What is mesomerism

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Area of ​​Expertise - General chemistry, quantum physics

The term mesomerism or resonance is used when a chemical structure (molecule, transition state) cannot be satisfactorily described by a single valence line formula (Lewis formula) - in this case also called limit formula or limit structure. For example, the allyl radical has two equivalent carbon-carbon bonds. None of the border structures

can explain this finding on its own. The term mesomerism is based on the valence bond method. This method uses the wave function of the electronic ground state as a linear combination of the electron wave functions of the boundary structures.

The coefficients are chosen so that the ground state energy is minimized. The ground state energy of the linear combination, the "mesomeric state", is lower than the energy of the boundary structures. The energy difference is called the mesomerism, resonance or delocalization energy. The linguistic usage that a chemical structure is mesomerism or resonance stabilized usually means that by delocalizing π-Electrons stabilization is achieved.

Learning units in which the term is discussed

Delocalized systems45 min.

chemistryGeneral ChemistryChemical bond

There are a large number of molecules that cannot be adequately described with localized molecular orbitals or with overlapping using the VB method. This is the case, among other things, when the binding electrons have areas of residence available that extend beyond two atoms. Since a valence line corresponds to a binding electron pair between two atoms, i.e. the electrons are seen to be localized, difficulties arise here when writing valence line formulas.

Chemical bond: resonance20 min.

chemistryOrganic chemistryChemical bond

In this learning unit, the concept of resonance is introduced using the example of the allyl cation. The MO and VB theory are also explained using the allyl cation.