Inversion of ammonia molecule

Ammonia molecule has a pyramidal structure with the basal plane as an equilateral triangle defined by the three HYDROGEN atoms and a NITROGEN atom at the apex on the line passing normally through the center of the triangle. This structure can be represented by a symmetric double well potential. The nitrogen atom can come closer to the basal plane. As it comes closer to the basal plane, it experiences a repulsive force due to the hydrogen atoms. Thus there is a potential barrier. The nitrogen atom then passes to the other side by tunneling effect. This can not be explained classically and is a purely a quantum mechanical problem. The frequency at which the nitrogen atoms oscillates back and forth about the basal plane is called the inversion frequency. Here are some references-Ref-1 , Ref-2.