode vibrations (τNM), transfer-path dynamics (τTP), and waiting times (τTW).
• Computed infrared difference absorption spectra from simulated trajectories and compared them to experimental THz and mid-IR data.
• Identified spectral contributions by associating specific frequency bands with distinct dynamical processes using time-scale decomposition.
• Validated results by comparing simulated difference spectra with experimental data from 4 M HCl, showing good agreement in both THz and mid-IR regions.

Experimental results

Research questions

• RQ1 What is the origin of the broad continuum band in the 2000–3000 cm⁻¹ range in IR difference spectra of aqueous HCl?
• RQ2 How do the 400 cm⁻¹ and 1200 cm⁻¹ spectral features relate to the dynamics of excess-proton transfer and solvation?
• RQ3 What are the characteristic time scales of proton waiting and transfer events, and how do they contribute to the IR spectrum?
• RQ4 Can trajectory decomposition techniques resolve spectral signatures beyond the scope of normal-mode analysis in proton-transfer dynamics?

Key findings

• The 2000–3000 cm⁻¹ continuum band arises from normal-mode vibrations of transient, asymmetric H₃O⁺ complexes formed during proton transfer.
• A prominent peak at 400 cm⁻¹ reflects coupling between excess-proton motion and relative vibrations of the two flanking water molecules.
• The waiting time for proton transfer is 200–300 fs, producing a broad, weak shoulder near 100 cm⁻¹ consistent with experimental THz spectra.
• The mean duration of a single proton transfer event is 14 fs, generating a well-defined spectral feature at 1200 cm⁻¹ matching experimental mid-IR spectra.
• The simulated IR difference spectra at various HCl concentrations agree quantitatively with experimental data in both the THz and mid-IR regimes.
• The study demonstrates that barrier-crossing proton transfer events—previously outside the scope of normal-mode theory—make dominant spectroscopic contributions.