The Complex Energy Spectrum of Isomeric Reactions
  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. 
A. Ugulavaa, Z. Toklikishvilia, S. Chkhaidze, L. Chotorlishvili, R.   Abramishvilia
physics.chem-ph