Chemistry Reference
In-Depth Information
0.2
0.15
0.1
0.05
0
0
0.05
0.1
0.15
0.2
DFT energy (kcal mol
-1
water
-1
)
Figure 31.
Test of the ability of the graph invariant expansion to fit the energies of the 62
symmetry-distinct H-bond isomers of a 12-water unit cell of ice XII. The arrows indicate those
symmetry-distinct configurations possessing
P
2
1
2
1
2
1
symmetry, the ordering of which is identical
to that previously reported [174].
At higher temperatures, but still below the melting point of ice, water molecules
on the surface of ice are more mobile than those in the bulk due to their lower H-
bond coordination. This topmost liquid-like (quasiliquid) layer has been the topic
of scientific discussion for more than a century [182]. In one recent study, the
surface of some ice samples were investigated by examining the elastic scattering
of low-energy helium atoms [183]. The angular distribution of the scattered He
atoms from the ice surface is sensitive to the degree of order on the ice surface. In
this study, an ordered surface corresponds to a crystalline surface, not necessarily
an ordered arrangement of H-bonds. It was observed that the intensity of the
angular distribution decreases linearly as the temperature is raised from 150-180 K
after which the intensity rapidly decreases. This rapid decrease was interpreted by
the authors in terms of strongly anharmonic surface vibrations and a first sign of
increased mobility of water molecules on the ice surface.
At temperatures below this premelting transition, where there is a solid-like
ice surface, one would expect distinguishable H-bond arrangements to exist at the
surface in conjunction with proton disorder in the bulk. One topological feature
of the ice surface is the distribution of dangling oxygen and hydrogen atoms (free
OH bonds). Surface sensitive techniques, such as sum-frequency generation spec-
troscopy, can detect these free OH bonds. In one recent study on the ice-vapor
interface, the maximum tilt angle and orientational-order parameters for the free
