Perturbation of the HOMO And LUMO

Chapter 12 Qualitative Molecular Orbital Theory Pericyclic Reactions 

The best electron sources have an accessible high-energy HOMO. When the energy of the HOMO of the nucleophile is high, the nucleophile is softer, and the stabilization resulting from overlap with the electrophile s LUMO is much greater. The best electron sinks have an accessible low-energy LUMO. 

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