A molecular polarizability effect occurring by an intramolecular electron displacement (sometimes called the 'conjugative mechanism' and, previously, the 'tautomeric mechanism') characterized by the substitution of one electron pair for another within the same atomic octet of electrons. It can be indicated by curved arrow symbolizing the displacement of electron pairs, as in:
which represents the hypothetical electron shift
The term has been deemed obsolescent or even obsolete (see mesomeric effect, resonance effect). It has long been custom to use phrases such as 'enhanced substituent resonance effect' which imply the operation of the electromeric effect, without using the term, and various modern theoretical treatments parametrize the response of substituents to 'electronic demand', which amounts to considering the electromeric effect together with the inductomeric effect.
PAC, 1994, 66, 1077. 'Glossary of terms used in physical organic chemistry (IUPAC Recommendations 1994)' on page 1109 (https://doi.org/10.1351/pac199466051077)