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called intensity and given the symbol I. See also isotopically substituted Describing a com-
photon irradiance. pound that has a composition such that essen-
tially all the molecules of the compound have
irradiation Exposure to ionizing radiation. only the indicated nuclide(s) at each designated
position. For all other positions, the absence of
isomer One of several species (or molecular nuclide indication means that the nuclide com-
entities) that have the same atomic composition position is the natural one.
(molecular formula) but different line formulae
or different stereochemical formulae and hence isotopically unmodified Describing a com-
different physical and/or chemical properties. pound that has a macroscopic composition such
that its constituent nuclides are present in the pro-
isomorphic Two sets are isomorphic if there portions occurring in nature.
is an isomorphism between them. Two graphs,

G and G, are isomorphic if their nodes can be
isotopologue A molecular entity that differs
labeled with the numbers 1, 2,...,p such that
only in isotopic composition (number of isotopic
whenever v is adjacent to v in G, ˙v and ˙v are substitutions), e.g., CH CH D, CH D .
i
j
i
j
4
3
2
2

adjacent in G ,1 ≤ i, j ≤ p, i = j.
isotopomer Isomers having the same num-
isomorphism (1)A bijection between two
ber of each isotopic atom but differing in their
sets which preserves all structure shared by the
positions. The term is a contraction of “iso-
sets (e.g., group structure).
topic isomer.” Isotopomers can be either con-
(2)A morphism between objects of a category
stitutional isomers (e.g., CH DCH = 0 and
2
which is both surjective and injective.
CH CD = 0) or isotopic stereoisomers (e.g.,
3
(R)- and (S) − CH CHDOH or (Z)- and
3
isotope effect The effect on the rate or equi-
(E) − CH CH = CHD).
librium constant of two reactions that differ only 3
in the isotopic composition of one or more of
isotropic (sub)manifold (in a symplectic mani-
their otherwise chemically identical components
fold [P, ω]) A submanifold M ⊂ P of a
is referred to as a kinetic isotope effect or a
symplecticmanifold(P, ω)suchthatatanypoint
thermodynamic (or equilibrium) isotope effect,
p ∈ M the tangent space contains its symplectic
respectively.
polar, i.e.,
o
(T M) ⊂ T M.
isotopically labeled Describing a mixture p p
ofanisotopicallyunmodifiedcompoundwithone The dimension of an isotropic manifold M is at
or more analogous isotopically substituted com- most half of the dimension of P .
pound(s).
isotropy group Let ) : G × M → M be a
isotopically modified Describing a com-
smooth action of the Lie group G on the manifold
pound that has a macroscopic composition such
M. For each x ∈ M the group
that the isotopic ratio of nuclides for at least one
element deviates measurably from that occurring G ={g ∈ G | )(g, x) = x}
x
in nature. It is either an isotopically substituted
compound or an isotopically labeled compound. is called the isotropy group of ) at x.

© 2003 by CRC Press LLC
© 2003 by CRC Press LLC