UV spectroscopy is a technique used to analyze the composition of a sample by measuring its absorption or reflection of ultraviolet light. The sample is placed in a UV spectrophotometer and exposed to a range of UV wavelengths. The amount of light absorbed or reflected at each wavelength is recorded and plotted as a UV spectrum. This spectrum can be used to identify specific compounds in the sample, as each compound absorbs or reflects light at different wavelengths. This technique is widely used in fields such as chemistry, biology, and environmental science to analyze a variety of samples such as drugs, food, and water.

1. MSC CHEMISTRY
ULTRAVOILET SPECTROSCOPY
BY
FARAH KHAN

2. PRESENTATION TOPIC:
WHAT TO LOOK FOR IN
ULTRAVOILET SPECTRUM?

3. Ultraviolet Spectroscopy
o UV spectroscopy involves absorption spectroscopy where molecules interact with UV
radiation and produce absorption spectra in the range of 200nm to 400nm.
o The UV range in electromagnetic spectra is subdivided into two.
Far or Vacuum UV region; 10nm-200nm
Near OR QUARTZ UV region; 200nm-400nm
o UV spectroscopy is a molecular spectroscopic method arising due to transition of
valence electrons in a molecule from the ground state energy (E) to the higher state (E1).
o Then the difference in change of energy is
E1-E2 = hv

4. What is ultraviolet radiation?
 Ultraviolet (UV) is a form of electromagnetic
radiation with wavelength from 10 to 400 nm shorter than that
of visible light, but longer than X-rays.
 UV radiation is present in sunlight, It is also produced by electric
arcs and specialized lights, such as mercury-vapor
lamps, tanning lamps, and black lights.

5. Principle of UV spectroscopy:
 UV spectroscopy measure the response of a sample to UV rang of
electromagnetic radiation.
 1. In UV spectroscopy, the sample is irradiated with the broad spectrum
of the UV radiation
 2. If a particular electronic transition matches the energy of a certain
band of UV, it will be absorbed
 3. The remaining UV light passes through the sample and is observed
 4. From this residual radiation a spectrum is obtained with “gaps” at
these discrete energies – this is called an absorption spectrum

6. o Absorption Spectra
When sample molecules are exposed
to light having an energy that
matches a possible electronic
transition within the molecule, some
of the light energy will be absorbed
as the electron is promoted to a
higher energy orbital.
The significant features:
1) Λ max
2) ε max intensity of maximum
absorption)

7. UV SPECTRUM OF ISOPRENE SHOWING
MAXIMUM ABSORPTION AT 222nm

8.  Energy absorbed in the UV region by valence electrons causes
transition from ground state to excited state.
 The valence electrons are excited from bonding to an antibonding
orbitals.
 The energy required for various transition are in the following order
 n→∏* < ∏→∏* < n→σ* < σ→σ*

9. TYPES OF ELECTRONS ARE INVOVLED IN UV
SPECTROSCOPY
σ electrons
 These electrons are involved in saturated bonds.
 The amount of energy required to excite electrons are high.
 σ Electrons do not absorbed near UV region, but absorbed in far UV
region.
Π electrons
 These electrons are involved in unsaturated compounds.
 Typically, compounds with π bonds are trienes and aromatic
compounds
n electrons
 These electrons are not involved in bonding between atoms in
molecules.
 So it is called not bonded electrons
 Eg organic compounds containing nitrogen oxygen or halogens.

10. Theory of UV Spectroscopy
:
 UV visible absorption spectra originate from electronic transitions
 These transitions involving promotion of valence electrons
 Since various energy levels of molecules are quantized,

11. [1] σ - σ * transition
The transition or promotion of an electron from a
bonding sigma orbital to the associated antibonding
sigma orbital is σ - σ * transition. It is a high energy
process because σ bonds are generally very strong
and absorption band occurs in the far UV region
(125_135nm).Eg Saturated Hydrocarbons.
 EXAMPLE:
Methane(125nm)
Ethane(135nm)
Cyclopropane(130nm)

12. [2] n - σ * transition
Transition or promotion of an electron from a non-
bonding orbital to an antibonding sigma orbital is
designated as n - σ * transition. Compounds
containing non bonding electrons on a heteroatom
are capable of absorption due to n - σ *
Transitions. These transitions require lower energy
than σ- σ* transitions.
 EXAMPLE
Saturated Hydrogen containing with
unshared electyrons pairs (Oxygen,
Nitrogen,Sulphur,or Halogen) OR non
bonding electrons are capable of n - σ *
transition.

13. [3] π- π* transition
The transition or promotion of an
electron from a π bonding orbital to a
π antibonding orbital is designated π-
π* transition.
 EXAMPLE
These type of transitions occur in
compounds containing
Alkene,carbonyl compound and aromatic
compounds. one or more covalently
unsaturated groups like C=C,C=O,NO2 etc.,
π- π* Transitions require lower energy than
n - σ * transitions.

14. Aromatic compounds show a number of bands.
B benzenoid (B) band:
 This band is due to π- π* Transitions in aromatic or hetro
aromatic molecules .
 The benzene shows a broad absorption band containing
peak between 230-270nm.
 When a chromophoric group is attached to benzene ring,
the B-band are observed at longer wavelength.
Compound Transition λ max 'ε max
Benzene π-π* 255 215
Phenol π-π* 270 1450

15. Ethylenic (E) band:
 This band is due to the electronic transition in the
benzenoid system of three ethylenic bonds are in closed
cyclic conjugation
 These are further characterized as E1 and E2 bands.
 The E1 band of benzene , which appears at lower
wavelength (184nm )
 Is more intense than E2 band occurs at longer
wavelength (204 nm).
compound E1 Band E2 Band E1 Band E2 Band
λ max Ε max λ max Ε max
Benzene 184 50,000 204 79,000
Naphthalene 221 133,000 286 9,300

16. K band:
 This band exhibited by aromatic compounds with the
benzene ring directly attached to a group containing
multiple bond.
 Eg: Styrene , Benzaldehydes etc.
Compound Transition λ max Ε max
Acetophenone π- π* 240 13,000
1,3-Butadiene π- π* 217 21,000

17. [4] n - π* transition
 The transition or promotion of an electron from a
non-bonding orbital to a π antibonding orbital is
designated n - π*. This transition reqires lowest
energy.
 These transition is exhibit a weak band in their
absorption spectrum.
 The peak due t this transition is called R band .
 In aldehyde and ketones the band due to this
transition occur in the rang of 270 to 300nm.
 In aldehydes and ketones this arises from excitation
of a lone pair of electrons in the 2py orbitals of the
oxygen atom into the antibonding orbitals of the
carbonyl group
 When hydrogen is replaced by methyl group in an
aldehyde group , this result in a shift of band due to
the shorter wavelength.

18. R Band:
 R-Band transition originate due to n-π* transition of a single
chromophoric group and having at least one lone pair of
electrons on the hetero atom
 These are less intense with ε max value below 100.
COMPOUND Transition Λ max
(nm)
Ε max
Acetone n-π* 270 15
Acetone n-π* 293 12

19. Formation of Band or
Why Band Form?
 The spectrum consist of sharp peaks and each peak will
correspond to the promotion of electron from one
electronic level to another.
 During promotion, the electron moves from a given
vibrational and rotational level within one electronic
mode to the other within the next electronic mode.
 Thus, there will be a large no of possible transitions
Hence, not just one but a large no. of wavelengths which
are close enough will be absorbed resulting in the
formation of bands .

20. APPLICATIONS OF UV:
 Detection of Impurities
 Structure elucidation of organic
compounds
 quantitative determination of
compounds that absorb UV radiation.
 Kinetics of reaction can also be studied
using UV spectroscopy.
 Molecular weight determination.
 As HPLC detector

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