Abstract
A possible role of quantum effects, such as tunneling and zero-point energy, in the structural dynamics
of supercooled liquids is studied by dielectric spectroscopy. The presented results demonstrate that the
liquids, bulk 3-methyl pentane and confined normal and deuterated water, have low glass transition
temperature and unusually low for their class of materials steepness of the temperature dependence
of structural relaxation (fragility). Although we do not find any signs of tunneling in the structural
relaxation of these liquids, their unusually low fragility can be well described by the influence of
the quantum fluctuations. Confined water presents an especially interesting case in comparison to
the earlier data on bulk low-density amorphous and vapor deposited water. Confined water exhibits
a much weaker isotope effect than bulk water, although the effect is still significant. We show that
it can be ascribed to the change of the energy barrier for relaxation due to a decrease in the zeropoint
energy upon D/H substitution. The observed difference in the behavior of confined and bulk
water demonstrates high sensitivity of quantum effects to the barrier heights and structure of water.
Moreover, these results demonstrate that extrapolation of confined water properties to the bulk water
behavior is questionable.
