atropisomer

right|300px|thumb|Atropisomers of 6,6'-dinitro-2,2'-diphenic acid were first experimentally described case, by Christie and Kenner (1922).

Atropisomers are a kind of stereoisomer arising because of hindered rotation about a single bond, where energy differences due to steric strain or other contributors create a barrier to rotation that is high enough to allow for isolation of individual rotamers. They occur naturally and are of occasional importance in pharmaceutical design. When the substituents are achiral, these conformers are enantiomers (atropoenantiomers), showing axial chirality; otherwise they are diastereomers (atropodiastereomers).