INTRODUCTION TO UV-VISIBLE SPECTROSCOPY

BY: MOHAMMED JUNAID KHAN
PhD Scholar
IOP, Pt. RSU
RAIPUR

 The UV-VIS spectrometry is one of the oldest
instrumental techniques of analysis and is the basis for
a number of ideal methods for the determination of
micro and semi-micro quantities of analytes in a
sample.
 UV-VIS spectrum results from the interaction of
electromagnetic radiation in the UV-VIS region with
molecules, ions or complexes. It forms the basis of
analysis of different substances such as, inorganic,
organic and biochemicals.
 These determinations find applications in research,
industry, clinical laboratories and in the chemical
analysis of environmental samples.
9/12/2013UV-VISIBLE SPECTROSCOPY 2

 The absorption of radiation in the UV-VIS region
of the spectrum is dependent on the electronic
structure of the absorbing species like, atoms,
molecules, ions or complexes.
 A given electronic energy level has a number
of vibrational energy levels in it and each of
the vibrational energy level has a number of
rotational energy levels in it.
 When a photon of a given wavelength
interacts with the molecule it may cause a
transition amongst the electronic energy levels
if its energy matches with the difference in the
energies of these levels.

The absorption of radiation in the UV-VIS region of the spectrum
causes the transitions amongst the electronic energy levels.

 In order to obtain a UV-VIS spectrum the sample is
ideally irradiated with the electromagnetic
radiation varied over a range of wavelength.
 A monochromatic radiation i.e., a radiation of a
single wavelength is employed at a time.
 This process is called scanning.
 The amount of the radiation absorbed at each
wavelength is measured and plotted against the
wavelength to obtain the spectrum.
 Thus, a typical UV spectrum is a plot of wavelength
or frequency versus the intensity of absorption.

 The UV spectra of substances are characterised by
two major parameters, namely, the position of the
maximum of the absorption band called λmax,
and the intensity of the bands.
 The λmax refers to the wavelength of the most
absorbed radiation and is a measure of the
difference in the electronic energy levels involved
in the transition.
 The intensity on the other hand is indicative of the
probability of the transition i.e., whether the
transition is allowed or not.
 It is also is a measure of the concentration of the
absorbing species.

 The electronic transitions do not occur in all
the species when a radiation in UV region is
passed through the sample.
 Generally three types of species show the
mentioned transitions, these are organic,
inorganic and complexes formed by
change transfer.

 The wavelength of the radiation absorbed by
an organic molecule is determined by the
difference in energy between the ground state
and the various excited electronic states of the
molecule.
 In organic molecules that the constituent
atoms are bonded through σ and π bonds.
 In addition, these have nonbonding electrons
on the atoms like, N,O,S and halogens etc.
 There are a number of transitions possible
involving the bonding and the nonbonding
electrons.

 As a rule, the transitions occur from the highest
occupied molecular orbital (HOMO) to the lowest
unoccupied molecular orbital (LUMO) in a molecule.
 Of the six possible transitions indicated in the figure,
only the two of the lowest energy ones (n → π ∗ and
π → π ∗ ) can be achieved by the energies available
in the 200 to 800 nm region.
 π → π ∗ transitions:
 These transitions are observed in molecules
containing π electrons and occur at wavelengths
approaching near ultraviolet regions .
 The hydrocarbons containing double and triple
bonds are typical examples. For example, ethylene
absorbs at 170 nm.
 The conjugation of unsaturation further increases λ
max; but-1, 3-diene absorbs at 217 nm.

 The effect of conjugation is quite important in
aromatic molecules. For example single ring
aromatics absorb in the vicinity of 250 nm,
Naphthalene in the vicinity of 300 nm and
Anthracene in the vicinity of 360 nm.
 n →π ∗ transitions:
 These transitions are observed in molecules containing
lone pairs or nonbonding electrons.
 In such transitions one of the nonbonding electrons
may be excited into an empty π ∗ orbital.
 The energies required for these transitions are lower
than that for π →π ∗ transitions and result in the
absorption in the ultraviolet and visible region.
 The presence of atoms or groups containing n =
electrons, can cause remarkable changes in the
spectrum.
 Thus, nitrogen, sulphur and halogens tend to move
absorption to higher wavelengths.9/12/2013UV-VISIBLE SPECTROSCOPY 14

 The π → π ∗ transitions are generally
intense while the n → π ∗ transitions are
weak.
 For example, acetone, exhibits a high
intensity π → π ∗ transition at 195 nm and
a low intensity n → π ∗ transition at 274
nm in its absorption spectrum.

 Only those molecules are likely to absorb
light in the 200 to 800 nm region which
contain π-electrons and may also have
atoms with non-bonding electrons.
 Such groups that absorb light in the UV-
VIS region are referred to as
chromophores.
 Chromophore in Greek means, “colour
bearing”.

INTRODUCTION TO UV-VISIBLE SPECTROSCOPY

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INTRODUCTION TO UV-VISIBLE SPECTROSCOPY