The electrostatic field and the molecular dipole moment in the polymorphs of ice

The magnitude and the direction of the electrostatic field at the site of a water molecule in hexagonal and cubic ice and in ice III, IV, V, and VI have been calculated. The calculations take into account neighbors up to the third shell and the probability of each orientation of these neighbors in a dendritic manner. The electrostatic field has the direction of the dipole moment of the central molecule in all the polymorphs, but its magnitude decreases with increase in the density of the polymorphs. The field at a molecular site is lower the lower the symmetry of the molecular site. The increase in the dipole moment of water molecule resulting from this field has been calculated for all molecules in the unit cell, assuming that the increase is directly proportional to the field strength. The weighted average dipole moment in the ices is 30%–45% higher than the dipole moment of a water molecule in the vapor phase. The largest increase is for cubic ice and the smallest for ice VI. All calculated values of the dipole moments substantially differ from values anticipated from the model of point dipole at the center of an isotropically polarizable spherical cavity. The contribution to relative permittivity due to orientation polarization has been calculated using these values of the dipole moment and the known dipolar correlation factor. The relative permittivity agree reasonably well with the measured values for all polymorphs of ice.