UV-VISIBLE SPECTROSCOPY

• UV – 200 to 400nm VISIBLE- 400 to 800nm

• It is also known as electronic spectroscopy

• The absorption of UV-Visible radiations by the molecules leads to electronic

excitations between molecular electronic energy levels.

• Transitions involving pi orbitals and lone pairs are important, So uv- visible
spectroscopy mostly used for identification of conjugated systems.
Classification of Electronic transitions
When a molecule absorb energy in UV-Visible region, its electrons are promoted from bonding molecular orbital to
an antibonding molecular orbital. Three types of electrons are involved in electronic transitions. They are σ
electrons, π electrons and non- bonding electrons (n). Electronic transitions are classified in to 4 types.
 • The unsaturated hydrocarbons containing double or
• The saturated hydrocarbons are transparent to uv- triple bonds shows this type of transitions.
visible light. Therefore they are colourless. The • In the case of molecules containing conjugated double
saturated hydrocarbons can give only σ ͢ σ* bonds , these transitions can be observed from HOMO
transitions of C-C bond and C-H bond. This type of to LUMO.
transition occurs below 150nm. The ordinary UV • Molecules with conjugated pi systems, the energy gap
spectrometer take spectra only from 200-700nm for π -π* transitions is smaller than for isolated double
and hence saturated hydrocarbons cannot be bonds, and thus the wavelength absorbed is longer.
detected using UV-Visible spectrometer.
Molecular energy diagram of ethene

C=C of ethylene molecule gives an absorption at 169 nm

So normal UV machines will not show such transitions
Molecular energy diagram of butadiene

It has four pi molecular orbitals formed by the linear combination of four atomic orbitals.
Ψ1 and ψ2 are bonding molecular orbitals . Ψ3 and ψ4 are antibonding molecular orbitals.
In the ground state four π electrons occupy bonding molecular orbitals.
The transition takes place from HOMO ψ2 to LUMO ψ3 with λmax = 217 nm
Molecular energy diagram of 1,3,5 hexatriene

The absorbance due to the π - π* transition in 1,3,5-

hexatriene, occurs at 247 nm.
There are three double bonds in conjugation.

It has six pi molecular orbitals formed by the linear

combination of six atomic orbitals.
Ψ1, Ψ2 and ψ3 are bonding molecular orbitals . Ψ4, Ψ5
and ψ6 are antibonding π molecular orbitals.

In the ground state the six pi electrons occupy bonding

molecular orbitals Ψ1, Ψ2 and ψ3

The transition takes place from HOMO Ψ3 to LUMO ψ4.

Λmax is observed at 247 nm
 505nm

As more and more double bonds come into conjugation the absorption comes to the visible region.
In Lycopene,a red colored natural dye of tomato 11 double bonds in conjugation, wavelength of absorption is
Observed at 505nm, red in color.
 Molecular energy diagram of benzene

There are 3 double bonds in conjugation.

It has six π molecular orbitals formed by the linear
combination of six atomic orbitals.

ᴪ1 ,ᴪ2, ᴪ3 are bonding molecular orbitals & ᴪ4, ᴪ5, ᴪ6 are

antibonding molecular orbitals .

ᴪ1 and ᴪ 6 have unique energy levels. Here ᴪ2 and ᴪ3 are

degenerate orbitals (same energy). ᴪ4 and ᴪ5 are also
degenerate.

In the ground state the six pi electrons occupy bonding

molecular orbitals.

The transition takes place from HOMO to LUMO.

Λmax is observed at 255 nm
 • The absorption takes place below 200nm

These two types of transitions are symmetry forbidden and occur in low intensity
 UV- Visible spectrum
• It is a plot of Absorbance
against wavelength
• The wavelength at which
absorbance is maximum is
designated as λmax.
• It is a characteristic value

Each electronic energy levels have vibrational sub levels and each vibrational levels have rotational sub-levels
Electronic transitions are usually accompanied by vibrational and rotational transitions. Therefore
discrete lines are not observed in the spectrum .It appears as band with an absorption maximum.
 Instrumentation of UV Spectroscopy
UV-Vis Spectrophotometer is the analytical instrument used for the UV-Vis spectroscopic analysis.

Components in a spectrophotometer

1)Radiation source: A stable continuous radiation is required.

• Tungsten filament Lamp
• Hydrogen discharge Lamp
• Xenon flash lamp

2)Wavelength selector (monochromator)-

• It is a device that breaks the polychromatic radiation (contains many different wavelengths of light.) into
component wavelengths.
• Prism , grating, filters
3) Sample holder (cuvette)-
• must be transparent containers to the radiation which will pass through them.
• Glass is suitable for visible but not for UV spectroscopy because it absorbs UV radiation.
• Quartz can be used in UV as well as in visible spectroscopy.

4)Detectors-
• The detectors are devices that convert radiant energy into electrical signal
• The photomultiplier tube is a commonly used detector in UV-Vis spectroscopy.

5)Recorder
• The signal coming from the detector is recorded by the recorder which
is displayed on the read out device
Double beam spectrophotometer

• In a double beam spectrometer a beam of light is split into two equal beams and one is
passed through sample (sample beam I) and the other through reference(reference beam I0).

• A very dilute solution of the sample is prepared using a suitable solvent and is taken in a
transparent cuvette. The pure solvent is taken as reference in another similar cuvette. The
most commonly used solvents are water, ethanol,hexane etc.

• The intensities of these light beams are then measured by photo detectors and compared.

• If the sample does not absorb light of a given wavelength then I=I0. However if the sample
absorbs the light then I is less than I0 and this difference is detected and recorded.

• The graph obtained is a plot of absorbance vs wavelength.

Double beam spectrometer
Applications of Electronic spectroscopy
• For the detection of aromatic compounds and conjugated dienes.

• For the characterization of dyes and colorants

• Detection of impurities- Example: benzene is the common impurity in cyclohexane and its
presence can be easily detected by the absorption at 255nm.

• Control of purification- The process of purification should be continued till the compound
which is being purified, stops showing bands due to impurity.
Applications of Electronic spectroscopy
• Determination of unknown concentration. Since absorbance is given by A=εCx. For a solution of same
compound in the same cell ε and x are constants. Absorbance of standard solution and unknown solution is
measured . Knowing the value of concentration of standard solution , concentration of unknown can be
determined. A1/C1 = A2/C2

• For quantitative estimation of blood sugar, cholesterol etc in medical lab – these tests are done by
measuring the absorbance after developing colour with suitable reagents.

• Study of kinetics of chemical reaction- It is done by measuring the change in concentration of reactant or
product with time provided one of the reactants or products exhibit suitable absorption in the UV-visible
region. As absorbance is directly proportional to concentration, UV spectrometry can be used to follow the
course of the reaction.