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Subject - Organic Chemistry

The term mesomerism or resonance is used if a chemical structure (molecule, transition state) cannot satisfactorily be described by one single Lewis formula - in this case also called mesomeric formula or mesomeric structure. For example, the allyl radical has two equivalent C-C bonds, but none of the mesomeric structures

CH2-CH=CH2CH2=CH-CH2

can solely account for this fact. The mesomerism term is based on the valence bond theory. This theory uses the wave function Ψ of the electronic ground state as a linear combination of electronic wave functions Ψi of the mesomeric structures.

Ψ=iCiΨi

The coefficients Ci are selected in such a way as to minimize the ground state energy. Therefore, the ground state energy of the the linear combination of the "mesomeric state" is lower than the energy of the mesomeric structures. The energy difference is named mesomerism- , resonance- or delocalization energy. Commonly, the wording that a chemical structure is mesomerism- or resonance stabilized often means that the delocalization of π electrons leads to stabilization.

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