A possible role of quantum effects, such as tunneling and zero-point energy,in the structural dynamics of supercooled liquids is studied by dielectricspectroscopy. Presented results demonstrate that the liquids, bulk3-methylpentane 3MP and confined normal and deuterated water have low glasstransition temperature and unusually low for their class of materials steepnessof the temperature dependence of structural relaxation, or fragility. Althoughwe do not find any signs of tunneling in structural relaxation of theseliquids, their unusually low fragility can be well described by the influenceof the quantum fluctuations. Confined water presents especially interestingcase in comparison to the earlier data on bulk low-density amorphous and vapordeposited water. Confined water exhibits much weaker isotope effect than bulkwater, although the effect is still significant. We show that it can beascribed to the change of the energy barrier for relaxation due to a decreasein the zero-point energy upon D-H substitution. The observed difference in thebehavior of confined and bulk water demonstrates high sensitivity of quantumeffects to the barrier heights and structure of water. Moreover, these resultsdemonstrate that extrapolation of confined water properties to the bulk waterbehavior is questionable.
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