Unimolecular reactions internal energy modes

The most widely accepted theory of unimolecular reactions of polyatomic ions remains the quasiequilibrium theory (QET) [591, 720, 883], which is a treatment in the spirit and tradition of absolute reaction rate theory. Thus it is assumed that the rate of reaction of an ion is slow relative to the rate of energy flow among its vibrational modes and that each reaction may be described as a motion along a reaction coordinate which is separable from all other internal coordinates and which passes through a critical configuration (the transition state ). It is further assumed that ions formed in excited electronic states rapidly redistribute such electronic energy over vibrational levels of the ground electronic state. One further assumption is necessary, and that is that the time involved in the ionization process is short compared with subsequent reaction times. The QET model is taken as the theoretical basis of this review. QET leads to... 

These high collision efficiencies have some interesting consequences for the theory of unimolecular reactions. In particular, they imply that the preferred path for fission reactions involve appreciable storage of internal energy in two rotational modes (5). For pyrolysis reactions, they suggest that atom-atom recombination will usually contribute very little to over-all recombination relative to atom-radical or radical-radical reactions. The reason is that the atom-atom recombination is third order. 

There is considerable support for the hypothesis that a molecule decomposes unimolecularly when a sufficient amount of energy becomes localized in a particular bond or mode of vibration.19 Knowing these various types of vibration as in the ethyl bromide molecule one is tempted to speculate as to which are the most likely ones to be involved in the chemical reaction. For example, if ethyl bromide decomposes to give a free ethyl radical and a bromine atom as the first step the action must occur by increasing the amplitude of vibration of the fundamental frequency Fi to the point of rupture. On the other hand if the molecule disrupts into ethylene and hydrobromic acid in a single internal operation the fundamental frequency Fq or F8 must be involved. These latter... 

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