The effect (on reaction rates, @[email protected] equilibria, etc.) attributed to a substituent due to overlap of its p- or π-orbitals with the p- or π-orbitals of the rest of the @[email protected] @[email protected] 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 @[email protected] of the molecule. When the molecule undergoes electronic excitation or its energy is increased on the way to the @[email protected] of a @[email protected], the mesomeric effect may be enhanced by the @[email protected], but this term is not much used, and the mesomeric and electromeric effects tend to be subsumed in the term resonance effect of a @[email protected]
See also:
electronic effect
field effect
inductive effect
PAC, 1994, 66, 1077. (Glossary of terms used in physical organic chemistry (IUPAC Recommendations 1994)) on page 1139 [Terms] [Paper]