  It is shown that the dipole moment of polar (water, methanol, formamide,
acetone and acetonitrile) molecules in the neighborhood of a cation is
increased primarily by polarization from the bare electrostatic charge of the
cation, although the effective value of the latter is somewhat reduced by "back
donation" of electrons from neighbouring polar molecules. In other words, the
classical picture may be viewed as if a point charge slightly smaller than the
nominal charge of the cation would be placed at the cation site. It was found
that the geometrical arrangement of the polar molecules in the first solvation
shell is such that their mutual polarization reduces the dipole moments of
individual molecules, so that in some cases they become smaller than the dipole
moment of the free protic or aprotic molecule. We conjecture that this behavior
is essentially a manifestation of the Le Chatellier-Braun principle.
