Aqueous proton transfer (hydronium ion dynamics) is a key event in numerous biological and industrial processes. Frequently, OH or OD stretch of HOD molecules are used in ultrafast IR spectroscopies to examine water dynamics. This approach is challenging in acidic solutions due to the strong and continuous absorption of hydronium ions from &lt;1000 cm-1 to 4000 cm-1. As an alternate approach, it is shown that methyl thiocyanate (MeSCN) is an accurate probe of water dynamics in pure water. MeSCN is then used in concentrated lithium chloride (LiCl) and hydrochloric acid (HCl) solutions to probe water/ion exchange. Two CN stretch absorption peaks of MeSCN were observed in both solutions. The lower frequency peaks, with peak positions almost the same as in pure water, were attributed to populations whose N lone pair is bonded to water. The blue-shifted peaks correspond to Li+-bonded and hydronium-bonded populations in the LiCl and HCl solutions, respectively. Chemical exchange is observed in both solutions as the growth of off-diagonal peaks in the 2D IR spectra. The dissociation time constants for the cation-bonded populations are ~60 ps in LiCl and ~8 ps in HCl solutions. Physical replacement of Li+/water bound to the N lone pair is the only pathway in the LiCl solutions. In HCl solution, however, proton transfer from a hydronium bound to the N lone pair to a neighboring water molecule, and vice versa is another possible pathway in addition to physical replacement. Ab initio molecular dynamics simulation demonstrates that proton shuffling is indeed the mechanism. Therefore, the chemical exchange dynamics reflects the proton transfer step time and extrapolation to dilute solution condition indicates that an irreversible proton transfer event takes ~ 2.7 ps. Further considerations of this value yield the reported bulk water proton diffusion constant.

Direct determination of proton transfer rate by 2D IR chemical exchange spectroscopy and ab initio molecular dynamics simulation

2019 Spring National Meeting

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