optical isomers(enantiomers) Are nonsuperimpos-
able mirror images of each other and are said to be chi-
ral (not superimposable on their mirror image).
See alsoCHIRALITY.
optically detected magnetic resonance (ODMR)
A double resonance technique in which transitions
between spin sublevels are detected by optical means.
Usually these are sublevels of a triplet, and the transi-
tions are induced by microwaves.
optical yield In a CHEMICAL REACTIONinvolving chi-
ral reactants and products, the term optical yieldrefers
to the ratio of the optical purity of the product to that
of the precursor, reactant, or catalyst. This should not
be confused with “enantiomeric excess.” The optical
yield is in no way related to the chemical yield of the
reaction.
See alsoCHIRALITY; STEREOSELECTIVITY.
orbital SeeATOMIC ORBITAL; MOLECULAR ORBITAL.orbital steering A concept expressing that the stereo-
chemistry of approach of two reacting species is gov-
erned by the most favorable overlap of their
appropriate ORBITALs.
See alsoSTEREOCHEMICAL.orbital symmetry The behavior of an atomic or
localized MOLECULAR ORBITALunder molecular symme-
try operations characterizes its orbital symmetry. For
example, under a reflection in an appropriate symmetry
plane, the phase of the orbital may be unchanged (sym-
metric), 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 con-
servation 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 SIGMA BOND—one that
has cylindrical symmetry. In contrast, a p-orbital or PI-
BONDorbital has pi symmetry—one that is antisymmet-
ric with respect to reflection in a plane passing through
the atomic centers with which it is associated. In ethene,
the pi-bonding orbital is symmetric with respect to
reflection in a plane perpendicular to and bisecting the
C–C bond, whereas the pi-star-antibonding orbital is
antisymmetric with respect to this operation.
Considerations of orbital symmetry are frequently
grossly simplified in that, for example, the pi 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 that indicate whether PERICYCLICorbital symmetry 201Optical isomerism occurs when a compound has no plane of sym-
metry and can exist in either left- or right-handed forms that are
mirror images of each other.