  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. Presented results demonstrate that the liquids, bulk
3-methylpentane 3MP 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, or fragility. Although
we do not find any signs of tunneling in structural relaxation of these
liquids, their unusually low fragility can be well described by the influence
of the quantum fluctuations. Confined water presents especially interesting
case in comparison to the earlier data on bulk low-density amorphous and vapor
deposited water. Confined water exhibits 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 zero-point 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.
