determining the pH of a solution (indicator paper,
mixed indicators) and in analytical chemistry in finding
the end point of an acid-base titration. Redox indica-
tors similarly have colors determined by the potential
of a solution. Complexometric indicators are used to
find the end points in titrations of metal ions by com-
plexing agents.
induced dipole moment Polar molecules have
bonds that have positive and negative ends, or poles,
and the molecules are often said to have a DIPOLE
MOMENT, a force of attraction between oppositely
charged particles. When an atom comes close to a
polar molecule, the electrons can shift to one side of the
nucleus to produce a very small dipole moment that
lasts for only an instant.
induction period The initial slow phase of a CHEMI-
CAL REACTIONthat later accelerates. Induction periods
are often observed with radical reactions, but they may
also occur in other systems (for example, before steady-
state concentration of the reactants is reached).
inductive effect In strict definition, an experimen-
tally observable effect (on rates of reaction, etc.) of the
transmission of charge through a chain of atoms by
electrostatic induction. A theoretical distinction may
be made between the FIELD EFFECTand the inductive
effect as models for the Coulomb interaction between
a given site within a MOLECULAR ENTITYand a remote
unipole or dipole within the same entity. The experi-
mental distinction between the two effects has proved
difficult, except for molecules of peculiar geometry,
which may exhibit “reversed field effects.” Ordinarily,
the inductive effect and the field effect are influenced
in the same direction by structural changes in the
molecule, and the distinction between them is not
clear. This situation has led many authors to include
the field effect in the term inductive effect.Thus the
separation of σ-values into inductive and RESONANCE
components does not imply the exclusive operation of
a through-bonds route for the transmission of the non-
conjugative part of the substituent effect. To indicate
the all-inclusive use of the term inductive,the phrase
“so-called inductive effect” is sometimes used. Certain
modern theoretical approaches suggest that the so-
called inductive effect reflects a field effect rather than
through-bonds transmission.
See alsoFIELD EFFECT; MESOMERIC EFFECT; POLAR
EFFECT.inductomeric effect A molecular polarizability effect
occurring by the inductive mechanism of electron dis-
placement. The consideration of such an effect and the
descriptive term have been regarded as obsolescent or
even obsolete, but in recent years theoretical
approaches have reintroduced substituent polarizability
as a factor governing reactivity, etc., and its parameteri-
zation has been proposed.inert STABLEand unreactive under specified conditions.
See alsoLABILE.inert s-pair effect A tendency of the outermost s
electrons to remain nonionized or unshared in com-
pounds, characteristic of the post-transition minerals.infrared radiation Long-wavelength radiation that
can be felt as heat but not seen.infrared spectroscopy Infrared (IR) spectroscopy is
a type of vibrational spectroscopy. It is the study of the
interaction of substances with infrared electromagnetic
radiation. IR spectroscopy can be used to determine the
concentration of the sample under study or to study the
spectral characteristics of the sample. The frequencies
of the vibrations excited by the infrared radiation are
characteristic of the functional groups in a molecule
and so provide information on molecular structure and
identification.inhibition The decrease in RATE OF REACTION
brought about by the addition of a substance
(inhibitor) by virtue of its effect on the concentration of
a reactant, CATALYST, or reaction INTERMEDIATE. For
example, molecular oxygen and p-benzoquinone can
react as inhibitors in many reactions involvinginhibition 139