Proton transfer in water is ubiquitous and a critical elementary event which, via proton hopping between water molecules, enables protons to diffuse much faster than other ions. The proton hopping mechanism for enhanced diffusion of protons compared to other small cations was first proposed by Grotthuss in 1806. While there have been a vast number of experiments and molecular dynamics simulations investigating proton hopping in water, an experimental determination of the proton hopping time has remained elusive due to its ultrafast nature and the lack of direct experimental observables. Here, 2D IR chemical exchange spectroscopy was used to extract the chemical exchange rates between hydronium and water in acid solutions using a vibrational probe, methyl thiocyanate (MeSCN), with the CN stretch used as the vibrational probe. 2D IR spectral diffusion measurement show that MeSCN accurately reports on the hydrogen bond (H-bond) dynamics in pure water, making it an excellent probe of dynamics in aqueous systems. Ab initio molecular dynamics simulations demonstrate that the chemical exchange is dominated by proton hopping. The observed experimental and simulated acid concentration dependence as well as a number of factors obtained from the simulations and spectral diffusion measurements make it possible to extrapolate the measured single step proton hopping time to the dilute limit, which, within error, gives the same value as inferred from measurements of the proton diffusion constant. It is found that the dilute limit proton hopping time is the same as the time for concerted H-bond rearrangement of the extended H-bond network in pure water. The results indicate that the H-bond rearrangement of the water network in which hydronium ions are embedded triggers proton hopping.

Direct observation of proton hopping in water/HCl solutions by 2D IR spectroscopy and ab initio simulations

2019 Fall National Meeting

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