Heteroatoms The Effect of an Electronegativity Perturbation

Earlier in this chapter we considered the effect of orbital interactions on a previously noninteracting system. But suppose now we take as starting point two interacting orbitals 4 A and f B of equal energy and we introduce a change in electronegativity at centers A and B. The qualitative results of such a perturbation are again well known from elementary quantum chemistry  

When an electronegativity perturbation occurs, two molecular orbitals originally containing equal components of two atomic orbitals localize so that the lower one is primarily on the more electronegative atom, the upper one on the less electronegative atom. 

Both orbitals are stabilized if the average electronegativity increases both destabilized if it decreases. If the average electronegativity remains constant, one orbital rises while the other one is lowered. 

It is generally ambitious to try and explain reactive pathways by sole consideration of the electron distribution in the reactant. 

Correlation diagrams include the product orbitals while perturbation approaches require knowledge of the empty orbitals of the reactant. However, the occupied molecular orbitals of diazirine (II), compared with those of cyclopropene (I), do seem to give some indication of a preferred thermal decomposition of (II) compared with the rearrangement of (I). Moreover these molecular orbitals are a typical illustration of the localization obtained in the presence of an electronegativity perturbation. 

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