isotonic solutions Solutions having identical
osmotic pressures, i.e., solutions where cells do not
swell or shrink.
isotope Different forms of a single element that have
the same number of protons but have different num-
bers of neutrons in their nuclei. Radioactive isotopes
are unstable and break down until they become stable.
Carbon-14 is a radioactive isotope of carbon that is
used to date fossilized organic matter.
isotope effect The effect on the rate or equilibrium
constant of two reactions that differ only in the iso-
topic composition of one or more of their otherwise
chemically identical components is referred to as a
kinetic isotope effect (seeISOTOPE EFFECT, KINETIC) or a
thermodynamic (or equilibrium) isotope effect (seeISO-
TOPE EFFECT, THERMODYNAMIC), respectively.
isotope effect, equilibrium See ISOTOPE EFFECT,
THERMODYNAMIC.
isotope effect, heavy atom An ISOTOPE EFFECTdue
to isotopes other than those of hydrogen.
isotope effect, intramolecular A kinetic ISOTOPE
EFFECTobserved when a single substrate, in which the
isotopic atoms occupy equivalent reactive positions,
reacts to produce a nonstatistical distribution of ISO-
TOPOLOGUEproducts. In such a case, the isotope effect
will favor the pathway with lower force constants for
displacement of the isotopic nuclei in the TRANSITION
STATE.
isotope effect, inverse A kinetic ISOTOPE EFFECT
wherein kl/kh < 1, i.e., the heavier substrate, reacts
more rapidly than the lighter one, as opposed to the
usual normal isotope effect, in which kl/kh> 1. The iso-
tope effect is typically “normal” when the frequency
differences between the isotopic TRANSITION STATEs are
smaller than in the reactants. Conversely, an inverse
isotope effect can be taken as evidence of an increase in
the corresponding force constants upon passing from
the reactant to the transition state.isotope effect, kinetic The effect of isotopic substi-
tution on a rate constant is referred to as a kinetic iso-
tope effect.
For example in the reaction
A + B →C
the effect of isotopic substitution in reactant A is
expressed as the ratio of rate constants kl/kh, where the
superscripts l and h represent reactions in which the
molecules A contain the light and heavy isotopes,
respectively.
Within the framework of TRANSITION STATEtheory,
in which the reaction is rewritten asA + B [TS]‡→Cand with neglect of isotopic mass on TUNNELINGand
the TRANSMISSION COEFFICIENT, kl/khcan be regarded
as if it were the equilibrium constant for an isotope
exchange reaction between the transition state [TS‡]
and the isotopically substituted reactant A, and can be
calculated from their vibrational frequencies, as in the
case of a thermodynamic isotope effect.
Isotope effects like the above, involving a direct or
indirect comparison of the rates of reaction of ISO-
TOPOLOGUEs, are called intermolecular, in contrast to
intramolecular isotope effects, in which a single sub-
strate reacts to produce a nonstatistical distribution of
isotopologue product molecules.
See alsoISOTOPE EFFECT, INTRAMOLECULAR.isotope effect, primary A kinetic isotope effect
attributable to isotopic substitution of an atom to
which a bond is made or broken in the RATE-CONTROL-
LING STEPor in a PRE-EQUILIBRIUMstep of a specified
reaction. The corresponding isotope effect on the equi-
librium constant of a reaction in which one or more
bonds to isotopic atoms are broken is called a primary
equilibrium isotope effect.
See alsoISOTOPE EFFECT, SECONDARY.isotope effect, secondary A kinetic isotope effect
that is attributable to isotopic substitution of an atom148 isotonic solutions