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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 "[s]tereoisomerization 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" and there, unless different positions are distinguished by… isotopic labeling."[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). 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). 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)


  1. ^ Here and following, see IUPAC, 1997, "Compendium of Chemical Terminology," 2nd ed. (AD McNaught & A Wilkinson, compilers; the "Gold Book"), Oxford, UK: Blackwell Scientific Publications, ISBN 0-9678550-9-8, DOI:10.1351/goldbook, see [1]; online XML version (2006-) created by M Nic, J Jirat & B Kosata with updates by A Jenkins, accessed 28 May 2014.

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