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ION–SOLVENT INTERACTIONS 213

But the general definition of a quadrupole moment is the magnitude of each charge of
quadrupole times the square of the distance of the charge from the origin. Thus,
is the y component of the quadrupole moment for the particular coordinate
system that has been chosen. Similarly, is the x component of the quadrupole
moment. Thus,

One can combine into a single symbol and talk of the quadrupole
moment of the water molecule in the particular orientation of Fig. A2.3.1.
Hence,

and therefore by substituting Eqs. (A2.3.22) and (A2.3.25) in Eq. (A2.3.18),

The ion–quadrupole interaction energy [cf. Eq. (A2.3.1)] thus becomes

When a negative ion is considered, the water molecule turns around through
and one obtains by an argument similar to that for positive ions

EXERCISES

1. The heat ofevaporation of water is Calculate the total H- bond energy
for 1 mole of water, and compare the two values.

2. The ion has a radius of 180 pm. Find how much bigger the ion–dipole term
is compared with the Born term. The dielectric constant in the Born equation is
to be taken as 80.
3. The energy of the ion-water dipole interaction is at 298 K.
Calculate the radius of a water molecule. Consider pm and
(Contractor)