The effect (on reaction rates, ionization equilibria, etc.) attributed to a substituent due to overlap of its p- or pi-orbitals
with the p- or pi-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.