Preferred Label : orbital symmetry;
IUPAC definition : The behaviour of an atomic or localized molecular orbital under molecular symmetry
operations characterizes its orbital symmetry. For example, under a reflection in
an appropriate symmetry plane, the phase of the orbital may be unchanged (symmetric),
or it may change sign (antisymmetric), i.e. the positive and negative lobes are interchanged.
A principal context for the use of orbital symmetry is the discussion of chemical
changes that involve 'conservation of orbital symmetry'. If a certain symmetry element
(e.g. the reflection plane) is retained along a reaction pathway, that pathway is
'allowed' by orbital symmetry conservation if each of the occupied orbitals of the
reactant(s) is of the same symmetry type as a similarly (e.g. singly or doubly) occupied
orbital of the product(s). This principle permits the qualitative construction of
correlation diagrams to show how molecular orbitals transform (and how their energies
change) during idealized chemical changes (e.g. cycloadditions ). An idealized single
bond is a σ-bond, i.e. it has cylindrical symmetry, whereas a p-orbital or π-bond
orbital has π-symmetry, i.e. it is antisymmetric with respect to reflection in a plane
passing through the atomic centres with which it is associated. In ethene, the π-bonding
orbital is symmetric with respect to reflection in a plane perpendicular to and bisecting
the C–C bond, whereas the π*-antibonding orbital is antisymmetric with respect to
this operation. Considerations of orbital symmetry are frequently grossly simplified
in that, for example, the p-orbitals of a carbonyl group would be treated as having
the same symmetry as those of ethene, and the fact that the carbonyl group in, for
example, camphor, unlike that in formaldehyde, has no mirror planes would be ignored.
These simplified considerations nevertheless afford the basis of one approach to the
understanding of the rules which indicate whether pericyclic reactions are likely
to occur under thermal or photochemical conditions.;
Origin ID : O04320;
See also
The behaviour of an atomic or localized molecular orbital under molecular symmetry
operations characterizes its orbital symmetry. For example, under a reflection in
an appropriate symmetry plane, the phase of the orbital may be unchanged (symmetric),
or it may change sign (antisymmetric), i.e. the positive and negative lobes are interchanged.
A principal context for the use of orbital symmetry is the discussion of chemical
changes that involve 'conservation of orbital symmetry'. If a certain symmetry element
(e.g. the reflection plane) is retained along a reaction pathway, that pathway is
'allowed' by orbital symmetry conservation if each of the occupied orbitals of the
reactant(s) is of the same symmetry type as a similarly (e.g. singly or doubly) occupied
orbital of the product(s). This principle permits the qualitative construction of
correlation diagrams to show how molecular orbitals transform (and how their energies
change) during idealized chemical changes (e.g. cycloadditions ). An idealized single
bond is a σ-bond, i.e. it has cylindrical symmetry, whereas a p-orbital or π-bond
orbital has π-symmetry, i.e. it is antisymmetric with respect to reflection in a plane
passing through the atomic centres with which it is associated. In ethene, the π-bonding
orbital is symmetric with respect to reflection in a plane perpendicular to and bisecting
the C–C bond, whereas the π*-antibonding orbital is antisymmetric with respect to
this operation. Considerations of orbital symmetry are frequently grossly simplified
in that, for example, the p-orbitals of a carbonyl group would be treated as having
the same symmetry as those of ethene, and the fact that the carbonyl group in, for
example, camphor, unlike that in formaldehyde, has no mirror planes would be ignored.
These simplified considerations nevertheless afford the basis of one approach to the
understanding of the rules which indicate whether pericyclic reactions are likely
to occur under thermal or photochemical conditions.