NMR Study of the Structure and Dynamics of Supercritical Water

A high-temperature and high-pressure NMR method to investigate the structure and dynamics of supercritical water is reviewed. In this method, a high-temperature condition is realized by introducing hot air directly into the sample portion of a specially designed probe, and a high pressure is achieved by sealing the material of interest into a quartz capillary. The method allows a high-resolution measurement up to 400°C and 0.6 g/cm3 (corresponding to 55 MPa) of water. It is found from the proton chemical shift measurement that the hydrogen bonding persists at supercritical temperatures and that the average number of hydrogen bonds is at least one in the supercritical densities. The measurement of the spin-lattice relaxation time also shows that while the reorientational relaxation proceeds on the order of picosecond in ambient water, it does on the order of several tens of femtoseconds in supercritical water.