Collective Proton Tunneling in Ice?
Final Report Abstract
The transfer of multiple protons in hydrogen-bonded networks usually occurs one proton at a time. At sufficiently high temperatures, such as room temperature, each proton transfers via thermally activated hopping along its hydrogen bond, thereby moving a charge defect through the network. At low temperatures, in stark contrast, quantummechanical tunneling might set in instead, thus avoiding to hop over the energy barriers. In the case of several protons to move, independent thermal hopping or quantum tunneling of the individual protons becomes less favorable because of a significant creation of charge defects. In individual molecules or hydrogen-bonded molecular complexes, for instance, double proton transfer is often found to be concerted. Multiple proton transfer that avoids charge defects can occur in cyclic, ring-like topologies built from several hydrogen bonds that allow for directional chains of proton transfer. This requires perfect proton order within these rings, which imposes handedness and thus chirality, and changes parity upon transfer of all protons. Ordinary ice, which is hexagonal ice Ih , is the most stable form of crystalline ice obtained upon freezing liquid water at ambient pressure and consists of interconnected six rings of oxygen atoms that host six protons each which form a cyclic hydrogen-bonded network (called hexamer). These hexagonal rings remain overall proton disordered even down to low temperatures, as heralded by the residual entropy of ice Ih . However, owing to combinatorics, a certain number of these six rings is proton ordered in macroscopic crystals. These chiral hexameric rings might support coherent tunneling of the hosted protons. Indeed, there is some evidence in the recent literature that correlated tunneling of all protons in chiral cyclic water clusters might be possible if temperatures are low enough. Our ab initio path integral simulations of ordinary ice Ih , which take the electronic structure into account as well as the quantum nature of all nuclei in the system, have disclosed a delocalized quasi-particle mechanism for correlated proton tunneling that involves all six protons in proton-ordered (chiral) hexamer rings that are embedded in overall proton-disordered crystalline ice Ih . Replacing only a single proton by a deuteron in such a chiral H-bonded cyclic network has been found to greatly suppress the collective tunneling process. Such a phenomenon has been detected subsequently by experimentalists when performing scanning tunneling microscopy experiments upon replacing a normal water molecule by a heavy water molecule in chiral water tetramers that have been deposited on surfaces.
Publications
- Exceptional Isotopic-Substitution Effect: Breakdown of Collective Proton Tunneling in Hexagonal Ice due to Partial Deuteration, Angew. Chem. Int. Ed. 53, 10937–10940 (2014)
C. Drechsel–Grau and D. Marx
(See online at https://doi.org/10.1002/anie.201405989) - Quantum Simulation of Collective Proton Tunneling in Hexagonal Ice Crystals, Phys. Rev. Lett. 112, 148302–1-5 (2014) [with 21 pages of Supplemental Material and 3 Videos]
C. Drechsel–Grau and D. Marx
(See online at https://doi.org/10.1103/PhysRevLett.112.148302) - Tunneling in chiral water clusters: Protons in concert (News & Views Article), Nat. Phys. 11, 216–218 (2015).
C. Drechsel–Grau and D. Marx
(See online at https://doi.org/10.1038/nphys3269) - Collective proton transfer in ordinary ice: local environments, temperature dependence and deuteration effects, Phys. Chem. Chem. Phys. 19, 2623–2635 (2017)
C. Drechsel–Grau and D. Marx
(See online at https://doi.org/10.1039/c6cp05679b)
