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SFB Publications by Brünig et al (SFB 1078 project C1, AG Netz) derive the spectral signatures related to excess-proton transfer in aqueous hydrochloric-acid solutions based on ab initio molecular-dynamics simulations and reaction-rate theory

Schematic trajectory of an excess proton

Schematic trajectory of an excess proton
Image Credit: Brünig et al (2022) Nat Comm under the CC-BY open access license.

In a recent publication in Nature Communications, Brünig et al. show how the infrared spectroscopic signatures of hydrochloric acid solutions are related to proton-transfer processes between water molecules, the elementary step of the long-range Grotthus transport.
In a second work that just appeared in the Journal of Chemical Physics, the IR band shapes produced by proton-transfer events are derived using analytical models for the barrier-crossing dynamics as a proton moves from one water molecule to a neighbouring one.

News from Nov 24, 2022

The theoretical basis for linking spectral signatures of hydrated excess protons with microscopic proton-transfer mechanisms has so far relied on normal-mode analysis. We introduce trajectory-decomposition techniques to analyze the excess-proton dynamics in ab initio molecular-dynamics simulations of aqueous hydrochloric-acid solutions beyond the normal-mode scenario. We show that the actual proton transfer between two water molecules involves for relatively large water-water separations crossing of a free-energy barrier and thus is not a normal mode, rather it is characterized by two non-vibrational time scales: Firstly, the broadly distributed waiting time for transfer to occur with a mean value of 200–300 fs, which leads to a broad and weak shoulder in the absorption spectrum 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.

Brünig, F.N., Rammler, M., Adams, E.M., Havenith, M. and Netz, R.R. (2022). Spectral signatures of excess-proton waiting and transfer-path dynamics in aqueous hydrochloric acid solutions. Nat Commun, 13, 1: 4210. doi: 10.1038/s41467-022-31700-x.

Brünig, F.N., Hillmann, P., Kim, W.K., Daldrop, J.O. and Netz, R.R. (2022). Proton-transfer spectroscopy beyond the normal-mode scenario. J Chem Phys, 157, 17: 174116. doi: 10.1063/5.0116686.

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  • proton-transfer, hydrochloric-acid solutions