Overview about Uv visible spectroscopy

1. UV - Visible Spectroscopy

2. Ultraviolet – Visible Spectroscopy
 UV-Visible spectroscopy deals with the study of absorption of light in the UV region (100-
400 nm) & visible region (400-800 nm) of the electromagnetic spectrum.
 Classification of UV-Visible spectrum :-
 Far UV region – 120-200 nm – Saturated organic compound (single bond)
 Near UV region – 200-380 nm – Quartz
 Visible UV region – 380-700 nm – Large no. of conjugated double bonds
(β-Carotene)

3. UV – Visible Absorption
 In UV/VIS spectroscopy, the transition which result in the absorption of EM radiation in
this region are transition between electronic energy levels . Hence it is also called
electronic spectroscopy.

4. Molecular Absorption
 In molecules, not only have electronic level but also consist of vibrational & rotational
sub-levels.
 This result in band spectra.

5. Principle of UV – Visible Spectroscopy
Basically, spectroscopy is related to the interaction of light with matter. As light is absorbed by matter, the result is an
increase in the energy content of the atoms or molecules.
 The Principle of UV-Visible Spectroscopy is based on the absorption of ultraviolet light or visible light by chemical
compounds. When a molecule absorbs a photon of UV-Vis light, molecule is excited from its ground state. In other
words, an electron is promoted from HOMO (Highest-energy Occupied Molecular Orbital) of the molecule to the
LUMO (Lowest-energy Unoccupied Molecular Orbital) of the molecule. The HOMO is commonly a π orbital of
conjugated functional group and the LUMO is commonly a π* orbital of conjugated functional group. The smaller
the energy difference between HOMO & LUMO, the less energy is needed & the longer the wavelength that will be
absorbed.

6. INSTRUMENTATION & WORKING
1. Light source
2. Monochromator
3. Sample & Reference
4. Detector

7. APPLICATIONS
 Qualitative & quantitative analysis :-
 Identifying the functional group
 Structure of chemical compound
 Detection of impurity
 Detection of isomer
 Determine molecular weight of sample

8. Electronic Transition

9. 1. σ → σ* transition
 σ electron from orbital is excited to corresponding anti-bonding orbital σ*.
 The energy required is large for this transition.
 Example :- Methane(CH4) has C-H bond only and can undergo σ → σ* transition and
shows absorbance maxima at 125 nm.

10. 2. π → π* transition
 π electron in a bonding orbital is excited to corresponding anti-bonding orbital π*.
 Compounds containing multiple bonds like alkenes, alkynes, carbonyl, nitriles, aromatic
compounds, etc undergo π → π* transition.
 Example :- Alkenes generally absorb in the region 170 to 250 nm.

11. 3. n → σ* transition
 Saturated compounds containing atoms with lone pair of electrons like O, N, S & halogens
are capable of n → σ* transition.
 These transition usually requires less energy than σ → σ* transitions.
 The number of organic functional groups with n → σ* peaks in UV region is small (150 –
250 nm).

12. 4. n → π* transition
 An electron from non-bonding orbital is promoted to anti-bonding π* orbital.
 Compounds containing double bond involving hetero atoms (C=O, C≡N, N=O) undergo
such transitions.
 n → π* transitions require minimum energy & show absorption at longer wavelength
around 300 nm.

13. Types of Absorption Bands
1. R – Bands (Radical) :- (Ɛmax < 100)
 Bands due to n → π* transition in compounds with single chromatographic groups
 Example :- Carbonyl group, nitro group
2. K – Bands (Conjugate) :- (Ɛmax > 10,000)
 Bands due to π → π* transition in a compound with conjugated π system
 Example :- Dienes, polyene, enones & aromatic rings
3. B – Bands (Benzenoid Bands) :- (Ɛmax = 230 – 270)
 Bands due to π → π* transition in aromatic/hetero aromatic compounds
 Example :- Benzene ring compounds
4. E – Bands (Ethylenic Bands) :- (Ɛmax = 2000 – 14,000)
 Bands in benzenoid system of ethylenic part enclosed in cyclic conjugation
 E1 & E2 of benzene are observed near 180 nm & 200 nm

14. Types of Shifts
1. Bathochromic Shift (Red Shift) :-
 Absorption maxima shifts to longer wavelength
 Due to presence of an auxochrome or by the change of solvent
 Example :-
 CH2-CH2 max = 175 nm
 CH3-CH2-CH=CH2 max = 185 nm
2. Hypsochromic Shift (Blue Shift) :-
 Absorption maxima shifts to shorter wavelength
 Due to presence of group causes removal of conjugation or by change of solvent
 Example :-
max = 245 nm

15. 3. Hyperchromic Shift :-
 Absorption intensity of a compound is increased
 If auxochrome introduces to the compound, the intensity of absorption increases
 Example :-
Pyridine 2-methyl pyridine
Ɛ= 257 nm Ɛ= 260 nm

16. 4. Hypochromic Shift :-
 Absorption intensity of a compound is decreased