Proton Resonance Shift of Water in the Gas and Liquid States

Abstract

Temperature-dependence measurements have been made on the chemical shift of the proton of a water molecule in the liquid state and in the gas state at varying pressure. The problem of relating these experimental data to the intermolecular forces leading to cohesion and to hydrogen-bond formation between water molecules is considered in detail. It is shown that a consistent treatment of the chemical shift, thermal, and dielectric data for water can be given based on a two-state model involving an equilibrium between a hydrogen-bonded ``icelike'' fraction and a ``monomer'' fraction whose interaction with the lattice arises entirely from London dispersion forces. Using semiempirically derived values of the chemical shift and energy associated with the condensation of water vapor to ``monomer,'' the magnitude of the shift associated with the transformation to ice is calculated. It is then shown that, on the assumption that the hydrogen bond is electrostatic in character, the ``polar'' contribution to this shift can be related through the appropriate shielding equations to the dipole moment of the water molecule in ice. The magnitude of the dipole moment derived from these relationships is found to be in excellent agreement with values derived from dielectric data. The possibility that the shielding changes may in part be due to processes other than the breaking of hydrogen bonds is considered. It is shown that the model leads to the conclusion that the chemical shift in the transformation of ice to water at 0°C could be entirely accounted for either by a stretching of the hydrogen bonds or a small amount of bending of the bonds. It is noted that if some bond breaking does occur, as required by the fact that water is a liquid, then the amount of stretching and/or bending will be limited.