A molecular @P04711@ effect occurring by an @I03130@ electron displacement (sometimes called the '@C01272@' 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 @M03844-1@, @M03844-2@). It has long been custom to use phrases such as 'enhanced substituent @M03844-2@' 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 @I03022@.