Pseudorotation
{{short description|Movement of molecular ligands into a conformation indistinguishable from the original}}
In chemistry, a pseudorotation is a set of intramolecular movements of attached groups (i.e., ligands) on a highly symmetric molecule, leading to a molecule indistinguishable from the initial one. The International Union of Pure and Applied Chemistry (IUPAC) defines a pseudorotation as a "stereoisomerization resulting in a structure that appears to have been produced by rotation of the entire initial molecule", the result of which is a "product" that is "superposable on the initial one, unless different positions are distinguished by substitution, including isotopic substitution."{{cite book |doi=10.1351/goldbook.P04934 |chapter=Pseudorotation |title=The IUPAC Compendium of Chemical Terminology |year=2014 }}
Well-known examples are the intramolecular isomerization of trigonal bipyramidal compounds by the Berry pseudorotation mechanism, and the out-of-plane motions of carbon atoms exhibited by cyclopentane, leading to the interconversions it experiences between its many possible conformers (envelope, twist).{{cite journal |doi=10.1007/s40828-015-0014-0 |url=https://link.springer.com/content/pdf/10.1007/s40828-015-0014-0.pdf |title=Conformational analysis of cycloalkanes |year=2015 |last1=Dragojlovic |first1=Veljko |journal=Chemtexts |volume=1 |issue=3 |pages=14–15 |bibcode=2015ChTxt...1...14D |s2cid=94348487 }} Note, no angular momentum is generated by this motion.{{citation needed|date=May 2014}} In these and related examples, a small displacement of the atomic positions leads to a loss of symmetry until the symmetric product re-forms (see image example below), where these displacements are typically along low-energy pathways.{{citation needed|date=May 2014}} The Berry mechanism refers to the facile interconversion of axial and equatorial ligand in {{chem2|MX5}} types of compounds, e.g. D3h-symmetric {{chem2|PF5}} (shown).{{cite journal |doi=10.1021/ja105306s |title=Stereomutation of Pentavalent Compounds: Validating the Berry Pseudorotation, Redressing Ugi's Turnstile Rotation, and Revealing the Two- and Three-Arm Turnstiles |year=2010 |last1=Couzijn |first1=Erik P. A. |last2=Slootweg |first2=J. Chris |last3=Ehlers |first3=Andreas W. |last4=Lammertsma |first4=Koop |journal=Journal of the American Chemical Society |volume=132 |issue=51 |pages=18127–18140 |pmid=21082808 |bibcode=2010JAChS.13218127C }} Finally, in a formal sense, the term pseudorotation is intended to refer exclusively to dynamics in symmetrical molecules, though mechanisms of the same type are invoked for lower symmetry molecules as well.{{citation needed|date=May 2014}}
