frequencies. The frequencies of most
interest range from 2.5 to 16 μm;
however, in IR spectroscopy it is
common to use the reciprocal of the
wavelength, and thus this range be-
comes 4000–625 cm–1. Examples of
typical vibrations are centred on
2900 cm–1for C–H stretching in alka-
nes, 1600 cm–1for N–H stretching in
amino groups, and 2200 cm–1for
C≡C stretching in alkynes. In an IR
spectrometer there is a source of IR
light, covering the whole frequency
range of the instrument, which is
split into two beams of equal inten-
sity. One beam is passed through the
sample and the other is used as a ref-
erence against which theÜrst is then
compared. The spectrum is usually
obtained as a chart showing absorp-
tion peaks, plotted against wave-
length or frequency. The sample can
be a gas, liquid, or solid. See also
fourier-transform infrared.
Ingold, Sir Christopher Kelk
(1893–1970) British organic chemist
who worked chieÛy in London. His
main work was on reaction mecha-
nisms and, particularly, on *elec-
tronic effects in physical organic
chemistry.
inhibition A reduction in the rate
of a catalysed reaction by substances
called inhibitors. Inhibitors may work
by poisoning catalysts for the reac-
tion or by removing free radicals in a
chain reaction. Enzyme inhibition af-
fects biochemical reactions, in which
the catalysts are enzymes. Competi-
tive inhibition occurs when the in-
hibitor molecules resemble the
substrate molecules and bind to the
active site of the enzyme, so pre-
venting normal enzymatic activ-
ity. Competitive inhibition can
be reversed by increasing the con-
centration of the substrate. In non-
competitive inhibition the inhibitor
binds to a part of the enzyme or *en-
zyme–substrate complex other than
the active site, known as an allosteric
site. This deforms the active site so
that the enzyme cannot catalyse the
reaction. Noncompetitive inhibition
cannot be reversed by increasing the
concentration of the substrate. The
toxic effects of many substances are
produced in this way. Inhibition by
reaction products (feedback inhibi-
tion) is important in the control of
enzyme activity. See also allosteric
enzyme.innerDescribing a chemical com-
pound formed by reaction of one
part of a molecule with another part
of the same molecule. Thus, a lactam
is an inner amide; a lactone is an
inner ester.inner-sphere mechanism See
electron-transfer reaction.inner transition seriesSee tran-
sition elements.inorganic chemistryThe branch
of chemistry concerned with com-
pounds of elements other than
carbon. Certain simple carbon
compounds, such as CO, CO 2 , CS 2 ,
and carbonates and cyanides, are
usually treated in inorganic chem-
istry.
insecticideSee pesticide.insertion reaction A type of
chemical reaction in which an atom
or group is inserted between two
atoms in a molecule. A common ex-
ample is the insertion of carbene:
R 3 C–X + CH 2 : →R 3 CCH 2 X
The opposite of an insertion reaction
is an extrusion reaction.insulinA protein hormone, se-
creted by the βcells of the islets of
Langerhans in the pancreas, that pro-
motes the uptake of glucose by body
cells, particularly in the liver and
muscles, and thereby controls its285 insulin
i