Collective Proton Tunneling in Ice?

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Proton transfer in hydrogen-bonded networks plays an important role in many chemical, biological, and industrial processes. Although quantum effects on proton transfer involving a single proton or two protons are well investigated by simulation they remain elusive when it comes to many-body concerted proton motion triggered by collective motion. Recent experiments have provided evidence that concerted tunneling involving as many as six protons might underly the non-Arrhenius proton dynamics observed at low temperatures in ordinary hexagonal ice - yet any mechanistic explanations are lacking beyond speculations. In this project ab initio path integral simulations will be used to investigate the mechanism of the alleged concerted tunneling of six protons in hexagonal ice. Here, quantum effects are qualitatively important to understand the phenomenon, and do not only broaden distribution functions and lower free energy barriers as usually found as a "trivial" consequence of small-amplitude zero-point vibrations. It is thus expected that the mechanistic insights gained into collective proton tunneling will be of broad importance beyond the specific case.