Potential energy surface methyl radical transition state

More sophisticated calculations of the semi-empirical SCF and ab initio types have been used to plot partial potential energy surfaces for methyl radicals with alkenes (55) (56) (57). They suggest that the lowest energy approach of the radical is from above the plane of the alkene directly above one or other of the carbons. The geometry of the transition state was found to be little changed from that of the reactants. So far this type of computation has not been attempted with fluoroalkyl radicals. 

Figure 5 shows the calculated electron density distribution as a slice through the C-H-C linkage of the transition state. Clearly, the H atom has equal electron overlap between the C atom of the parent butane molecule and the C atom of the incoming methyl radical. As such, it can go downhill either way along the energy potential surface. This is an important feature, it keeps the radical alive and therefore makes the chain reaction propagate. 

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