π-electron acceptor/donor group

https://doi.org/10.1351/goldbook.E04391
A @S06076@ capable of a +R (e.g. NO2) or −R (e.g. OCH3) effect, respectively.
See:
σ-constant
,
electronic effect of substituents: symbols and signs
,
polar effect
,
substituent atom (group)
Source:
PAC, 1994, 66, 1077. (Glossary of terms used in physical organic chemistry (IUPAC Recommendations 1994)) on page 1155 [Terms] [Paper]