engleski

Proton dynamics of a Zundel ion in an aysmmetric environment

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). 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. An ideal system for study of the proton dynamics in a Zundel ion is nitranilic acid hexahydrate. It is a small molecule which contains only light atoms (C, N, O), which makes location of the proton using X-rays (especially CuKα radiation) not only reliable, but also very accurate. 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. Here we present a study of proton dynamics in such a perturbed Zundel ion by a combination of variable-temperature X-ray diffraction, IR, Raman and solid-state NMR spectroscopy and quantum chemical methods. X-ray diffraction experiments showed that the Zundel ion is almost temperature-invariant: between 100 K and room temperature O5∙∙∙O6 distance barely changes, from 2.433(2) Å to 2.438(2) Å, respectively. IR spectra display a similar temperature invariance ; a Zundel-like continuum between 1400 and 400 cm-1 corroborated a short, strong hydrogen bond with a dynamic proton. Solid-state 1H NMR revealed a peak at 16.5 ppm, which is assigned to the H1 proton of the Zundel ion.

Zundel ion; hydronium ion; proton transfer; proton dynamics

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