Spectroscopy of uv visible

1. UV-Vis spectroscopy

2. Objectives
 Regions of Ultraviolet and visible spectrum
 Nature of electronic excitations
Principle of absorption spectroscopy: Beer Lambert law
Theory of UV-Vis spectroscopy: mode of electronic transitions
Intensities of absorptions
Instrumentation
Sample preparation : chromophores, solvent cut off wavelengths,
auxochrome, bathochromic effect etc
How to take UV-Vis spectras
 Applications

3. Introduction
Regions between 10nm to 800nm
Far UV region 10-200nm ( vaccum UV region)
10-400nm UV region
Near UV region 200-400nm (ordinary or quartz region)
400 to 800nm Visible region

4. Nature of electronic excitations
When the continuous radiation passes through the transparent material, a portion may be
absorbed. The residual radiation when passed through prism, yields a spectrum with gaps
in it. This is termed as absorption spectra.
As a result of energy absorption, atoms or molecules passes the state of low energy
(ground state) to a state of high energy (excited state).
This excitation process is quantized. HOW????
In case of UV-Visisble spectroscopy, the absorption of radiation in this wavelength range causes
the electron transition from highest occupied molecular orbital (HOMO) to lowest unoccupied
molecular orbital
(LUMO)

5. Principle of absorption spectroscopy
The greater the number of molecules capable of absorbing light of given wavelength, greater will be the extant of light
absorption.
Beer Lambert Law:
Lambert Law:
First law by Lambert in 1768 states that “the thickness of the medium is directly proportional to the fraction of radiation
absorbed”
Mathmatically: A α b or l
Beer Law:
In 1852 Beer stated that “the absorption of radiation is directly proportional to the concentration of sample”
Mathmatically: A α C
Combining both beer and lambert law we get

6. The intensity of absorption may be measured as Transmittance(T) or Absorbance (A)
Transmittance is defined as:
T = I/ Io
Then absorption is
A= 1/T,
A = Io/I

7. Theory of UV-Vis spectroscopy:
When a molecule is irradiated with UV or visible light the valence shell electrons are excited from HOMO(
ground state) to LUMO (excited state)
For example: Ethylene …?
Mode of electronic transitions:
After the absorption of energy in the form of UV-Vis light, the lowest energy molecular orbital (HOMO) are
sigma bonds and Highest energy molecular orbital (LUMO) are pi orbitals.
Different electronic transitions:
o n-π carbonyl compounds
o n-σ* nitrogen, halogen, oxygen and sulphur compounds
o π-π* carbonyl, alkenes, azo like alkynes
o σ-π* carbonyl compounds
o σ-σ* in alkanes

8. Relative energy positions of these transitions:
unoccupied
levels
occupied levels
The energy required to bring transitions froms Highest energy occupied energy levels (HOMO)
to lowest energy least occupied energy levels (LUMO) is less than the energy required to bring
about the transitions from lowest (E) occupied energy level.
For example:
n-π* have low energy difference
π-π* have greater E difference hence greater energy for transition.

9. Possible electronic transitions:
σ-σ* :
 Saturated hydrocarbons containing strongly bondede sigma electrons ex. ethane
 Highest energy hence in vaccum UV region. Can we see these in spectrum?
 Propane absorption band =135nm
n-σ* :
 Saturated compounds having heteroatom with lone pair of electrons Ex. Oxygen, nitrogen,
halogen
 The excitation of electron from non-bonding p-orbitals of heteroatom to an sigma antibonding
orbital
 less energy i.e λmax 150-250nm
 Example: CH3OH at 183nm, (CH3)3N at 277nm
 Can organic alcohols (ROH), ethers (ROR), amines (RNH), alkyl halde (RX) show these
transitions?
π-π* :
 Molecules containing pi bond
 Promotion of electron from pi-pi star orbital
 Absorption band 160-190nm Ex. Ethylene=171nm
n-π* :
 Molecules containing double triple bond with heteroatoms ex C=O, -C≡N
 Less energy or longer wavelength
 Aldehydes and ketones at 275-295nm

10. Intensities of absorption
Absorption band
two characteristics
Position Intensity
Information
The radiation wavelength whose E Number of molecules that absorbs
is equal to ∆E required for electron the radiation of that wavelength.
transition.
Dependence
Intensity of absorption depends on
Probability of transition which depends
symmetry of orbitals in ground and excited state.
Measurement
In terms of transmittance and absorbance
Expressed
molar absorptivity (ε)

11. Probability transition and molar absorptivity (ε)
Probability of π-π* transition high than n-π* transition.
Reason
π-π* transition: Give high intensity band due to symmetry.
The absorption with molar absorptivity greater than 10,000 is high intensity absorption and
transition associated with this absorption is called “allowed”.
n-π* transition: Low intensity absorption.
The absorption with “ε” less than 10,00 and the transition with this absorption is “forbidden”

12. Instrumentation
Spectrophotometer:
An instrument that resolve polychromatic radiation in different wavelength.
Consist of
1. Source (continuous spectra)
2. Monochromator (selecting narrow band of wavelength from source)
3. A sample cell
4. Detector (converting radiant energy to electrical energy)
5. Readout device
Arrangement
Light source Monochromator Sample detector read out device

13. 1. Source:
Continuous sources as these emit radiation of wide bands of wavelength
Tungsten filament lamp Hydrogen or deuterium discharge lamp
* for visible and near IR * UV region (approx 185-375nm)
* 350-2500nm * large amount of continuous radiation

14. 2- Wavelength selection:
a-Filters
b-Monochromators
c-Diffraction grating
a.Filters:
Device that allows to pass through the radiation of certain wavelength while absorbing the
rest partially or wholly.
Two types:-
Two types
Absorption filter Interference Filter
Manufactured from dyed glass Composed of transparent material
Or pigmented gelatin resin. and thin silver film.

15. b. Monochromators
Successfully isolates the band of wavelength much narrower than a filter.
Composition:
•A prism or diffraction grating
(In UV, made of silica or alumina while of optical glass in visible region)
•Lenses
•Entrance and exit slit
Working:
Radiation energy enter through a slit and made parallel by lens, falls on prism. The dispersed
radiation from prism is focused by a second lens and the desired wavelength is centrelized on exit
slit.

16. c. Diffraction grating
Dispersion of radiant energy may also be obtained by passing through a diffraction
grating.
Consist of large number of parallel lines ruled on a highly polished surface such as
alumina.

17. 3- Sample container
Must be transparent to the wavelength
Cells employed in UV-Visible spectrophotometers are usually square cuvettes.

18. 4- Detectors
a- Phototube
Consist of
•An evacuated glass envelope (quartz window)
•A semi cylindrical cathode with inner surface coated with compound loosely bound electrons
( alkali or alkaline earth metals)
•A central metal wire anode
Working
When radiant energy is incident on cathode photoelectrons are
emitted which pass over the anode, where these are collected and
return the external circuit.

19. b- Photomultiplier tube
• For the radiations of small intensity.
•Consist of cathodes(more than one) that causes the amplification of radiation
A photon strikes to a series of electrodes (dynodes) each at a more positive potential
than before (50 to 90V)

21. Single beam and Double beam Instruments
a.Single Beam instruments:

22. b. Double beam instruments.

23. Sample preparation
UV-Vis spectra of compounds are determined in solution form.
•0.1 to 100mg of substance
•0.00001 to 0.01 M concentration
Solvents for making sample:
Choice of solvent should depends on following:
1. High purity i.e. of “spectrograde”
2. Transparent over the desired range of wavelength. Ex water, 95% ethyl alcohol and n-hexane
3. Not react chemically
4. Polarity of solvent i.e. a non polar solvent is preferred. WHY???
Hint: Beer-Lambert law is not
obeyed for complexes

24. Important terms:
Solvent cut off wavelength:
Wavelength below which solvent absorbs too much radiation .
Chromophores:
The functional groups showing absorption in UV-Vis region of spectrum
OR
The presence of one or more unsaturated groups responsible for electronic absorption and giving
colors are termed as chromophores.
Ex: C=C, C≡C, C=N, C≡N, C=O, N=N etc.
Auxochromes:
Substituent that increase the intensity of absorption and possibly the wavelength when attached to
the particular chromophore.
Ex. Methyl, hydroxyl, alkoxy, halogen etc.

25. Important points:
Bathochromic shift (Red shift) :
A shift to the longer wavelength .
Example. Phenol (λmax = 210.5nm)
Phenolate anion (λmax= 235nm)
Hypsochromic shift (blue shift):
A shift of absorption maximum to shorter wavelength
Factors:
Change of medium. For example: Acetone absorbs at 279nm in hexane and 264.5 nm in water.
Hyperchromic Effect:
An increase in absorption intensity
Hypochromic Effect:
A decreased absorption intensity

26. Hyperchromic
Bathochromic
Hypochromic
Hypsochromic
100 800nm
Absorptionintensity

27. How to take UV-Vis spectra
Absorbance & Transmittance displayPower indicator light
Sample
holder
Wavelength
selector
Absorbance & Transmittance control
Power switch
Zero control
Spectronic 20 spectrophotometer

28. How to take UV-Vis spectra
Procedure
1) Power on
2) Select wavelength
3) 0% T adjustment
(Calibration)
4) Blank (Reference cell) is
inserted into cell holder
5) 100% T adjustment
6) Sample cell is placed in
the cell compartment
7) Readout absorbance
8) Power off

29. Applications:
1. Qualitative and Quantitative study:
A. Qualitative:
The formation of coloured solutions
Color that arises from selective absorption in visible region act as
qualitative identification.
For example: in KMnO4
B. Quantitaive:
Quantitative estimation of KMnO4 can be done by taking the known
concentration against absorbance. The value of unknown (KMnO4)
can be determined spectroscopically.

30. 2. Identification of compound:
compairing the spectrum with the standard spectrum samples
3. Quantitative estimation of any compound:
concentration of unknown can be determined
4. Kinetic measurements:
Rate of many reactions that involve the change of absorbing group. Rate of reaction can be
measured by either the decrease ( reactant is absorbing specie) or increase in absorbance as a
function of time.
5. Study of chemical equilibria:
acid base, tautomeric and complex formaation.
6. Stereo chemical studies: