INFRARED SPECTROSCOPY
Infrared rays is the region between red rays in the visible rays and
microwave spectrum in electromagnetic radiation.
The energy of infrared rays less than red rays but more than
microwaves. The frequency is vice versa.
Infrared is thermal radiation and could be emitted from the lamp, or
any heating body as well as emitted from the earth, and the sun or
from the human body, animals and plants.
Infrared cannot be seen with the naked eye, but can imaging in total
darkness, because it depends on the thermal radiation emitted from
objects.
Infrared application:
1. Doctors use infrared to treat skin diseases, and pain relief, which
may affect the muscles, where they are shed infrared on the
patient's body, as implemented through the skin and works to a
certain degree warmed to stimulate circulation.
2. Infrared ovens used in some special paint dry surfaces such as
leather, metal, and paper, and fabrics.
3. Some photographers use films sensitive to infrared imaging in
the dark using infrared spectrum.
The infrared portion of the electromagnetic spectrum is usually divided
into three regions, named for their relation to the visible spectrum.
1. Near- infrared
The higher-energy near-IR, approximately 14000–4000 cm−1 (0.8–
2.5 μm wavelength) can excite overtone or harmonic vibrations.
2. Mid- infrared
The mid-infrared, approximately 4000–400 cm−1 (2.5–25 μm)
may be used to study the fundamental vibrations and
associated rotational-vibrational structure.
3. Far- infrared
The far-infrared, approximately 400–10 cm−1 (25–1000 μm), lying
adjacent to the microwave region, has low energy and may be
used for rotational spectroscopy.
The names and classifications of these sub regions are conventions,
and are only loosely based on the relative molecular or
electromagnetic properties.
The Mid IR region is the most commonly used devices in the spectral
analysis of the infrared.
Infrared spectroscopy (IR spectroscopy) is the spectroscopy that deals
with the infrared region of the electromagnetic spectrum, that is light
with a longer wavelength and lower frequency than visible light. It
covers
a
range
of
techniques,
mostly
based
on absorption
spectroscopy. As with all spectroscopic techniques, it can be used to
identify and study chemicals. A common laboratory instrument that
uses this technique is a Fourier transform infrared (FTIR) spectrometer.
Theory
Sample of an IR spec. reading; this one is from BROMOMETHANE,
showing peaks around 3000, 1300 and 1000(on the x-axis).
Infrared spectroscopy exploits the fact that molecules absorb specific
frequencies that are characteristic of their structure. These absorptions
are resonant frequencies, i.e. the frequency of the absorbed radiation
matches the transition energy of the bond or group that vibrates. The
energies are determined by the shape of the molecular potential
energy surfaces, the masses of the atoms, and the associated vibronic
coupling.
IR absorption and the number of vibrational modes
A molecule can vibrate in many ways, and each way is called
a vibrational mode.
For molecules with N atoms in them, linear molecules have 3N – 5
degrees of vibrational modes, whereas nonlinear molecules have 3N –
6 degrees of vibrational modes (also called vibrational degrees of
freedom).
As an example H2O, a non-linear molecule, will have 3 × 3 – 6 = 3
degrees of vibrational freedom, or modes.
Simple diatomic molecules have only one bond and only one
vibrational band. If the molecule is symmetrical, e.g. N2, the band is
not observed in the IR spectrum, but only in the Raman spectrum.
Asymmetrical diatomic molecules, e.g. CO, absorb in the IR spectrum.
More complex molecules have many bonds, and their vibrational
spectra are correspondingly more complex, i.e. big molecules have
many peaks in their IR spectra.
The
atoms
in
a
CH2X2 group,
commonly
found
in organic
compounds and where X can represent any other atom, can vibrate in
nine different ways.
Six of these involve only the CH2 portion: symmetric and antisymmetric
stretching, scissoring, rocking, wagging and twisting,
as
shown below. (Note, that because CH2 is attached to X2 it has 6 modes,
unlike H2O, which only has 3 modes. The rocking, wagging, and twisting
modes do not exist for H2O, since they are rigid body translations and
no relative displacements exist.)
Symmetrical
stretching
Anti-symmetrical
stretching
Scissoring
Rocking
Wagging
Twisting
Practical IR spectroscopy
The infrared spectrum of a sample is recorded by passing a beam of
infrared light through the sample. When the frequency of the IR is the
same as the vibrational frequency of a bond, absorption occurs.
Examination of the transmitted light reveals how much energy was
absorbed at each frequency (or wavelength). This can be achieved by
scanning the wavelength range using a monochromator.
Vibrational energy levels in water molecule:
Water molecule is non-linear and contains 3 atoms, and therefore the
number of vibrational changes is 3 according to the previous law.
3N-6 = (3 X 3) -6 = 3
 Symmetrical stretching s OH (3652 cm-1)
 Asymmetrical stretching As OH (3756 cm-1)
 Bending scissoring s HOH (1596 cm-1)
Benzoic acid
Acetone
Absorption bands
IR spectroscopy is often used to identify structures because functional
groups give rise to characteristic bands both in terms of intensity and
position (frequency). The positions of these bands is summarized in
correlation tables as shown below.
Wavenumbers listed in cm−1.
IR spectrometer
Source of IR radiation
1. Nernest glower
2. Globar
3. Incandescent wire
4. High pressure mercury arc lamp
‫السلك المتوهج‬
‫لمبة نرنست المتوهجة‬
‫القضيب المتوهج‬
‫لمبة الزئبق القوسية ذات الضغط العالي‬
MONOCHROMATORS:
Grating: The most modern equipment use GRATING to separate
different wavelengths of infrared rays after passing through the
sample.
One disadvantage in GRATING is the increase of the amount of
radiation scattered. To overcome this problem PRISM or FILTER is used
with GRATING at the same time.
PRISM is made from flint glass to be IR transparent.
Sample cell
Samples can be used as liquid or solid or gaseous. The thickness of the
sample should be too small, therefore minute metal cells fitted with
two windows are used to permit the passage of radiation through the
sample. The material used of windows should not absorb IR in the
measurement area.
Windows material
The wavelength of the rays that pass without absorption
NaCl
40,000 - 625 cm-1
KBr
40,000 - 400 cm-1
AgCl
25,000 - 435 cm-1
Cesium bromide
10,000 - 270 cm-1
Cesium iodide
10,000 - 200 cm-1
Germanium
20,000 - 600 cm-1
Polyethylene
625 - 33 cm-1
Windows must be cleaned by organic solvents only and keep them
away from water because they melt in water. In case of aqueous
samples, AgCl windows could be used.
Gas samples: Cylindrical cells made of Pyrex glass with length 10 cm
are used. The windows of cells are made of sodium chloride or calcium
fluoride or potassium bromide
Liquid samples: Placed a thin film of pure neat sample of about 0.01
mm (1-10 mg sample) between two disks of material salts of sodium
chloride, or calcium fluoride, or potassium bromide. In case of very
small samples, ultra micro cavity cells fitted with beam condenser are
used.
Solid samples:
Sample (2 – 5 mg)
is crushed in carborundum
container, attached in a liquid high molecular weight (mulling oil
named Nujol), and then placed them thin film called mulls.
Solid sample can also prepared in a form of a compact disc. 1 mg
sample is
mixed carefully and homogeneously with approximately
with 100 mg of potassium bromide by dry ball mill, and then
compressing the mixture under a pressure of up to 20,000 -50,000 lb /
in2
Stuff used in the processing of samples for IR spectral
analysis:
Metal blocks
KBr Die sets for KBr Discs
Laboratory hydrolyic press product
Detector
1. Thermocouple detector
2. Bolometer detector
3. Golay cell detector
IR gas sampling supplies cells
Thermocouple detector
Bolometer
Golay Cell
Recorder
Recording unit is used to record the absorption either at different
wavelengths wavelength (nm), or different wave numbers (cm-1), and
thus can be recorded absorption spectrum in the long-desired
In general, IR spectrum of organic compounds can be divided into two
parts:
High frequency portion: absorption of function groups appeared in this
region 1300-3600 cm-1
Low frequency portion: absorption of aromatic groups appeared in this
region 650-909 cm-1
More distinct division into four areas, namely:
1- Region between 2700 – 3600 cm-1
A special area for stretching between hydrogen atom and other high
atomic weight atoms such as oxygen or nitrogen or carbon, O-H, N-H,
C-H
2- Region between 1850 – 2700 cm-1
A special area for triple bonds CC , CN
3- Region between 1500 – 1850 cm-1
A special area for double bonds C=N, C=O, C=C
4- Region between 700 – 1500 cm-1
A special area for finger print. This region for single bonds between
carbon atoms and atoms other than hydrogen, such as C-C, C-O, C-Cl
and others, any links that are the basic structure of the molecule.
Compound
Group
IR Absorption
νmax cm-1
C=C
Alkene
1690 – 1600
CC
Alkyne
3300
C=O
Ketone
1700 – 1750
-COH
Aldehyde
1700 – 1750
-COOH
Carboxyl
3520
-NH2
Amido
3400 – 3180
-NO2
Nitro
1850 – 1555
-CN
Nitrile
2250 – 2225
-S=O
Sulfoxide
2600 – 2550
O=S=O
Sulfone
2600 – 2550
R-OH
Alcohol –OH
3650 – 3584
Ar-OH
Phenol –OH
3650 – 3584
Phenyl
Aromatic structure
909 – 650
INTERPRETING AN INFRA-RED SPECTRUM
This explains how to use an infra-red spectrum to identify
the presence of a few simple bonds in organic compounds.
Ethanoic acid
Ethanoic acid has the structure:
You have seen that it contains the following bonds:
carbon-oxygen double, C=O
carbon-oxygen single, C-O
oxygen-hydrogen, O-H
carbon-hydrogen, C-H
carbon-carbon single, C-C
The carbon-carbon bond has absorptions which occur over
a wide range of wavenumbers in the fingerprint region that makes it very difficult to pick out on an infra-red
spectrum.
The carbon-oxygen single bond also has an absorbtion in
the fingerprint region, varying between 1000 and 1300cm-1
depending on the molecule it is in. You have to be very
wary about picking out a particular trough as being due to a
C-O bond.
The other bonds in ethanoic acid have easily recognised
absorptions outside the fingerprint region.
The C-H bond (where the hydrogen is attached to a carbon
which is singly-bonded to everything else) absorbs
somewhere in the range from 2853 - 2962 cm-1. Because
that bond is present in most organic compounds, that's not
terribly useful! What it means is that you can ignore a
trough just under 3000 cm-1, because that is probably just
due to C-H bonds.
The carbon-oxygen double bond, C=O, is one of the really
useful absorptions, found in the range 1680 - 1750 cm-1. Its
position varies slightly depending on what sort of
compound it is in.
The other really useful bond is the O-H bond. This absorbs
differently depending on its environment. It is easily
recognised in an acid because it produces a very broad
trough in the range 2500 - 3300 cm-1.
The infra-red spectrum for ethanoic acid looks like this:
The possible absorption due to the C-O single bond is
queried because it lies in the fingerprint region. You
couldn't be sure that this trough wasn't caused by
something else.
Ethanol
The O-H bond in an alcohol absorbs at a higher
wavenumber than it does in an acid - somewhere between
3230 - 3550 cm-1. In fact this absorption would be at a
higher number still if the alcohol isn't hydrogen bonded for example, in the gas state. All the infra-red spectra on
this page are from liquids - so that possibility will never
apply.
Notice the absorption due to the C-H bonds just under 3000
cm-1, and also the troughs between 1000 and 1100 cm-1 one of which will be due to the C-O bond.
Ethyl ethanoate
This time the O-H absorption is missing completely. Don't
confuse it with the C-H trough fractionally less than 3000
cm-1. The presence of the C=O double bond is seen at about
1740 cm-1.
The C-O single bond is the absorption at about 1240 cm-1.
Whether or not you could pick that out would depend on
the detail given by the table of data which you get in your
exam, because C-O single bonds vary anywhere between
1000 and 1300 cm-1 depending on what sort of compound
they are in. Some tables of data fine it down, so that they
will tell you that an absorption from 1230 - 1250 is the C-O
bond in an ethanoate.
The infra-red spectrum for a ketone
Propanone
You will find that this is very similar to the infra-red
spectrum for ethyl ethanoate, an ester. Again, there is no
trough due to the O-H bond, and again there is a marked
absorption at about 1700 cm-1 due to the C=O.
Confusingly, there are also absorptions which look as if
they might be due to C-O single bonds - which, of course,
aren't present in propanone. This reinforces the care you
have to take in trying to identify any absorptions in the
fingerprint region.
Aldehydes will have similar infra-red spectra to ketones.
The infra-red spectrum for a hydroxy-acid
2-hydroxypropanoic acid (lactic acid)
This is interesting because it contains two different sorts of
O-H bond - the one in the acid and the simple "alcohol"
type in the chain attached to the -COOH group.
The O-H bond in the acid group absorbs between 2500 and
3300, the one in the chain between 3230 and 3550 cm-1.
Taken together, that gives this immense trough covering
the whole range from 2500 to 3550 cm-1. Lost in that trough
as well will be absorptions due to the C-H bonds.
Notice also the presence of the strong C=O absorption at
about 1730 cm-1.
The infra-red spectrum for a primary amine
1-aminobutane
Primary amines contain the -NH2 group, and so have N-H
bonds. These absorb somewhere between 3100 and 3500
cm-1. That double trough (typical of primary amines) can be
seen clearly on the spectrum to the left of the C-H
absorptions.