4. Infrared (IR) spectroscopy
Molecular Structure and Organic Chemistry
• The structure of a molecule refers to the arrangement of atoms within
the molecule. The structure of a molecule is critical to the chemical and
physical properties of a substance. In fact so vital is structure to
molecular identity that the same molecular formula may represent
more than one substance based upon their differing structures.
• Example: Two totally different substances share the same molecular
formula, C2H6O, but are different because of their differing molecular
structures.
Ethyl Alcohol C2H6O
Dimethyl Ether C2H6O
Isomers
• Molecules that have the same molecular formulas but different
structures are called isomers
• There are 2 isomers corresponding to the molecular formula C4H10 ,
Spectroscopy
And 3 corresponding to C5H12 and 39 corresponding to the molecular
formula C9H20
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The IR Region
• Just below red in the visible region.
• Wavelengths usually 2.5-25 mm.
• More common units are wavenumbers, or cm-1, the reciprocal of the
wavelength in centimeters.
Wavenumbers are proportional to frequency and energy.
Infrared Spectrophotometry
• using molecular vibrations as a key to structure
• Although ball and stick models of molecules are very effective at
approximating the actual shapes of molecules, they do have one
major flaw they leave you with the false impression that molecules
are rigid objects
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Some Vibrational Modes
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Covalent bonds vibrate at only certain allowable frequencies.
Each type of vibration has a frequency that depends upon the: the mass of
the vibrating atoms and the nature of the bond between them.
• For a constant bond type (single, double or triple) the frequency of
the vibration is low for a bond between heavy atoms. Conversely,
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for a given bond type the frequency of vibrations is high for light
atoms.
• Multiple bonds vibrate at a higher frequency than do single bonds.
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Stretching Frequencies
 Frequency decreases with increasing atomic weight.
 Frequency increases with increasing bond energy.
Infrared Absorptions
Vibrations as a key to structure- the entire range of vibrations for all
organic
molecules
falls
within
the
Infrared
Region
of
the
Electromagnetic Spectrum (2500nm – 25000nm). If a beam of IR
radiation is directed at a molecular sample and if the beam has the
same frequency as one of the vibrational modes of the molecule then
the molecule will absorb the energy of the IR radiation and the
molecular vibration will increase in intensity.
If in order for absorption to occur, the IR frequency must match the
frequency of the vibrating atoms, and if the frequency of the vibrating
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atoms is dependent upon the mass of the atoms and the bond type;
then the frequency at which absorbance occurs is dependent upon the
mass of the atoms and the bond type. Therefore, the same two bonded
atoms, regardless of the molecule that they are in, will have the same
absorbance frequency in the IR region. This is the major strength of IR
Spectrophotometry. IR Spec identifies the functional groups present in
an organic molecule.
Functional groups
Functional Group- this is an atom or group of atoms that imparts a unique
set of chemistry to whatever organic molecule it is bonded to. If the same
functional group is attached to two different organic molecules then the
two organic molecules will have similar chemistry’s and have similar
absorbance values of the IR Specs
THE FINGERPRINT REGION OF AN INFRA-RED SPECTRUM
What is the fingerprint region
This is a typical infra-red spectrum:
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Spectroscopy
The region to the right-hand side of the diagram (from about 1500 to 500
cm-1) usually contains a very complicated series of absorptions. These are
mainly due to all manner of bending vibrations within the molecule. This is
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called the fingerprint region.
The importance of the fingerprint region is that each different compound
produces a different pattern of troughs in this part of the spectrum.
Using the fingerprint region
Compare the infra-red spectra of propan-1-ol and propan-2-ol. Both
compounds contain exactly the same bonds. Both compounds have very
similar troughs in the area around 3000 cm-1 - but compare them in the
fingerprint region between 1500 and 500 cm-1.
The pattern in the fingerprint region is completely different and could
therefore be used to identify the compound.
So . . . to positively identify an unknown compound, use its infra-red
spectrum to identify what sort of compound it is by looking for specific
bond absorptions. That might tell you, for example, that you had an
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alcohol because it contained an -OH group. You would then compare the
fingerprint region of its infra-red spectrum with known spectra measured
under exactly the same conditions to find out which alcohol (or whatever)
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you had.
INTERPRETING AN INFRA-RED SPECTRUM
The infra-red spectrum for a simple carboxylic acid (Ethanoic acid)
You will see 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 1300 cm-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 singlybonded to everything else) absorbs somewhere in the range from 2853 -
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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.
The infra-red spectrum for an alcohol (Ethanol) .
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Dr. Hisham E Abdellatef
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Spectroscopy
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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.
The infra-red spectrum for an ester (Ethyl ethanoate)
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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.
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Spectroscopy
Aldehydes will have similar infra-red spectra to ketones.
The infra-red spectrum for a hydroxy-acid (lactic acid).
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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.
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Table of IR Absorptions
Functional Group
Characteristic Absorption(s) (cm-1)
Alkyl C-H Stretch
2950 - 2850 (m or s)
Alkenyl
C-H
Stretch
Alkenyl C=C Stretch
3100-3010(m).
1680 - 1620 (v)
Alkynyl
C-H
Stretch
Alkynyl C=C Stretch
~3300(s)
2260 - 2100 (v)
Aromatic
C-H
Stretch
Aromatic C-H Bending
Aromatic C=C Bending
~3030(v)
860-680(s)
1700 - 1500 (m,m)
Alcohol/Phenol O-H Stretch
3550 - 3200 (broad, s)
Carboxylic Acid O-H Stretch
3000 - 2500 (broad, v)
Amine N-H Stretch
3500 - 3300 (m)
Nitrile C=N Stretch
2260 - 2220 (m)
Aldehyde C=O Stretch
Ketone
C=O
Stretch
Ester
C=O
Stretch
Carboxylic Acid C=O Stretch
Amide C=O Stretch
1740
1750
1750
1780
1690 - 1630 (s)
Amide N-H Stretch
3700 - 3500 (m)
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1690
1680
1735
1710
(s)
(s)
(s)
(s)
Dr. Hisham E Abdellatef
Spectroscopy
IR Spectrophotometers
• Because every molecule has a unique set of atoms and bonds that
compose it, each molecule will absorb IR radiation only at certain
frequencies. These frequencies are related to the types of bonds and
arrangements of atoms in a molecule.
• An IR Spectrophotometer is an instrument that measures the
absorbance of IR radiation by a sample as a function of frequency.
Instruments
• IR spectrometer consist of
1. Source of IR light (radiation) Nernst Glowers and Globars.
2. Monochromator: Using either prism or Grating system
3. Sample-compartment (liquid o solid with KBr)
4. Light-detector Thermal transducer
5. Signal processor The same as in spectrophotometer
Dispersive (Double Beam)
IR Spectrophotometer
IR Source Lenz
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Prism
or
Diffraction
Grating
Split
Beam
Slit
Air
Sample
Photometer
Recorder
Dr. Hisham E Abdellatef
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