Pseudorotation

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."[1]

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).[2] Note, no angular momentum is generated by this motion.[citation needed] 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] The Berry mechanism refers to the facile interconversion of axial and equatorial ligand in MX5 types of compounds, e.g. D3h-symmetric PF5 (shown).[1][3] 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]

Pseudorotation of Iron-Pentacarbonyl-Complex (Berry mechanism)
Pseudorotation of Iron-Pentacarbonyl-Complex (Berry mechanism)
  1. ^ a b "Pseudorotation". The IUPAC Compendium of Chemical Terminology. 2014. doi:10.1351/goldbook.P04934.
  2. ^ Dragojlovic, Veljko (2015). "Conformational analysis of cycloalkanes" (PDF). Chemtexts. 1 (3): 14–15. doi:10.1007/s40828-015-0014-0. S2CID 94348487.
  3. ^ Couzijn, Erik P. A.; Slootweg, J. Chris; Ehlers, Andreas W.; Lammertsma, Koop (2010). "Stereomutation of Pentavalent Compounds: Validating the Berry Pseudorotation, Redressing Ugi's Turnstile Rotation, and Revealing the Two- and Three-Arm Turnstiles". Journal of the American Chemical Society. 132 (51): 18127–18140. doi:10.1021/ja105306s. PMID 21082808.