Structural study of supercritical water. I. Nuclear magnetic resonance spectroscopy

The proton chemical shift of water is measured at temperatures up to $\text{400°}\mathrm{C}$ and densities of $\text{0.19}$ , $\text{0.29}$ , $\text{0.41}$ , $\text{0.49}$ , and $\text{0.60}\mathrm{g}/{\mathrm{cm}}^3$ . The magnetic susceptibility correction is made in order to express the chemical shift relative to an isolated water molecule in dilute gas. The chemical shift is related to the average number of hydrogen bonds in which a water molecule is involved. It is found that the hydrogen bonding persists at supercritical temperatures and that the average number of hydrogen bonds is more than one for a water molecule in the supercritical densities. The density and temperature dependence of the chemical shift at supercritical temperatures are analyzed on the basis of statistical thermodynamics. It is shown that the hydrogen bonding is spatially more inhomogeneous at lower densities.