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

21.
References
1. Sooväli, L.; Rõõm, E.-I.; Kütt, A.; et al. (2006). "Uncertainty sources in UV–Vis
spectrophotometric measurement". Accreditation and Quality Assurance. 11 (5): 246–
255. doi:10.1007/s00769-006-0124-x. S2CID 94520012.
2. ^ reserved, Mettler-Toledo International Inc. all rights. "Spectrophotometry Applications
and Fundamentals". www.mt.com. Retrieved 10 July 2018.
3. ^ Forensic Fiber Examination Guidelines, Scientific Working Group-Materials,
1999, http://www.swgmat.org/fiber.htm
4. ^ Standard Guide for Microspectrophotometry and Color Measurement in Forensic Paint
Analysis, Scientific Working Group-Materials, 1999, http://www.swgmat.org/paint.htm
5. ^ Horie, M.; Fujiwara, N.; Kokubo, M.; Kondo, N. (1994). "Spectroscopic thin film
thickness measurement system for semiconductor industries". Conference Proceedings.
10th Anniversary. IMTC/94. Advanced Technologies in I & M. 1994 IEEE Instrumentation
and Measurement Technology Conference (Cat. No.94CH3424-9). pp. 677–
682. doi:10.1109/IMTC.1994.352008. ISBN 0-7803-1880-3. S2CID 110637259.
6. ^ Sertova, N.; Petkov, I.; Nunzi, J.-M. (June 2000). "Photochromism of mercury(II)
dithizonate in solution". Journal of Photochemistry and Photobiology A:
Chemistry. 134 (3): 163–168. doi:10.1016/s1010-6030(00)00267-7.
7. ^ Mekhrengin, M.V.; Meshkovskii, I.K.; Tashkinov, V.A.; Guryev, V.I.; Sukhinets, A.V.;
Smirnov, D.S. (June 2019). "Multispectral pyrometer for high temperature measurements
inside combustion chamber of gas turbine engines". Measurement. 139: 355–
360. doi:10.1016/j.measurement.2019.02.084.
8. ^ UC Davis (2 October 2013). "The Rate Law". ChemWiki. Retrieved 11 November 2014.