https://doi.org/10.1351/goldbook.F02520
In a stereochemical context the rotation about a bond is called 'free' when the @[email protected] is so low that different @[email protected] are not perceptible as different chemical species on the time scale of the experiment. The @[email protected] of rotation of groups about a bond due to the presence of a sufficiently large @[email protected] to make the phenomenon observable on the time scale of the experiment is termed hindered rotation or restricted rotation.
Source:
PAC, 1996, 68, 2193. (Basic terminology of stereochemistry (IUPAC Recommendations 1996)) on page 2209 [Terms] [Paper]