racemic Pertaining to a racemate, an equimolar mix-
ture of a pair of ENANTIOMERs. It does not exhibit opti-
cal activity.
radiation Energy released that travels through space
or substance as particles or electromagnetic waves and
includes visible and ultraviolet light, heat, X-rays, cos-
mic rays. Radiation can be nonionizing, such as infrared,
visible light, ultraviolet, electromagnetic, or it can be
ionizing, such as alpha, beta, gamma, and X-rays. Ioniz-
ing radiation can have severe effects on human health
but is used in medical diagnostic equipment. Radiation
has a host of other economic benefits ranging from elec-
trical power generation to smoke detectors.
radical(free radical) A MOLECULAR ENTITYsuch as
.CH 3 , .SnH 3 , Cl. possessing an unpaired electron. (In
these formulae the dot, symbolizing the unpaired elec-
tron, should be placed so as to indicate the atom of high-
est spin density, if this is possible.) PARAMAGNETICmetal
ions are not normally regarded as radicals. However, in
the “isolobal analogy,” the similarity between certain
paramagnetic metal ions and radicals becomes apparent.
At least in the context of physical organic chem-
istry, it seems desirable to cease using the adjective free
in the general name of this type of CHEMICAL SPECIES
and MOLECULAR ENTITY, so that the term free radical
may in future be restricted to those radicals that do not
form parts of radical pairs.
Depending upon the core atom that possesses the
unpaired electron, the radicals can be described as car-
bon-, oxygen-, nitrogen-, metal-centered radicals. If the
unpaired electron occupies an orbital having consider-
able sor more or less pure pcharacter, the respective
radicals are termed sigma or pi radicals.
In the past, the term radicalwas used to designate
a SUBSTITUENTgroup bound to a MOLECULAR ENTITY,
as opposed to “free radical,” which currently is
called “radical.” The bound entities may be called
GROUPs or substituents, but should no longer be called
radicals.
See alsoBIRADICAL.radical anion SeeRADICAL ION.radical center(s) The atom (or group of atoms) in a
polyatomic radical on which an unpaired electron is
largely localized. Attachment of a monovalent atom to
a radical center gives a molecule for which it is possible
to write a LEWIS FORMULAin which the normal stable
valencies are assigned to all atoms.radical combination SeeCOLLIGATION.radical ion A RADICALthat carries an electric charge.
A positively charged radical is called a “radical cation”227