Title: mesomeric effect
Long Title: IUPAC Gold Book - mesomeric effect
DOI: 10.1351/goldbook.M03844
Status: current

Definition
The effect (on reaction rates, ionization equilibria, etc.) attributed to a substituent due to overlap of its p- or $\ce{\uppi\!\mbox{-}orbitals}$ with the p- or $\ce{\uppi\!\mbox{-}orbitals}$ of the rest of the molecular entity. delocalization is thereby introduced or extended, and electronic charge may flow to or from the substituent. The effect is symbolized by M. Strictly understood, the mesomeric effect operates in the ground electronic state of the molecule. When the molecule undergoes electronic excitation or its energy is increased on the way to the transition state of a chemical reaction, the mesomeric effect may be enhanced by the electromeric effect, but this term is not much used, and the mesomeric and electromeric effects tend to be subsumed in the term resonance effect of a substituent.

Related Terms
- chemical reaction: https://goldbook.iupac.org/terms/view/C01033
- delocalization: https://goldbook.iupac.org/terms/view/D01583
- electromeric effect: https://goldbook.iupac.org/terms/view/E01973
- electronic effect: https://goldbook.iupac.org/terms/view/E01996
- electronic state: https://goldbook.iupac.org/terms/view/ET07026
- field effect: https://goldbook.iupac.org/terms/view/F02358
- inductive effect: https://goldbook.iupac.org/terms/view/I03021
- ionization: https://goldbook.iupac.org/terms/view/I03183
- molecular entity: https://goldbook.iupac.org/terms/view/M03986
- substituent: https://goldbook.iupac.org/terms/view/S06076
- transition state: https://goldbook.iupac.org/terms/view/T06468

Source
- PAC, 1994, 66, 1077. 'Glossary of terms used in physical organic chemistry (IUPAC Recommendations 1994)' on page 1139 (https://doi.org/10.1351/pac199466051077)

Other Outputs
- html: https://goldbook.iupac.org/terms/view/M03844/html
- json: https://goldbook.iupac.org/terms/view/M03844/json
- xml: https://goldbook.iupac.org/terms/view/M03844/xml

Citation: Citation: 'mesomeric effect' in IUPAC Compendium of Chemical Terminology, 5th ed. International Union of Pure and Applied Chemistry; 2025. Online version 5.0.0, 2025. 10.1351/goldbook.M03844
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Accessed: 2026-05-10T19:07:26+00:00