The unimolecular decomposition of molecules involving rupture of a bond and formation of unstable radicals (i.e., ones which decompose exothermally) is discussed. It is shown, by using contour maps, that two cases should be considered. If the decomposition of the unstable radical does not involve activation energy, then the process starts as expansion of one bond, but eventually the reaction path turns toward the center and the process proceeds as a simultaneous decomposition of the molecule into 3 or more fragments. The transition point is represented by a saddle point and the activation energy is lower than the expected bond dissociation energy. In a loose language, the exothermicity of the radical decomposition helps in the primary dissociation process.On the other hand, if the decomposition of the unstable radical does involve activation energy, then the unimolecular rupture of the first bond proceeds normally.
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