## Spectroscopy

It is a branch of science which deals with the study of interaction of matter with 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

 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 types

### 1.     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 molecules

### 2.     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
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).