2. Spectroscopy is a term that describes the interaction of matter with
electromagnetic radiation.
The electromagnetic spectrum, and as the word spectrum implies, is a
range of frequencies of the electromagnetic radiation and the
corresponding wavelengths and photon energies is a schematic portrayal of
the electromagnetic spectrum, along with the molecular processes that can
occur in each region, e.g.,
rotation (microwave)
vibration (infrared)
electronic excitation (ultraviolet-visible) and
bond breaking and ionization (X-rays).
3. 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
Infrared Spectroscopy is the analysis of infrared light
interacting with a molecule.
It is based on absorption spectroscopy
The wavelengths found in infrared radiation are a little
longer than those found in visible light.
IR spectroscopy is useful for finding out what kinds of
bonds are present in a molecule, and knowing what the
structure could be.
4. NATURE OF IR SPECTRA
IR spectrum is a graph of band intensities on y axis versus position of band
on x-axis.
Band intensities can be given in terms of transmittance(T) or
absorbance(A).
Position of band can be expressed in terms of wave number (n) or
wavelength(λ).
In IR spectra, wave numbers (n) are used instead of wavelength (λ) for
mentioning the characteristic peak as this unit has advantage of being linear
with energy of radiation (E) .
E = h c/ λ or, E= h c n
[ n = 1/λ, c= velocity of light, h= Planck’s constant ]
5. TRANSMISSION vs. ABSORPTION
When a chemical sample is exposed to the action of IR LIGHT, it can absorb (retain)
specific frequencies and allow the rest to pass through it (transmitted light).
Some of the light can also be reflected back to the source.
Transmittance ( T) is defined as the ratio of radiant power transmitted by a sample
to the radiant power incident on the sample.
Whereas, Absorbance (A) = log 10 (1 /T)
An IR spectrum is usually plotted using transmittance, hence absorption band
appears as dips rather than maxima. Each dip is called band or peak.
6. Regions of the Infrared spectrum
Functional Group Region-Most of the bands that indicate what functional
group is present are found in the region from 4000 cm-1 to 1400 cm-1. Their
bands can be identified and used to determine the functional group of an
unknown compound.
Fingerprint Region-Bands that are unique to each molecule, similar to a
fingerprint, are found in the fingerprint region, from 1400 cm-1 to 900 cm-1.
These bands are only used to compare the spectra of one compound to
another.
7. IR region of electromagnetic spectrum: λ : 780 nm – 1000 μm
Wavenumber : 12,800 – 10 cm-1
IR region is subdivided into 3 sub-regions:
1. Near IR region (Nearest to the visible) - 780 nm to 2.5 μm
(12,800 - 4000 cm-1 )
2. Mid IR region - 2.5 to 50 μm (4000 – 200 cm-1 )
3. Far IR region - 50 to 1000 μm (200 – 10 cm-1 )
8. When IR radiations are passed,it causes-
1.Changes in the shape of molecules such as
stretching of bonds, bending of bonds, or
internal rotation around single bonds.
2.IR absorption only occurs when IR radiation
interacts with a molecule undergoing a change
in dipole moment as it vibrates or rotates.
9. For a molecule to be IR active there must be a
change in dipole moment as a result of the
vibration that occurs when IR radiation is absorbed.
In homonuclear diatomic molecule like H2, O2, N2
etc., the dipole moment does not change during
vibration. Hence these molecules do not give
vibration spectra i.e. they are said to be infrared-
inactive
10. PRINCIPLE OF IR SPECTROSCOPY
When the energy in the form of IR is applied and if the applied IR frequency =
Natural frequency of vibration, the absorption of IR takes place and a peak is
observed.
Molecules are excited to the higher energy state from the ground state when they
absorb IR radiation.
When a compound is exposed to IR radiation, it selectively absorbs the radiations
resulting in vibration of the molecules of the compound, giving rise to closely
packed absorption bands, called as IR absorption spectrum.
The bands correspond to the characteristic functional groups and the bonds present
in a chemical substance. Thus, an IR spectrum of a compound is considered as the
fingerprint for its chemical identification.
11. A molecule in its ground state posses 3 energy levels,
1. Electronic,
2. Vibrational and
3. Rotational energy level.
• Absorption in the IR regions is due to the changes in the vibrational and rotational
levels.
• When an electron absorbs less energy, rotational transitions takes place. The
spectrum observed is called rotational spectrum observed in far IR region (25 to 300-
400 µ).
• When the electrons absorbs still higher energy, vibrational transitions take place
which is accompanied with rotational transitions. The spectrum observed is called
vibrational- rotational spectrum obtained in near IR region (2.5 to 25 µ).
• Electronic transitions take place at higher energies, these are not seen in IR.
12. Modes of Vibration
The interaction of infrared radiations with matter can be
understood in terms of changes in molecular dipoles associated
with vibrations.
Vibrations can involve either changes in-
bond length (stretching) or
bond angle (bending).
Some bonds can stretch in-plane (symmetric stretching) or
out-of-plane (asymmetric stretching).
Bending vibrations can be either in-plane (as; scissoring,
rocking) or out-of-plane (as; wagging, twisting) bending
vibrations.
18. Some General Trends:
i) Stretching frequencies are higher than corresponding bending
frequencies. (It is easier to bend a bond than to stretch or
compress it.)
ii) Bonds to hydrogen have higher stretching frequencies than
those to heavier atoms.
iii) Triple bonds have higher stretching frequencies than
corresponding double bonds, which in turn have higher
frequencies than single bonds.
(Except for bonds to hydrogen).
19. In order for a vibrational mode in a molecule to be "IR
active," it must be associated with changes in the permanent
dipole.
A molecule can vibrate in many ways, and each way is called a vibrational
mode or Fundamental Vibrations.
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.
20. Sampling in Infrared Spectroscopy
The samples used in IR spectroscopy can be either in the solid, liquid, or gaseous state.
Solid samples can be prepared by crushing the sample with a mulling agent which has
an oily texture or in the form of KBr pellets. A thin layer of this mull can now be applied
on a salt plate to be measured.example-Nujol mull
Liquid samples are generally kept between two salt plates and measured since the
plates are transparent to IR light. Salt plates can be made up of sodium chloride,
calcium fluoride, or even potassium bromide.
Gaseous samples are filled inside NaCl cell.
Since the concentration of gaseous samples can be in parts per million, the sample cell
must have a relatively long pathlength, i.e. light must travel for a relatively long distance
in the sample cell.
21. REGIONS OF IR SPECTRUM
IR spectra is divided into 2 regions.
Functional Group Region(4000- 1400 cm-1)
1. It consists of absorption bands of vibrational states of various types of bonds present
in the molecule.
2. The important groups accounted for include NH, OH, C=O, C=C, C=N, etc.
3. The presence of aromatic nucleus (2000-1670 cm-1) and hydrogen bonding O-H, N-H,
etc are also encountered in this region.
22. 2. Fingerprint region
• This region accounts for many absorption bands characteristic of functional group.
Since numbers of sharp bands of varying intensities are encountered, close
examination is needed.
• This region is useful for the identification of compounds since no two compounds
can have identical IR spectra under identical conditions.
• Regions present below 1400 cm-1 shows absorption bands due to bending
vibrations and stretching vibrations of C-C, C-O and C-N bonds.
• Regions less than 1250 cm-1 consists of complex vibrational and rotational spectra
of the complete molecule.
23.
24. APPLICATIONS OF IR SPECTROSCOPY
•Identification of functional groups & structure elucidation of organic compounds.
•Quantitative analysis of a number of organic compounds.
•Study of covalent bonds in molecules.
•Studying the progress of reactions.
•Detection of impurities in a compound.
•Ratio of cis-trans isomers in a mixture of compounds.
•Shape of symmetry of an inorganic molecule.
•Study the presence of water in a sample.
•Measurement of paints and varnishes
25. Numerical on Lambert Beer Law
1.The percentage transmittance of an aqueous solution of unknown compound
is 40% at 25degree centigrade and 300nm..Calculate the absorbance of the
solution.
Given-
I o=100
I t=40
A=log Io/It
=(log100-log40)
=0.3979
26. 2. A solution of Tryptophan has an absorbance at 280 nm of 0.54 in a 0.5 cm length
cuvette. Given the absorbance coefficient of trp is 6.4 × 10 3 LMol-1 cm-1 . What is the
concentration of solution?
Solution: As
ε = A / l c
l= 0.5 cm,
A= 0.54
ε = 6.4 × 10 3 LMol-1 cm-1
C=?
c = A/ε l = 0.54 / 6.4 × 10 3 × 0.5
Answer = 0.000168 M 2.
27. 3.In a spectrometric cell of 2 cm path length the solution of a substance shows the
absorbance value of 1.0.If the molar absorptivity of the compound is 2x10 4
litre/mole.cm,calculate the concentration of the substance
