  The proton dynamics of a 2D water monolayer confined inside a graphene slit
pore is studied in Cartesian and molecular frames of reference using molecular
dynamics simulations. The vibrational density of states of the proton was
calculated versus temperature and was further used to deduce the mean kinetic
energy of the hydrogen atoms, Ke(H), in both frames of reference. The
directional components of Ke(H) are in good agreement with experimental
observations for bulk as well as nanoconfined water. Nonetheless, while in the
molecular frame of reference the effect of temperature on the anisotropy ratios
of Ke(H) (the ratio between its directional components) are practically
invariant between the 2D and 3D cases, those in the Cartesian frame of
reference reveal a rather notable reduction across 200K, indicating the
occurrence of an order-disorder transition. This result is further supported by
the calculated entropy and enthalpy of the confined water molecules. Overall it
is shown that Ke(H) anisotropy ratios may serve as a valuable order parameter
for detecting structural transformations in hydrogen bonds containing molecular
systems.
