Why can the boundary between chemical relaxation and molecular relaxation become unclear during isomerization?

In the study of relaxation phenomena, it is common to distinguish between chemical relaxation—which involves the transformation between chemically distinct species—and molecular relaxation, which typically refers to physical processes like the transfer of energy between translational and vibrational states.1

However, this boundary becomes unclear when considering structural isomerizations. In these cases, the distinction between chemical and molecular relaxation depends largely on the height of the energy barrier separating the different structural forms of a molecule. Because an isomerization involves a change in the internal structure of a molecule rather than a fundamental change in chemical identity, it can be categorized as either a chemical or a molecular process depending on the definition used.1

For example, the interconversion between the axial and equatorial chair forms of methylcyclohexane is a structural change that can be viewed through either lens, illustrating why the classification of such relaxation processes is often a matter of definition rather than a rigid physical distinction.1