IR spectroscopy
IR = Infra-red
Functional
groups
• Functional groups are collection
of atoms that attach the carbon
skeleton of an organic molecule
and confer specific properties
• Each type of organic molecule has
its own specific type of functional
group
• Functional groups in biological
molecules play an important role
in the formation of molecules like
DNA, proteins, carbohydrates
etc….
An example and basic principle
• Carbonyl functional group in formaldehyde.
• Carbonyl means C double bond O
• In fact, this molecule (others as well are not static as it looks in this
molecular representation.
• In fact, the bonds are vibrating with some frequency at room temperature.
• The vibrational frequency of the bonds is temperature dependent
• When the IR radiations with the range of frequency is incident on to such
sample, the radiations with the frequency equivalent to the vibrational
frequency of the bonds will be absorbed (the concept of resonance)
Actual IR spectra
Use of Wavenumber instead
Of frequency.
Wavenumber tells you about
how many waves fit into one
Centimeter.
Wavenumber is units of energy
and therefore you can directly
deduce the difference of energy
between states
Functional group region vs the
fingerprint region
• Functional group region: gives information about the
presence of functional group present in the molecule
• Fingerprint region: gives information about the exact
central molecule. This is called the finger print because
every individual molecule has different spectra in the finger
print region.
• Every individual human being have distinct finger prints.
That’s why as an analogy, this region is called a finger print
region where every individual molecule have distinct
spectra (or metaphorically, the finger print). These finger
prints can be checked through the database.
Principle of IR spectroscopy and modes of
molecular vibrations
• IR active compounds will get IR spectra gets excited and shows vibrational, rotational
spectra; which is characteristic spectra for a given functional group.
• Two fundamental types of vibrations in IR spectroscopy:
1. Stretching
I.
Symmetrical stretching
II. Asymmetrical stretching
2.
I.
II.
III.
IV.
Bending
Scissoring
Rocking
Wagging
Twisting
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ules_(Physical_and_Theoretical_Chemistry)/Spectroscopy/Vibrational_Spectroscopy/Vibrational_Modes/Number_o
f_Vibrational_Modes_in_a_Molecule#:~:text=Molecular%20Vibrations,A%20molecule%20has&text=For%20a%20mode%20to%20be,(i.e.%20not%20diatomic%20molecules).&text=The%20
normal%20modes%20of%20vibration,and%20rocking%20for%20polyatomic%20molecules.
Infrared spectroscopy
• It is a type of absorption spectroscopy.
• It is used to determine the functional groups in a given organic compound.
(e.g., the detection of aldehydes, ketones, carboxylic groups etc….)
• Infrared radiations are directed onto the sample. Some infrared photons
are absorbed by the sample while the rest pass through (or transmit
through) the sample
• Vibrational rotational spectra occurs.
• The essential requirements of the sample if it has to be analyzed using IR
spectroscopy  it should possess dipole moments
• If the sample does not contain dipole moments, it means that it is IR
inactive compound. Otherwise, it is IR active (means, IR spectroscopy can
be used for the analysis of that compound). It will be able to absorb IR
radiations of an appropriate wavenumber.
• The molecules such as HF has a centre of positive charge and a centre of
negative charge is said to be dipolar or to have a dipole moment
• The dipolar character is represented by an arrow pointing towards the
negative charge centre (the tail of the arrow points towards positive
charge centre)
Dipoles and Dipole moments
• Dipole is the measure of separation of charge. The dipole in a bond is formed due to the difference in
electronegativity of the individual atoms forming the bond.
• Dipole moment is a measure of change in the dipole moment
• Dipole moment of a molecule can be calculated using the following relationship
• Why only polar bonds are IR active?
Because the polar molecules have dipole moments. Under normal conditions, these molecules are in
continuous vibrations e.g., stretching and bending. Any change in dipole moments of radiations means
absorption of it by the molecule. It have been observed that this takes place by IR radiations.
• Why not the vice versa?
• How does the IR radiations interact with molecules based on the presence of dipoles and dipole moments?
IR radiations are electromagnetic radiations
• Every compound will have its distinct IR spectra. In other words, no
two different compounds will have similar IR spectra. Enantiomers are
exceptional in this case.
• Categories of IR radiations:
1. Near IR
2. Mid IR (the one used in IR spectroscopy)\
3. Far IR
Wavelength Vs Wavenumber
Wavelength = the distance
between two successive crests
of troughs in a wave
Wavenumber = number of
waves (or number of cycles) in a
unit distance.
IR spectroscopy vs FT IR spectroscopy
• The principle in both is same
• The prime difference is that in case of IR spectroscopy, the intensity of
incident IR radiations is kept constant.
• In case of FTIR, the intensity of incident radiations fluctuates to
ensure complete detection of the functional groups and individual
bonds.