Liquid water has a primary role in ruling life on Earth in a wide temperature and pressure range as well as a plethora of chemical, physical, geological, and environmental processes. Nevertheless, a full understanding of its dynamical and structural properties is still lacking. Water molecules are associated through hydrogen bonds, with the resulting extended network characterized by a local tetrahedral arrangement. Two different local structures of the liquid, called low-density (LDW) and high-density (HDW) water, have been identified to potentially affect many different chemical, biological, and physical processes. By combining diamond anvil cell technology, ultrafast pump−probe infrared spectroscopy, and classical molecular dynamics simulations, we show that the liquid structure and orientational dynamics are intimately connected, identifying the P−T range of the LDW and HDW regimes. The latter are defined in terms of the speeding up of the orientational dynamics, caused by the increasing probability of breaking and reforming the hydrogen bonds.
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