Broadband 2D IR spectroscopy reveals dominant asymmetric H5O2+ proton hydration structures in acid solutions

Abstract

Given the critical role of the aqueous excess proton in redox chemistry, determining its structure and the mechanism of its transport in water are intense areas of experimental and theoretical research. The ultrafast dynamics of the proton’s hydration structure has made it extremely challenging to study experimentally. Using ultrafast broadband two-dimensional infrared spectroscopy, we show that the vibrational spectrum of the aqueous proton is fully consistent with a protonated water complex broadly defined as a Zundel-like H5O2+ motif. Analysis of the inhomogeneously broadened proton stretch two-dimensional lineshape indicates an intrinsically asymmetric, low-barrier O–H+–O potential that exhibits surprisingly persistent distributions in both its asymmetry and O–O distance. This structural characterization has direct implications for the extent of delocalization exhibited by a proton’s excess charge and for the possible mechanisms of proton transport in water.

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Fig. 1: Infrared spectrum of the aqueous proton.
Fig. 2: 2D IR difference spectra for 2 M HCl and neat water.
Fig. 3: 2D IR spectrum and lineshape analysis of the proton stretching mode.
Fig. 4: Polarization dependence of cross-peak intensity.
Fig. 5: Model of the aqueous proton structure and vibrational assignments consistent with our measurements.

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Acknowledgements

This work was supported by the Office of Basic Energy Sciences, US Department of Energy under grant DOE DE-SC0014305. J.A.F. thanks the Arnold O. Beckman Foundation for support through a postdoctoral fellowship. N.H.C.L. acknowledges support from the Yen Fellowship programme.

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J.A.F. and A.T. conceived and designed experiments. J.A.F., W.B.C. and N.H.C.L. performed the experiments. All authors discussed the results, analyses and interpretations. J.A.F. and A.T. wrote the paper with input from all authors.

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Correspondence to Andrei Tokmakoff.

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Supplementary Data and Analysis, Supplementary Figures 1–14

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Fournier, J.A., Carpenter, W.B., Lewis, N.H.C. et al. Broadband 2D IR spectroscopy reveals dominant asymmetric H5O2+ proton hydration structures in acid solutions. Nature Chem 10, 932–937 (2018). https://doi.org/10.1038/s41557-018-0091-y

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