Resonance stabilization, merging

Whereas in examples 1 and 2 above it may be argued that the direction and ease of reaction is (partly) due to merging resonance stabilization during product formation, this cannot be a positive factor in cases where OH or OR function as the nucleophile. Valuable examples are seen in the reaction of some o- and p-halogenoanisoles. o-Fluoroanisole and especially p-chloroanisole give smooth photosubstitution upon illumination in the presence of various nucleophiles (Brasem and Lammers, 1972 den Heyer, 1973). 

In the case of adiabatic electron transfer reactions, it is found that the potential energy profiles of the reactant and product sub-systems merge smoothly in the vicinity of the activated complex, due to the resonance stabilization of electrons in the activated complex. Resonance stabilization occurs because the electrons have sufficient time to explore all the available superposed states. The net result is the attainment of a steady, high, probability of electron transfer. By contrast, in the case of -> nonadiabatic (diabatic) electron transfer reactions, resonance stabilization of the activated complex does not occur to any great extent. The result is a transient, low, probability of electron transfer. Compared with the adiabatic case, the visualization of nonadiabatic electron transfer in terms of potential energy profiles is more complex, and may be achieved in several different ways. However, in the most widely used conceptualization, potential energy profiles of the reactant and product states... 

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