In a stereochemical context the rotation about a bond is called 'free
' when the rotational barrier
is so low that different conformations
are not perceptible as different chemical species on the time scale of the experiment.
of rotation of groups about a bond due to the presence of a sufficiently large rotational barrier
to make the phenomenon observable on the time scale of the experiment is termed hindered
rotation or restricted rotation.
PAC, 1996, 68, 2193
(Basic terminology of stereochemistry (IUPAC Recommendations 1996))
on page 2209
IUPAC. Compendium of Chemical Terminology, 2nd ed. (the "Gold Book"). Compiled by
A. D. McNaught and A. Wilkinson. Blackwell Scientific Publications, Oxford (1997).
XML on-line corrected version: http://goldbook.iupac.org (2006-) created by M. Nic,
J. Jirat, B. Kosata; updates compiled by A. Jenkins. ISBN 0-9678550-9-8. https://doi.org/10.1351/goldbook