Discrimination of diverse (pressure/temperature-dependent/independent) inherent sub-structures in liquid water (D2O) from difference vibrational spectroscopy

The new kind of inherent (interstitial) sub-structure of liquid D2O under high pressure up to 300 MPa, compared to those found under ambient liquid conditions (5-85 degreesC) and for D2O ice Ih (-10 degreesC), have been revealed by using of difference and double-difference near-infrared O-D stretching overtone spectroscopy and the subsequent band shape treatment. These procedures rigorously disclosed a variety of component sub-bands exhibiting either dependence on or independence of pressure and temperature. An extended analysis based on the well established correlation between the stretching frequency shift and the hydrogen-bond energy for liquid binary systems (along with the geometric parameters discussed earlier), including also the available results for X-ray and neutron scattering of liquid water, allowed for the identification of basic (Ih ice-like), temperature-induced remote defect (transport-related "tetrahedrally displaced"), temperature-independent dense liquid-phase (probably noncyclic tetramer), and pressure-induced denser (probably cube-like) inherent substructures, in accordance with the modern concepts on the liquid-liquid transitions (including the "fragile-to-strong transition" and the "second critical point" hypotheses).