  The internal motion in a molecule, in which isomerization processes occur, is
characterized by two essentially different modes of motion - oscillatory and
rotational. The quantum equation of motion which describes an isomerization
process is reduced to the Mathieu-Hill equation. As is known, this equation is
able to describe both modes. In the paper, it is shown that the chaotic region
of the energy spectrum characterizing an isomerization process corresponds to
the region where two modes of motion are merged together.
