Das, S.K.; Winghart, M.-O.; Han, P.; Rana, D.; Zhang, Z.-Y.; Eckert, S.; Fondell, M.; Schnappinger, T.; Nibbering, E.T. J.; Odelius, M.: Electronic Fingerprint of the Protonated Imidazole Dimer Probed by X-ray Absorption Spectroscopy. The Journal of Physical Chemistry Letters 15 (2024), p. 1264–1272
10.1021/acs.jpclett.3c03576
Open Access Version
Abstract:
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 protonated imidazole dimer, a prototypical building block enabling effective proton transport in biology and high-temperature fuel cells. Using nitrogen core excitations as a sensitive probe for the protonation status, 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.