Ultraviolet spectroscopy is a technique that measures how molecules absorb UV radiation between 200-400nm. It can be used to identify organic compounds based on their characteristic absorption peaks. UV absorption occurs through electronic transitions between molecular orbitals, such as n-π* and π-π* transitions. The intensity of absorption follows Beer's law, where absorbance is directly proportional to concentration. UV spectroscopy has applications in qualitative and quantitative analysis of pharmaceutical compounds.

1. ULTRAVIOLET
SPECTROSCOPY
(UV- SPECTROSCOPY)
By – Dr. Umesh Kumar Sharma
& Arathy S.A
DEPARTMENT OF PHARMACEUTICS
MAR DIOSCORUS COLLEGE OF PHARMACY
THIRUVANANTHAPURAM, KERALA, INDIA

2. INTRODUCTION
 Spectroscopy is the science that deals with the measurement and
interpretation of Electro Magnetic Radiations (EMR) absorbed or
emitted when the molecules / atom / ions moves from one energy
state to another.
 UV- Spectroscopy is deals with the study of absorption of UV
radiations ranges from 200nm to 400nm. Only colourless
compounds absorb radiation in this UV region.
 Earlier interactions of a electro magnetic radiations with matter
alone were considered in spectroscopy but now interactions
between matter and other forms of energy like acoustic waves,
electron beams etc.

3.  Absorption spectroscopy is widely used for the quantitative
determination of large number of inorganic, organic and
biological species.
 It is based on the measurement of the transmittance or the
absorbance of solutions contained in transparent cell.
 The ultraviolet region is subdivided into two spectral regions.
 Near UV region (2000A 0- 4000A 0 )
 Vacuum UV region (below 2000A 0)

4. THEORY
 UV absorption spectra arise from transition of electrons
from a lower to a higher electronic energy.
 From a bonding molecular orbital to antibonding molecular
orbital.
 For a radiation to cause electronic excitation.
 UV emission spectra arise from reverse type of transition.

5.  The frequency of radiation falling on the substance and energy
difference between various electronic levels is given by the
equation.
E = hv, E= h.C/λ
 Energy absorbed in the UV region produces changes in the
electronic energy of the molecule resulting transitions of valence
electrons in the molecule.
 Three distinct types of electrons are involved in organic
molecules.
 They are σ, n, π electrons.

6. σ - electrons
 These electrons are involved in saturated bonds, such as those
between carbons and hydrogen's in paraffin's. These bonds are
known as σ bonds.
 The amount of energy required to excite electrons in σ bonds
is much more than that produced by UV light.
 Paraffin compounds are frequently very useful as solvents
because they do not absorb UV radiation.
 Ethane

8. π- electrons
 These electrons are involved in unsaturated hydrocarbons.
 Typical compounds with π bonds are trienes and aromatic
compounds.

10. n- electrons
 These are the electrons which are not involved in the bonding
between atoms in molecules
 Organic compounds containing nitrogen, oxygen , or halogens.

12. Types of transitions in organic molecules
 Energy absorbed in the UV region by complex organic
molecules causes transitions of valence electrons in the
molecules. The transitions are n - σ *, σ - σ *, n - π* , π- π*

13. 1. n - π* transitions
 Spectral region of 200 - 700nm
 These type of transition are shown by unsaturated molecules
which contain atoms such as oxygen, nitrogen, and sulphur
Eg: Acetone 279nm
2. σ - σ *transitions
 These transitions can occur in such compounds in which all the
electrons are involved in single bonds and there are no lone pair of
electrons.
 Energy required is very large.
Eg: methane has C-H bonds and its absorption maxima at 125nm

14. 3. n - σ * transitions
 Saturated compounds with lone pair of electrons under go this
transitions.
 Energy required is less than σ - σ * transitions.
 Absorption bands appear at longer wavelength, in the near uv
region 180-200 nm
 Molar absorptivities- 100-3000<cm-1 mol-1
Eg: methanol 183nm
4. π- π* transitions
 The promotion of an electrons from a bonding π orbital to an anti
bonding π* orbital.
 The transition can in principle occur in any molecule having a π
electron system.
 Molar absorptivity – 1000-10000<cm-1 mol-1
Eg: ethylene 175nm ,benzene 254nm

15. •Selection rules are in fact the back bone of spectroscopy and are
obtained from the quantum theory of interaction of radiation with
matter.
•The spectral transitions which obey selection rules are called
allowed transitions and those violate selection rules are called
forbidden transitions
•The Beer-Lambert’s law is applicable to UV radiations also.

17. INSTRUMENTATION:
Source of Light: Spectrum Range 180 -360nm 9UPTO -400)
Hydrogen discharge lamp/ Deuterium Lamp/Xenon Discharge lamp / Mercury
arc.
Monochromator: Grating Monochromators made up of quartz- provide band
pass of 0.4 to 2nm.
Sample cell: Quartz cell only
Solvents: Solvents which neither absorbs in the measurement region nor affect
the absorption of sample are used. Eg- water-191nm, methanol -203nm etc.
Detector: Photomultiplier Tubes – due to high accuracy of measurement.
Recorders: Photo Voltaic cell / Photo Tubes / Photomultiplier tubes.

18. The shape of UV absorption curve-
• The total range of the absorption wavelength may stretch over
10mm.
• UV radiation absorbed in absorption bands rather than at
discrete wavelength.
• An absorption band is made up of numerous absorption lines.
• The effect of the rotational energy of the molecule is to add
more absorption lines to the band the lines even closer together.
• It does not increase the total range of the band ,this is because
the energy involved in rotation is very small compared to
vibrational energy and extremely small compared to electronic
excitation energy.

19. UV spectrum of zinc oxide

20. Hyprochromic shift or blue shift
 A shift of absorption peak towards shorter wavelength
 Peaks associated with n - π* transition are generally shifted.
Eg: aniline in acidic solution
Bathochromic shift or red shift
 A shift of absorption peak towards longer wavelength.
 This shift usually seen in π- π* transitions
Eg: conjugation between the doubly bonded oxygen of
aldehyde, ketones and carboxylic acids.

21. Chromophore
 Is a covalently bonded unsaturated group.
 It has π electrons and n electrons which may under go π- π* and n -
π* transitions.
Auxochrome
 Molar absorbitivities are very large for strongly absorbing
chromophores and very small for weakly absorbing ones.
Eg: ethylenic,acetylenic ,carbonyls, acids ,esters
 They are fully saturated groups with lone pair of electrons.
 When attached to the chromophore shifts the absorption peak
towards longer wavelength and also increase the intensity.

22. LAWS OFABSORPTION
 There are two fundamental laws related to the absorption
LAMBERT’S LAW and BEER’S LAW.
 The lamber’s law states that “ when monochromatic light passes
through a transparent medium, layers of equal thickness of that
homogenous absorbing medium absorb equal proportions of incident
radiations”.
-dI/dt Io ………. 1
-dI/dt =kIo ………. 2
On integration we get
I t=I o e –kt ………. 3
The beer’s law states that “The fraction of the monochromatic
radiant energy absorbed on passing through a solution is
directly proportional to the concentration of the absorber”.
-dI/dc I ………4
-dI/dc= kI ………. 5

23. On integrating
It = I o e –kc ........... 6
Combination equation 3 & 6
It = I o e –kct ..……7
Converting natural logarithm to the base 10
I t /I o = 10 –kct (k= k x 0.4343)
Taking inverse on both side = I t /I o =10 kct
log on both side = log I t /I o =kct
i.e. absorbance A= kct
 The quantity log I t /I o is called absorbance.
 The value of k depends on concentration when c is in moles/
liter the constant is called molar absorptivity.
A = єbc
 Substance with є value less than 100 are weakly absorbing and
more than 10000 are strongly absorbing.

24.  Deviations from beers law
 According to beers law a straight line passing through the origin,
when a graph is plotted between absorbance and concentration.
 Absorption curve usually changes with changes in concentration of
solution and apparent failure of beers law.
 The real limitation of the law is it only describing the absorption
behavior of dilute solutions only.

25. Chemical deviation
 The nature of absorbing units change with concentration.
 Deviation from beers law are frequently encountered as a consequence of
association, dissociation or reaction of the absorbing species with the solvent.
Instrumental deviations
 Use of truly monochromatic beam for absorbance measurements is seldom
practical.
 Polychromatic radiation may lead to deviations from the beers law

27. Applications in Pharmacy:
1. Qualitative analyses. -Detection of Impurities, Structure
elucidation of organic compounds, Structural analysis of organic
compounds.
2, Quantitative analysis:
3. Determination of Molecular weight.
4. Determination of Dissociation constant.
5. Assay of medicinal substances.
6. Chemical kinetics.
7. Keto-enol Tautomerism.

28. REFERENCE
 INTRODUCTION AND THEORY OF UV
SPECTROMETRY
Instrumental methods of chemical analysis, Gurdeep
.R. chatwal, page no. 2.149 - 2.155.
 LAWS OF ABSORPTION
Instrumental analysis, Skoog, page no. 379 – 382,
& V.K Sharma, page no. 71 – 78.

29. THANK YOU
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