Definition of Resonance Effect

This is the term most commonly used to describe the influence (on reactivity, spectra, etc.) of a substituent through electron delocalization into or from the substituent. The use of the term obviates the need to attempt to distinguish between the operation of the mesomeric effect and the electromeric effect. (An alternative term with essentially the same meaning is "conjugative effect". At one time "tautomeric effect" was also used, but was abandoned because tautomerism implies reorganization of the atomic nuclei.) The effect is symbolized by R.