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.
Journal
Nature