Computational Physics Unravels The Mysteries Of Ice

March 30, 1998

A scientific team from the Max Planck Institute for Solid-State Research in Stuttgart and the University of Montpellier II report in the 19 March issue of Nature (volume 392, issue 6673, page 258) a ground-breaking study of the behaviour of hydrogen bonds -- a rather weak chemical bond that can be switched by thermal motion -- which is of importance from materials science (e.g. fast proton conductors) to biochemistry (e.g. enzyme catalysis).

The behaviour of ice under pressure offers a unique tool to investigate the properties of one of the most important interactions in nature: the hydrogen bond. A hydrogen bond is the interacion which gives water its extraordinary properties and is at the heart of most biochemical processes and many chemical phenomena.

As is well known, water is composed of one oxygen and two hydrogen atoms. In the hydrogen bond the hydrogen of one water molecule points towards the oxygen of another. By applying pressure one can continually vary the water-water distance and bring the water molecules so close together that the proton can be shared between two molecules, leading to the so-called symmetric hydrogen bond. In these circumstances, the water molecule ceases to exist as a separate entity and the protons become transferred from one molecule to another.

Symmetrization of the hydrogen bond and proton transfer are indeed important biological and chemical processes and the study of high pressure ice can shed some light on these phenomena as well. However, the study of high pressure ice is not without difficulty, both experimentally and theoretically.

Experimentally it is very difficult at high pressure to determine the proton position directly, since protons are invisible to X-ray and neutron scattering requires much larger samples than those that can be brought at high pressure. Indirect information about the ice phase transition has so far been obtained only via infrared and Raman scattering experiments.

Theoretical investigations are difficult because it is hard to model the interatomic interactions and describe the quantum nature of the proton. Very recent progress (Marx and Parrinello) has overcome these limitations and allows the proton behaviour to be described accurately.

Using these techniques Benoit et al. have shown that when it is subject to sufficient high pressure, the proton can jump from a molecule to a neighbouring one, due to a typically quantum phenomenon called tunnelling, which allows light particles like protons to overcome energy barriers. At even higher pressures the proton sits on average at the middle of the bond (symmetric hydrogen bond), leading to the so-called ice X phase.

The calculations of Benoit et al. confirm an old conjecture that such a phase can be reached and illuminate its subtle behaviour.
The insight gained will be of help in biology and chemistry, as discussed by Teixeira in the accompanying "News and Views" article on p. 232 in the same issue.


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