infrared spectrometry helps chemists to
identify the functional groups present
in a compound they can therefore help in
finding the structure of a compound a
pair of atoms joined by a covalent bond
can be thought of as being like balls on
the end of a vibrating spring the bond
can vibrate with different amounts of
energy at a frequency that depends on
the masses of the atoms and the strength
of the bond light atoms give higher
frequencies heavier atoms move more
slowly and so give lower frequencies
stronger bonds give higher frequencies
and weaker bonds lower ones at room
temperature most bonds will vibrate with
the lowest possible amount of energy but
if radiation of the right frequency is
supplied the bond can absorb energy and
vibrate with greater amplitude most
bonds absorb energy in the infrared
region of the electromagnetic spectrum
which corresponds to heat each bond such
as Oh H C double bond o NH and so on
absorbs at a particular frequency and
this allows it to be identified
for historical reasons and because the
numbers are easy to work with chemists
dealing with IR spectra use units called
wave numbers measured in centimeters to
the minus 1 which are proportional to
frequency all IR instruments have a
source of IR radiation this is a coil of
wire surrounded by a ceramic capsule
which is heated electrically so that it
gives out IR radiation in other words
heat over a whole range of frequencies
the IR radiation goes via a series of
mirrors into the sample which is placed
here in an appropriate holder the
radiation not absorbed by the sample
arrives at a detector situated here
modern instruments such as this one use
a device called an interferometer
consisting of a beam splitter here and a
pair of mirrors at right angles to one
another this produces what is called an
interferer gram from the source
radiation the interferogram holds
information about the intensity of all
infrared radiation at all frequencies
simultaneously this passes through the
sample and to the detector the
interferogram that arrives at the
detector can be decoded by a
mathematical technique called a Fourier
transformation this gives the intensity
of the infrared radiation at each
frequency separately the transformation
is handled by the computer and produces
a graph of percentage transmission
against wave number this is what the
chemist interprets the essential point
is that pulses of infrared radiation
consisting of a range of frequencies
simultaneously a pass through the sample
and arrive at the detector some
frequencies are absorbed more than
others depending on what bonds are
present in the sample
this is the sample holder the sample is
placed here on a crystal which is made
of diamond or germanium the IR beam is
directed into the sample by a mirror it
is reflected back from the upper surface
of the sample before being guided into
the detector by a second mirror this
method is called attenuated total
reflection or ATR here we will run the
IR spectrum of solid benzoic acid the
instrument is switched on and a blank is
run with no sample in place this is to
find the absorption of the air which
must be subtracted from that of the
sample a little benzoic acid is placed
on the ATR crystal this is a holder for
solid samples less than a milligram of
solid sample is required the torque
wrench is used to squash the sample to
ensure a good contact with the surface
of the crystal for most samples no
preparation is required details of the
scan are entered into the computer the
spectrum of the sample is obtained
within a few seconds the wave numbers of
the more significant Peaks can be
labeled onto the spectrum this helps the
chemists to interpret the data this peak
is caused by a stretching vibration of
the C double bond o and this by an O H
stretch these two peaks together suggest
that the sample may have a carboxylic
acid functional group which has both C
double bond o and O h bonds an unknown
sample may be identified by matching its
IR spectrum with a database of spectra
of known compounds particularly in the
region of the spectrum below about 1500
centimeters to the minus one the peaks
in this area are due to complex
vibrations of the whole molecule and the
area is referred to as the fingerprint
region because it is unique
a particular compound
for a liquid sample the procedure is the
same but a few drops of liquid are
placed in this type of sample holder the
cover is to prevent the evaporation of
volatile liquid samples