K-003
Erik T.J. Nibbering [1]
nibberin@mbi-berlin.de
Marc-Oliver Winghart [1], Sambit Das [2], Debkumar Rana [1], Zhuang-Yan Zhang [1], Peng Han [1], Maria Ekimova [1], Carlo Kleine [1], Jan Ludwig [1], Miguel Ochmann [3], Thomas E. G. Agrenius [2], Eve Kozari [4], Dina Pines [4], Mattis Fondell [5], Rolf Mitzner [5], Sebastian Eckert [5], Ehud Pines [4], Nils Huse [3], Philippe Wernet [6], Michael Odelius [2]
[1] Max Born Institute for Nonlinear Optics and Short Pulse Spectroscopy, Berlin, Germany
[2] Department of Physics, Stockholm University, Sweden
[3] Institute for Nanostructure and Solid State Physics, CFEL, Hamburg, Germany
[4] Ben Gurion University of the Negev, Beersheva, Israel
[5] Institute for Methods and Instrumentation for Synchrotron Radiation Research, Helmholtz-Zentrum Berlin für Materialien und Energie GmbH, Berlin, Germany
[6] Department of Physics, Uppsala University, Sweden.
On the Nature of Shared Protons in Water and in Imidazole: Mid-Infrared and Soft-X-Ray Spectroscopy
Protons in low-barrier superstrong hydrogen bonds are typically delocalized between two electronegative atoms. Conventional methods to characterize such superstrong hydrogen bonds are vibrational spectroscopy and diffraction techniques. We introduce soft X-ray spectroscopy to uncover the electronic fingerprints for proton sharing in the hydrated proton complex, important constituent in mediating proton transport in bulk water (known as the von Grotthuss mechanism) and in protonated imidazole dimer, a prototypical building block enabling effective proton transport in biology and high-temperature fuel cells. While geometries and stoichiometry have been widely addressed in both experiment and theory, the electronic structure of these specific hydrated proton species, however, has remained elusive to date. Here we show how by utilizing novel flatjet technology for accurate X-ray spectroscopic measurements and with a combination of infrared spectral analysis and calculations, we find orbital-specific markers that distinguish two main electronic-structure effects: Local orbital interactions determine covalent bonding between the proton and neigbouring water or imidazole molecules, while orbital-energy shifts measure the strength of the extended electric field of the proton. A hierarchy in electronic structure changes of water molecules involved in proton hydration, as evidenced by the local oxygen K-edge XAS contributions, is directly correlated with the strength of nearest neighbour hydrogen bond interactions and associated O···O distances. Using nitrogen core excitations as a sensitive probe for the protonation status in protonated imidazole dimer, we identify the X-ray signature of a shared proton in the solvated imidazole dimer in a combined experimental and theoretical approach. The degree of proton sharing is examined as a function of structural variations that modify the shape of the low-barrier potential in the superstrong hydrogen bond. We conclude by showing how the sensitivity to the quantum distribution of proton motion in the double-well potential is reflected in the spectral signature of the shared proton.
M. Ekimova et al., Angew. Chem. Int. Ed. 61, e202211066 (2022)
S. K. Das et al., J. Phys. Chem. Lett. 2024, 15, 1264−1272.