UV-Visible spectroscopy involves using electromagnetic radiation in the UV-Visible range to analyze molecules based on their absorption characteristics, which are determined by electronic transitions between molecular orbitals. Different types of transitions like σ→σ*, n→π*, and π→π* occur at different wavelengths and can be used to identify functional groups in compounds. This technique provides information about the structure and bonding of molecules based on their absorption spectra.

1. Evaluation seminar on
Basics of UV/Visble
spectroscopy
By
Mallappa. Shalavadi,
Lecturer,
Department of Pharmacology,
HSK College of Pharmacy,
Bagalkot.

2. Contents:
1. Radiation.
2. Characteristics of electromagnetic radiation.
3. Electromagnetic Spectrum.
4. Visible light.
5. Interaction of radiation with matter.
6. Basic principle of UV/visible spectroscopy.

3. Radiation:
• Radiation is the energy travelling trough space as a
series of waves or a stream of particles.
• Visible light can be explained by two theories
Corpuscular theory and Wave theory.
• Corpuscular theory- light travels in the form of
particles called photons.
• Wave theory- light travels in the form of wave.
• Radiant energy has wave nature and being
associated with electric as well as magnetic fields,
these radiations are called electromag- netic
radiations.
• Ex – Visible light, UV light, Infra-red, X-rays, Radio-waves ect.

4. Characterization of
electromagnetic radiation:
1. These are produced by the oscillation of electric charge and
magnetic field residing on the atom and are perpendicular to
each other.

5. 2. These are characterized by their wavelengths or frequencies
or wavenumbers.
3. The energy carried by an electromagetic radiation is directly
proportional to its frequency.
4. When visible light is passed through a prism, it split up into 7
coloures which correspond to definite wavelengths. This
phenomenon is called dispersion.

6. WHAT IS Wavelength ?
• It is the distance between the two adjacent
crests or troughs in a perticular wave.
• Denoted by λ and expressed in Angsrtom, nm
or milli micrones.

7. Frequency-
• Defined as number of waves which can pass
though a point in one second.
• Denoted by ν (nu) and expressed in cycles per
second or in Hertz (Hz).
1
Frequency α
Wavelength

8. Wave number-
• Defined as the total number of waves can pass
trough a space of one cm.
• It is resiprocal to the wave length. Denoted by
ν and expressed in per cm or cm-1.
1
wave number =
wavelength in cm.

9. Energy-
• Energy of a wave of the perticular radiation
can also be calculated by applying relation:
E = hv = h . c/λ
Wher,
h = Plank’s constant 6.626 X 10-34 Joules sec
v = Frequency of radiation in cycles per
sec.
c = Velocity of light 2.98 X 108 m/sec
λ = Wavelength in mtr.

10. • Calculated in joules/mole which can also be
converted into kcal/mole.
• HOW TO CALCULATE ENERGY FOR PERTICULAR WAVE
LENGTH?
• Ex- calculation of energy associated with radiation
having wave length 200 nm.
E = hv = h . c/λ
h= 6.626 X 10-34
c= 2.98 X 108
Avogadro number N= 6.02 X 1023
E= Nhc/ λ in mtr.

11. 6.626 X 10-34 X 6.02 X 1023 X 2.98 X 108
E=
200 X 10-9
= 6,00,000 J/mole
= 600 KJ/mole
Since 4.1855= 1 K cal,
600
4.1855
= 143 K cal/mole for 200 nm.

12. Electromagnetic spectrum:
• The arrangement of all radiations in order of
their increasing wavelength or decreasing
frequencies is known as complementary
spectrum.
• The portion above visible region is called Infra-
red while that below it is called ultra-violet.
Ultra violet -------- 200-400nm.
Visible--------------- 400-800nm.
IR--------------------- 667-4000/cm or 2.5-15 µ.

14. Relationship between energy, wavelength and
frequency:

15. Visible light:
• The visible spectrum is the electromagnetic
spectrum that is visible to the human eye.
• The longest wavelength is red and the shortest
is violet.

16. • Violet: 400 - 420 nm
• Indigo: 420 - 440 nm
• Blue: 440 - 490 nm
• Green: 490 - 570 nm
• Yellow: 570 - 585 nm
• Orange: 585 - 620 nm
• Red: 620 - 780 nm

17. Relationship between Absorption of radiation
and colors:
Newton’s wheal-
• When white light passes through or is
reflected by a colored substance, a
characteristic portion of the mixed
wavelengths is absorbed. The remaining light
will then assume the complementary color to
the wavelength(s) absorbed.

18. • Here, complementary colors are diametrically opposite each other.
• Thus, absorption of 420-430 nm light renders a substance yellow, and
absorption of 500-520 nm light makes it red.
• Green is unique in that it can be created by absoption close to 400 nm as
well as absorption near 800 nm.

19. Matter:
• All organic compounds are capable of
absorbing ECM radiation because all contain
valency electrons that can be excited to higher
energy levels.
• The electrons that leads to absorption are
a. Sigma electrons (σ): These associated with
the saturated bonds. located in sigma bond
ex- C-C, C-H, O-H, C-N,N-N ect.
b. Pi electrons (π): These electrons are involved
in unsaturated compounds

20. Ex- Alkenes, Alkynes and Aromatic compounds
C=C, C=N, C=O, C=S, ect.
c. Non bonding electrons (n): n electrons are
less firmly held or non bonding electrons and
found on nitrogen, oxygen, sulphur and
halogens.
ex- :O: N: :S: ect.

21. Interaction of matter with radiation:
• electromagnetic radiation interacts with
materials because electrons and molecules in
materials are polarizable
• Types of interactions
• Absorption
• Reflection
• Transmission
• Scattering
• Refraction

22. UV/Visible spectroscopy
• The alternate name for this technique is
Electronic Spectroscopy since it involves the
promotion of electrons from the ground state
to higher energy state.
• This involves the radiations range from 200nm
to 800nm.
• It is absorption spectroscopy.

23. Principle-
• Any molecule has either n, π and σ or a
combination of these electrons .
• These bonding (π and σ ) and non bonding
electrons absorb the characteristic radiation
and undergoes transition from ground state to
excited state.
• By the characteristic absorption peaks, the
nature of the electrons present.

24. TYPES OF ELECTRONIC TRANSITIONS

25. σ → σ* Transitions
• The energy required is large because σ- electrones.
• The transition occurs in mainly in saturated
compounds.
Examples:
Methane -122nm
Ethane -135nm
Propane -135nm
Cyclopropane- 190nm
• Below 200nm O2 and N2 from air absorb thus whol
path is evacuated thus called Vacum UV region.

26. • Why Hydrocarbons are called UV transparent?
• Because they require high energy for
excitation i.e below 200nm.
• Ex- Propane – 135 nm.

27. n → σ* Transitions
• Saturated compounds containing atoms with lone
pairs (non-bonding electrons) are capable of show
n → σ* transitions.
• These transitions usually need less energy than
σ → σ* transitions.
• Examples:
methanol-203
ethanol -204
ccl4 -257
methyl iodide -258
methyl chloride -172-175

28. • Why methyl iodide has loger wave length
compare to methyl chloride?

29. π → π* Transitions
• This type of transition occurs in unsaturated
compounds contain double bonds or triple bonds
and also in aromatics.
• The excitation of π electron requires smaller
energy hence transition occurs at longer
wavelength.
• Mainly in alkenes, alkynes, carbonyl compounds,
cyanides, azo compounds, etc.
• Unconjugated or isolated alkenes-below 200 nm.
and conjugated compounds-above 200 nm.

30. • Examples-

31. n → π*
• In this type of transition, an
electron of unshared electron
pair on hetero atom gets
excited to pi * anti bonding
orbital.
• This requires least energy
hence occurs at longer
wavelength.
• Examples: aldehydes and
ketones

32. References:
Instrumental analysis by B. K. Sharma
Elementary organic spectroscopy by Y. R.
Sharma
www.Google.com