Stereoisomerization resulting in a structure that appears to have been produced by
rotation of the entire initial molecule and is
superposable
on the initial one, unless different positions are distinguished by substitution,
including isotopic substitution. One example of pseudorotation is a facile interconversion
between the many envelope and twist conformers of a cyclopentane due to the out of
plane motion of carbon atoms. Another example of pseudorotation (Berry pseudorotation)
is a
polytopal rearrangement that provides an
intramolecular mechanism for the
isomerization of trigonal bipyramidal compounds
(e.g.
-
phosphanes), the five bonds to the
central atom E being represented as
,
,
,
and
.
Two
equatorial bonds move apart and become
apical bonds at the same time as the apical bonds move together to become
equatorial.
A related conformational change of a trigonal bipyramidal structure is described as
turnstile rotation. The process may be visualized as follows.
An apical and an
equatorial bond rotate as a pair ca. 120° relative to the other three bonds. (Doubts have been
expressed about the distinct physical reality of this mechanism.)
Source:
PAC, 1996, 68, 2193
(Basic terminology of stereochemistry (IUPAC Recommendations 1996))
on page 2215
