Spectroscopy
It is a branch of science which deals with the study of interaction of matter with electromagnetic radiation.
Electromagnetic Radiation
Electromagnetic Radiations are a certain amount of energy depending upon its wavelength. Hence these radiations are the sources of energy so called as electromagnetic energy or radiant energy.
Electromagnetic Radiations are of
many types like radio waves, UV rays, IR rays, visible light etc. these are all
electromagnetic radiations with different energy, wavelength and frequency. All
types of electromagnetic radiations travel with the speed of light but differ
in wavelength and frequency.
Energy of electromagnetic radiations
can be expressed as
Where E = Energy
n = Frequency
l = Wavelength
c
= Velocity of light = 3*108 m/s.
h
= planks constant
Types of electromagnetic radiations
Type
|
Frequency (Hz)
|
Wavelength (1/m)
|
Radio wave (low energy)
|
3*105
|
103
|
Microwave
|
3*109
|
10-1
|
Infrared
|
3*1010
|
10-2
|
Visible light
|
3*1014
|
10-6
|
Ultraviolet
|
3*1016
|
10-8
|
X-rays
|
3*1018
|
10-10
|
Gamma rays (high energy)
|
>3*1018
|
<10-10
|
Type of spectra :
Spectra are of two types1. Emission spectra :
When a substance is exposed to intense heat or light its atoms or molecules absorbs energy and get excited. When these atoms or molecules return to their initial state they emit radiations which when pass through a prism it produce a spectrum. This spectrum is known as a emission spectrum. When this type of spectrum is recorded on a photographic plate then bright lines are formed on a black background in case of atoms and bands are formed in case molecules2. Absorption Spectra :
When a substance or matter is exposed to intense heat or light it absorbs energy. Due to this intensity of absorption varies as function of the frequency. This variation is known as absorption spectrum. This type of spectra contains dark (black) lines or bands on light coloured background as some radiations are absorbed by the medium.
In this
process only those photons of radiation are absorbed whose energy is equal to
energy difference (DE) between two energy levels of molecules of substance.
Where h =
planck’s constant = 6.63 * 10-34 Js
n = Frequency
c = Velocity of light = 3 * 108
m/s
l = Wavelength of radiation
NA
= Avogadro’s number = 6.02 * 1023 mol-1
Difference between Emission and Absorption Spectrum :
In Emission Spectrum molecules come back to lower energy state from higher energy state while in Absorption Spectrum The molecule goes to Higher energy state from lower energy state.Absorption Spectrum and its types :
In addition to the nuclear energy, total internal energy of a molecule consist of three types of energies.
Einternal = Eelectronic
+ Evibrational + Erotational
Since, electromagnetic radiation is a
form of energy so its absorption by a molecule, increases the internal energy
of the molecule. Also when a molecule is exposed to electromagnetic radiation,
molecule does not absorbs all the radiations rather it absorb a particular
portion of radiation depending upon the structure of the molecule and amount of
absorption of energy depend upon the frequency of the radiation also. So it is
clear that different molecules absorb different type of energies and undergo
different excitations. Depending upon the absorption of energy molecule may go
under Electronic excitation, Vibrational excitation and/or Rotational
excitation. So produces different Spectra.
Table : Some Components of the Electromagnetic Radiation and Absorption
Spectra
Absorbed
Radiation and Type of Spectra
|
Wavelength
|
Energy
(kJ/mol)
|
Types
of Excitation
|
Effect
on Molecule
|
Application
|
Microwave
|
1 cm
|
0.01 - 1
|
Rotational
|
Changes in the Rotational energy levels of the
molecule
|
Calculation of bond distance and bond length
|
Infrared
|
2 – 15 mm
|
1 – 100
|
Vibrational and Rotational
|
Changes in Vibrational and rotational energy levels
|
Identification of functional groups, Calculation of
Bond length, Bond angle and Qualitative analysis
|
Electronic
(i)
Visible
(ii)
Ultraviolet
|
200-400 nm
400-800 nm
|
150-300
300-600
|
Electronic
Vibrational and rotational transitions also take
place but their resolution is not measurable
|
Change in Electronic energy level
|
Qualitative and quantitative analysis
|
The Absorption energy is measured
with the help of Spectrophotometer and expressed as Frequency, Wavelength or
Wave-number.
Law of Light Absorbance :
According to the Lambert Beer’s Law “The Absorption of light is directly proportional to the concentration of the solution and the length of the cell containing sample.
Where, I0 = Intensity of
incident Radiation
I = Intensity of transmitted radiation
c
= Molar concentration
l
= Length of the cell in centimetre
e(epcylon) = Molecular extinction
coefficient
The Wavelength at which molecule has maximum
Absorption coefficient e (max) is expressed as l (max). in above
expression log I0/I is
known as absorbance or optical density of the solution. And log I/I0 is known as
Transmission of the solution.
Ultraviolet and Visible Spectroscopy
It is a type of Absorption
Spectroscopy in which Electromagnetic Radiation of UV region (l = 200 – 400 nm) or visible region (l = 400 – 800 nm) when passed through
a sample containing a multiple bond, a part of incident radiation is absorbed
by the sample (compound). Amount of absorbed Radiation Energy depends upon
Wavelength of radiation and nature of the sample (compound). The absorbed
radiation excites electron from lower energy level to higher energy level so
electrons transferred from bonding orbital to the anti-bonding orbital. The
amount of absorbed radiation is measured with spectrophotometer.
Spectrophotometer
Tungsten filament Lamp and hydrogen discharge lamp are used as source of
light (energy) for visible and Ultraviolet region respectively. Sample is hold
in a cell, generally cell is placed between slit of spectrophotometer.
And
Chromohore :
Those isolated groups which exhibit characteristic absorption in UV region. So these group Absorb UV radiation and known as chromophores. Generally they are covalently unsaturated groups (contains double bonds).
Actually, those functional groups
that involve n-π*
and π-π* transitions are known as chromophores.
Auxochrome :
An auxiliary group which shifts absorption band towards longer wavelength is known as auxochrome. Auxochrome is a saturated group having non-bonding or n-electrons which when attached to chromophore changes both the intensity of bond and absorption maxima.
Some of the shifts in absorption
maxima have characteristic names like
1. Bathochromic shift :
The shifting of absorption bands towards the longer wavelength is known as Bathochromic shift. It is also known as Red shift.2. Hypsochromic shift :
The shifting of absorption bands towards the shorter wavelength is known as Hypsochromic shift. It is also known as blue shift.3. Hyperchromic shift :
If the presence of a group increases the intensity of the intensity of the band. It is known as hyperchromic shift.4. Hypochromic shift :
If the presence of a group decreases the intensity of the intensity of the band. It is known as hypochromic shift.Application of ultraviolet spectroscopy
1. Identification of a compound :
The absorption spectra of a compound is its characteristic property. Value of l max at which maximum absorption take place is note same for two compounds i.e. every compound have a particular wavelength at which maximum absorption takes place. This property is used to identify a compound. So a spectrum of unknown compound is compared with standard spectra to identify a compound.2. Identification of geometrical isomers :
cis and trans isomer are differentiated by the study of UV spectrum. In trans isomer π-π* transition take place at the higher wavelength while in cis isomer it take place at lower wavelength.3. Calculating molecular weight of a compound :
Prepare 1% solution of organic compound and fill it in 1 cm thick cell and determine its absorbance. Suppose A is absorbance, M is molecular weight then molar concentration is c = 10/M, cell length l = 1 cm. According to Lambert-Beer’s law-
A = ecl
A = e * (10/M) * 1
M = 10e/A
4. Study of reaction kinetics :
Study of reaction kinetics can be made by studying the absorption spectrum of products and reactants time to time. As l max for product and reactant is different so their concentration can be measured at any stage of the reaction.5. Functional group Analysis :
UV Spectroscopy is used in identification of some functional groups.6. Ascertaining of purity :
Every compound have a particular Spectrum if additional bands are found then given compound is impure (on comparison with standard Spectrum).Join us at Social Media
