A study of the hydrogen bonds effect on the water density and the liquid-liquid transition

We study the hydrogen bonds effect on the water density as a function of temperature and pressure from the supercritical region to the metastable supercooled and amorphous phases. We identify two important thermodynamic thresholds, that is P* similar or equal to 2 kbar and T* similar or equal to 315 K, that separate two different water behaviors in terms of hydrogen bonding capability. For T < T * and P < P * the formation and stability of hydrogen bonded local structures are enhanced. The additional analyses of the proton NMR chemical shift and of the relaxation time con firm this evidence and highlight the structure breaking effects of the pressure. The investigation of both structural and dynamical quantities allow us to draw a complete picture of the water properties in terms of the temperature pressure dependence of hydrogen bonding.