chemistry Chemistry is the scientific study of the properties and behavior of matter. It is a physical science within the natural sciences that studies the chemical elements that make up matter and chemical compound, compounds made of atoms, molecules a ...

, a pseudorotation is a set of intramolecular movements of attached groups (i.e.,

ligand In coordination chemistry, a ligand is an ion or molecule with a functional group that binds to a central metal atom to form a coordination complex. The bonding with the metal generally involves formal donation of one or more of the ligand's el ...

s) on a highly symmetric molecule, leading to a molecule indistinguishable from the initial one. The International Union of Pure and Applied Chemistry (

IUPAC The International Union of Pure and Applied Chemistry (IUPAC ) is an international federation of National Adhering Organizations working for the advancement of the chemical sciences, especially by developing nomenclature and terminology. It is ...

) defines a pseudorotation as a " stereoisomerization resulting in a

structure A structure is an arrangement and organization of interrelated elements in a material object or system, or the object or system so organized. Material structures include man-made objects such as buildings and machines and natural objects such as ...

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." Well-known examples are the intramolecular

isomerization In chemistry, isomerization or isomerisation is the process in which a molecule, polyatomic ion or molecular fragment is transformed into an isomer with a different chemical structure. Enolization is an example of isomerization, as is tautomer ...

of trigonal bipyramidal compounds by the Berry pseudorotation mechanism, and the out-of-plane motions of carbon atoms exhibited by

cyclopentane Cyclopentane (also called C pentane) is a highly flammable alicyclic compound, alicyclic hydrocarbon with chemical formula C5H10, C5H10 and CAS number 287-92-3, consisting of a ring of five carbon atoms each bonded with two hydrogen atoms above and ...

, leading to the interconversions it experiences between its many possible

conformers In chemistry, rotamers are chemical species that differ from one another primarily due to rotations about one or more single bonds. Various arrangements of atoms in a molecule that differ by rotation about single bonds can also be referred to as ...

(envelope, twist). Note, no

angular momentum Angular momentum (sometimes called moment of momentum or rotational momentum) is the rotational analog of Momentum, linear momentum. It is an important physical quantity because it is a Conservation law, conserved quantity – the total ang ...

is generated by this motion. 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. The

Berry mechanism The Berry mechanism, or Berry pseudorotation mechanism, is a type of vibration causing molecules of certain geometries to isomerization, isomerize by exchanging the two axial ligands (see the figure) for two of the equatorial ones. It is the most ...

refers to the facile interconversion of axial and equatorial

in types of compounds, e.g. D_3h-symmetric (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.

References

Chemical bonding Stereochemistry {{Stereochemistry-stub

See also

References