  We propose a model to describe a gas of pyramidal molecules interacting via
dipole-dipole interactions. The interaction modifies the tunneling properties
between the classical equilibrium configurations of the single molecule and,
for sufficiently high pressure, the molecules become localized in these
classical configurations. We explain quantitatively, without free parameters,
the shift to zero frequency of the inversion line observed upon increase of the
pressure in a gas of ammonia or deuterated ammonia. For sufficiently high
pressures, our model suggests the existence of a superselection rule for states
of different chirality in substituted derivatives.
