engleski

X-ray charge density study of a Zundel ion

The Zundel ion, H5O2+, occupies a central place in the theory of hydrogen bonding and proton transfer. It is formed as an intermediate during the proton transfer processes in aqueous solutions (and all other protic solutions, too) and biologial systems. One can hardly imagine fundamental biological reactions, such as enzyme kinetics, respiration and photosynthesis without transfer of protons and/or electrons. However, it is an ultrafast process, with a timescale of pico- and femtoseconds, which can be directly studied only by the most advanced time-resolved spectroscopic methods (femtosecond laser UV/vis and IR/Raman) and femtosecond X-ray diffraction using an X-ray free-electron lasers (XFEL). Zundel-like states, with dynamic proton disorder, can be 'trapped' in otherwise 'static' crystals, extending their lifetime by a dozen orders of magnitude - from pico- and femtoseconds to days and months. Then it becomes possible to focus on a single hydrogen bond, using a variety of experimental and theoretical methods.[1] An ideal system for study of the proton dynamics in a Zundel ion is nitranilic acid hexahydrate.[1] It is a small molecule which contains only light atoms (C, N, O), elliminating the problem of absorption and allowing accurate location of hydrogen atoms. Moreover, its crystals comprise H5O2+ cations located in the general position, thus artificial centrosymmetricity (which is usually the case with Zundel ions in the crystal state) can be avoided.[1] Thus, nitranilic acid hexahydrate is an ideal system for X-ray charge density study of a Zundel ion. We present the first X-ray charge density study of a Zundel ion, which reveals electron density and bond order of its central O∙∙∙H∙∙∙O group. To accurately model low electron density on the protons, we have combined X-ray and neutron diffraction (experiment was performed at Laboratoire Léon Brillouin in Saclay, France, with support by prof. Alexandre Bataille): position and anisotropic displacement parameters of the protons used in the refinement of charge density are taken from the structure determined by neutron diffraction.

Zundel ion ; proton transfer ; X-ray charge density

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