UV-Vis spectroscopy is an analytical technique that measures the amount of discrete wavelengths of UV or visible light that are absorbed by or transmitted through a sample in comparison to a reference or blank sample.

1. Ultraviolet Spectroscopy
By
Dr. Neetu Soni
SHUATS, Prayagraj, UP

2. UV-Vis spectroscopy is an analytical technique that measures the
amount of discrete wavelengths of UV or visible light that are
absorbed by or transmitted through a sample in comparison to a
reference or blank sample.

4. A simplified schematic of the main components in a UV-Vis spectrophotometer.

5. ORIGIN AND THEORY OF ULTRAVOILET SPECTRA
Ultra violet absorption spectra arise from transition of
electron or electrons with in a molecule or an ion from a
lower to a higher electronic energy level.
Both organic and inorganic species exhibit electronic
translations in which outermost of bonding electrons are
promotes to higher energy levels.

7. The difference E1 – E0 = ΔE is given by
ΔE = E1 – E0= hv = h c/λ (v = c/λ)
Where, h = Planck’s constant
c = Velocity of light
λ = wave length of the absorbed radiation.
Δ E depends upon how tightly the electrons are
bound in the bonds and accordingly the absorption
will occur in UV or particular region of visible range.
The main function of absorbed energy is to raise the molecule
from the ground state energy E0 to the higher excited state
(energy E1)

8. Three distinct type of are involved in organic
molecules these are as follows-
(a)σ-Electrons:
These electrons are involved in saturated bonds
such as those between carbon & hydrogen in
paraffin.
As the amount of energy required exciting electrons
in s bands is much more than that produced by UV
light.
Compounds containing s bands do not absorb UV
radiation.
 For this reason, paraffin’s compounds are
frequently very useful as solvents.

9. (b) π-electrons:
These electrons are involved in unsaturated
hydrocarbons,
The compounds with double or triple bonds and
aromatic compounds.
(c) n-electrons:
 These are the electrons which are not involved in
any of the bonding between atoms in molecules.
Examples are organic compounds containing nitrogen,
oxygen, or halogens. As n electrons can be excited by
UV radiation, any compound that contains atoms like
nitrogen, oxygen, sulphur, halogen compounds or
unsaturated hydrocarbons may absorb UV radiation.

10. Electronic transitions
Sigma to sigma star transition (σ → σ∗)
 n to sigma star transition (n → σ∗)
pi to pi star transition (π → π∗)
n to pi star transition (n → π∗)
A transition of an electron from bonding sigma orbital to higher energy antibonding
sigma orbital is designated σ → σ∗ e.g. Alkane.
n to sigma star transition (n → σ∗) involves saturated compounds with one hetero atom
like oxygen, nitrogen, fluorine, chlorine, etc. e.g. saturated halides, alcohols, ethers,
aldehyde, ketones, and amines.
pi to pi star transition (π → π∗) in uv vis spectroscopy is available in compounds with
unsaturated centers like unsaturated hydrocarbons and carbonyl compounds.
In n to pi star transition (n → π∗), an electron in unshared pair on a hetero atom is excited
to π∗ antibonding orbital. It involves the least amount of energy than Therefore, the n →
π∗ transition gives the absorption with a longer wavelength.
e.g. saturated ketones.

11. The R-band (from German radikalartig 'radical-like');
The K-band (from German konjugiert 'conjugated');
The B-band (from benzoic);
The E-band (from ethylenic).
hromophore Example Excitation λmax, nm ε Solvent
C=C Ethene π __> π* 171 15,000 hexane
C≡C 1-Hexyne π __> π* 180 10,000 hexane
C=O Ethanal
n __> π*
π __> π*
290
180
15
10,000
hexane
hexane
N=O Nitromethane
n __> π*
π __> π*
275
200
17
5,000
ethanol
ethanol
C-X X=Br
X=I
Methyl bromide
Methyl Iodide
n __> σ*
n __> σ*
205
255
200
360
hexane
hexane

12. σ → σ*​ > n → σ* > π → π* > n → π*

13. Transition probability
(i) Allowed transitions
(ii) Forbidden transitions
Allowed transitions – these are transitions having molar coefficient 104 or
more. These are generally designated as π→ π transitions. For example in 1,3-
butadiene which exhibits absorption at 217nm has €max value 21000 represents
an allowed transition. These transition are mainly favoured due to symmetry
relationship. For e.g. 1,3- butadiene absorbs at 217nm and has molar
absorptivity of 21000
Forbidden transitions – these are transitions for which €max is generally less
than 104 . for example transition of saturated aldehyde showing weak
absorption near 290nm and having €max 100 has been a forbidden transition.
For e.g. Carbonyl group absorbs at 300nm and a molar absorptivity of 10-100.5

14. Change in position and intensity of absorption
Bathochromi shift: The position shift of a peak or signal to longer wavelength ( Red shift).
Hypsochromic shift: The shift of a peak or signal to shorter wavelength (Blue shift).
Hyperchromic: an increase in the molar absorptivity.
Hypochromic: an decrease in the molar absorptivity.

15. All spectrophotomete, Photometers and Colorimeters have the following basic
components.
 Source of continuous radiation over the wavelengths of interest.
 A Filter or Monochromator for selecting a narrow band of
wavelengths from the source spectrum.
Sample cell A container for the sample.
A Detector for converting radiant energy into electrical energy, and
 A Read out system device to read out the response of the
detector.

16. Single beam uv- visible spectrophotometer

17. Double beam uv- visible spectrophotometer