  By confining water in nanopores, so narrow that the liquid cannot freeze, it
is possible to explore its properties well below its homogeneous nucleation
temperature TH ~ 235 K. In particular, the dynamical parameters of water can be
measured down to 180 K approaching the suggested glass transition temperature
Tg ~ 165 K. Here we present experimental evidence, obtained from Nuclear
Magnetic Resonance and Quasi-Elastic Neutron Scattering spectroscopies, of a
well defined decoupling of transport properties (the self-diffusion coefficient
and the average translational relaxation time), which implies the breakdown of
the Stokes-Einstein relation. We further show that such a non-monotonic
decoupling reflects the characteristics of the recently observed dynamic
crossover, at about 225 K, between the two dynamical behaviors known as fragile
and strong, which is a consequence of a change in the hydrogen bond structure
of liquid water.
