Observing Aqueous Proton Transfer Dynamics.

The intermolecular structure of water has garnered significant attention from both theoretical and experimental physical chemists within the past 20 years. While early studies focused extensively on hydrogen bonding in pure water, advancements in the last two decades have been answering more questions about the role of the excess proton.[1] Characterizing how bulk water adapts its structure to adopt the excess proton is a pertinent question; however, understanding how these structures interconvert to accommodate proton transfer dynamics, first observed over 200 years ago, still requires further elucidation.[1] To date, numerous contributions have commented on the solvation structure of the proton defect, its transfer, and transition states associated with that transfer.[2−4] However, the time scales of these proton transfer dynamics are not experimentally well quantified.[2,3] A recent contribution by Yuan et al. circumvents experimental challenges to quantify the rates of the most fundamental aqueous proton transfer mechanism: the von Grotthuss mechanism.[5]

Like many others in the field, Yuan et al. employ 2D IR spectroscopy to study this phenomenon. 2D IR spectroscopy is well suited for studying structural dynamics of water for a variety of reasons (Figure ). First, by using femtosecond pulses, 2D IR is capable of measuring the ∼1 ps intermolecular structural dynamics of water, including proton transfer. Second, 2D IR is particularly sensitive to the solvent environment of a vibrational probe and can be used to monitor ultrafast exchange between structures.[6] By tagging a population of molecules with a pair of pump pulses, 2D IR can track how this population interconverts between structures over time using a probe pulse.

Figure 1

A three pulse 2D IR sequence is shown above their corresponding spectra at two different waiting times. An equilibrium of species A and species B are pumped. The excitation tags the molecules in species A and species B such that they appear at their respective frequencies along the pump axis. At a short waiting time, not enough time has elapsed for molecules in species A to convert to species B; therefore, all molecules are probed at the same frequency at which they are pumped. By waiting time t2, a portion of species A has converted to species B and vice versa; therefore, both species A and species B appear along the probe axis for a given pump frequency (i.e., cross peaks appear).

The von Grotthuss mechanism, or the proton hopping mechanism, can be described as the simple transfer of a proton from a hydronium ion to an adjacent water molecule. To probe the hydronium ion, highly acidic solutions are required. In such solutions, the broad and overlapping spectra of the O–H stretch in H2O and H3O+ prohibit tracking interconversion between the two species. Yuan et al. cleverly evade this limitation by employing a methyl thiocyanate (MeSCN) probe that hydrogen bonds with both water and hydronium. The IR absorption of the MeSCN probe is relatively narrow, and the MeSCN·H3O spectrum is noticeably shifted from the MeSCN·H2O spectrum. In this manner, proton hopping is observed by monitoring the transfer of a proton to an adjacent water molecule from a hydronium ion bound to MeSCN.

In addition to monitoring the exchange between the MeSCN bound hydronium ion and water molecule, this study provides a detailed molecular level picture of their experiment using ab initio molecular dynamics (AIMD) simulations. All experiments were performed at high hydronium concentrations, but a value of particular interest is the rate of proton transfer in pure water. Yuan et al. combine hydronium concentration dependent measurements and AIMD simulations to extrapolate their results to the dilute limit. Their estimate provides important information for those studying processes related to proton transfer dynamics under neutral pH conditions.

A particular strength of this work is how AIMD simulations were used to corroborate their results and rigorously investigate potential caveats to their experiment and analysis, as well as derive a method to extrapolate their results to the dilute limit. For example, even though MeSCN·H2O and MeSCN·H3O+ absorption features are narrow and discernible relative to H2O and H3O+ absorption bands, spectral overlap still adds ambiguity to interpretation. Yuan et al. justify their interpretation by performing a spectral decomposition using the N–H radial distribution function yielded from AIMD simulations. A particularly important caveat was that the time constants obtained from their chemical exchange measurements could be influenced by replacement of water bound to MeSCN with hydronium rather than proton hopping. The authors again turn to their simulations to correct for the influence of water replacement on their obtained rates. Several other important corrections are made on the basis of their theoretical results as they derived the rate equations required to perform the extrapolation.

The results obtained from this study fit well with past experiments, provide details relevant to others studying structural dynamics of water, and pose questions for further experimental research. The determined proton hopping time between H3O+ and H2O species matches results from NMR line shape analysis and proton mobility measurements.[7,8] Furthermore, the time scale of proton hopping in the dilute limit matches the time scale of hydrogen bond reorganization in bulk water, suggesting that hydrogen bond reorganization drives proton hopping. These details provide motivation for future studies investigating what drives proton hopping dynamics. Lastly, the proton hopping time constants obtained from this study provide a temporal reference for rearrangement of the solvation structure of the proton defect. Therefore, this work may assist in future characterization of solvent structure rearrangement occurring upon proton transfer. A more concrete understanding of proton transfer dynamics and solvation of the proton defect in bulk water is a crucial step toward characterizing more complicated systems. Significant attention has been given to understanding the behavior of the hydronium and hydroxide ions at water interfaces.[1] Furthermore, proton transfer dynamics and water solvation structure play a role in many biophysical processes such as proton migration through ion channels and proton transfer in amyloid proteins and enzymes.[9,10] Therefore, recent advances in characterizing bulk water, including those by Yuan et al., take an important step toward unravelling many chemical and biophysical processes in the future.