free rotation (hindered rotation, restricted rotation)

https://doi.org/10.1351/goldbook.F02520
In a stereochemical context the rotation about a bond is called 'free' when the @R05408@ is so low that different @C01258@ are not perceptible as different chemical species on the time scale of the experiment. The @I03034@ of rotation of groups about a bond due to the presence of a sufficiently large @R05408@ 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]