An in situ Raman spectroscopy study of subcritical and supercritical water: The peculiarity of hydrogen bonding near the critical point

The Raman spectra of water are measured at temperatures up to 510 °C and at pressures up to 40 MPa. The peak frequency increases with temperature, indicating the break of hydrogen bonding, and it changes only slightly at higher temperatures above the critical point. The peak frequency has a maximum near the critical pressure, and the extent of hydrogen bonding significantly changes with pressure in this near-critical region. The deviation of the maximum frequency $\mathrm{\Delta f}$ relative to that of the monomer is in good agreement with the chemical shifts with the literature NMR data. The extent of the hydrogen bonding can be estimated from the $\mathrm{\Delta f}$ values. The $\mathrm{\Delta f}$ values at the near-critical points are significantly lower compared with those in super- or sub-critical conditions, and the strength of the hydrogen bonding weakens uniquely in the near-critical region.