NMR Study of Water Structure hi Supercritical Conditions

Abstract

The proton chemical shift of water is measured at temperatures up to 400°C and densities of 0. 19, 0. 29, 0. 41, 0. 49, and 0. 60 g/cm³. 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 at least one for a water molecule in the supercritical densities . The density dependence of the chemical shift at supercritical temperatures is analyzed on the basis of statistical thermodynamics. It is shown that the hydrogen bonding is spatially more inhomogeneous at lower densities. The dipole moment of water at supercritical states is estimated from the number of hydrogen bonds. we perform the proton chemical shift measurements for a water molecule as a function of the density. than that in the gas phase, and this enhancement of supercritical water by fitting the experimentally determined number of hydrogen bonds.