Pyridinium cations frontier orbitals

Fig, 4-47 Frontier orbitals of the pyridinium cation as a model for the Diels-Alder reaction of 208 with acrylonitrile... 

Fig. 5-8 Interactions of frontier orbitals for the reaction of alkyl radicals with substituted pyridinium cations...

These reactivity trends clearly show that polar effects are involved in these radical substitution reactions. The transition state is thought to include a charge transfer 9) from the radical (electron donor) to the pyridinium ion (electron acceptor) [13], Frontier Molecular Orbital Theory (FMO) [14] has been applied to explain the reactivity differences which have been observed upon varying the substituents at the pyridinium ion and upon altering the nucleophilicity of the attacking radical. Moreover, FMO can be used to explain the regioselectivities obtained in these homolytic aromatic substitutions. The LUMO of the substituted pyridinium cation... 

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