Uv visible spectroscopy absorption methods. INSTRUMENTAL METHODS OF ANALYSIS, B.PHARM 7TH SEM. AND FOR BSC,MSC CHEMISTRY. This is Geeta prasad kashyap (Asst. Professor), SVITS, Bilaspur (C.G) 495001

1. Presented by: Mr. G.p Kashyap
(Asst. Prof.)
SiddhiVinayaka Inst.of Technology &
Sciences(College of Pharmacy) Bilaspur,(C.G)
UV VISIBLE SPECTROSCOPY
ABSORPTION METHODS

2. Introduction
 Absorption spectroscopy in the UV and visible region is
considered as one of the oldest physical methods used for
analysis quantitative & structural elucidation.
 It s the measurement of electromagnetic radiation (EMR)
absorbed or emitted when molecules or ions or atoms of a
sample move from one energy state to another energy state.
 NMR & IR techniques mainly used for structural elucidation
and qualitative analysis.
 UV Spectroscopy concerned the 4000 Ǻ- 2000Ǻ.
 UVVisible Spectroscopy- 8000Ǻ-4000Ǻ.

3. Photometer Vs Spectrophotometer
PHOTOMETER SPECTROPHOTOMETER
 An instrument which
measure the ratio or some
function of the two , of the
radiant power of two
electromagnetic beam.
 Expansive, employing a filter
to isolate narrow
wavelength region. E.g.
Photocell/Phototube.
 This instrument measure
the ratio or a function of
the two , of the radiant
power of two
electromagnetic beam
over the large wavelength
region.
 Monochromators used to
isolate is used instead of a
filter

4. ELECTRO MAGMAGNETIC RADATION
 Electromagnetic radiation is, classically speaking, a wave
of electric and magnetic fields propagating at the speed of
light c through empty space.
 Electromagnetic radiation is a form of energy that
includes radio waves, microwaves,X-rays and gamma rays,as
well as visible light.

5. Properties of EMR
•Wavelength:
Electromagneticwaves
are arrangedon the
electromagneticspectrum
by wavelength.
•All types of electromagnetic
radiationtravel at the speed
of light,but differ in
wavelength.
•The electromagnetic
spectrumincludesgamma
rays,X-rays, ultraviolet,
visiblelight,infrared,
radio and microwaves.
As wavelengthincreases,
frequency decreases.
•There is an inverse
relationshipbetween frequ
ency and wavelength.

6. Radiant
•Radiant energy travels
in straight lines until it strikes an object
where it can be reflected, absorbed,
or transmitted.
•As visiblelight travels
through different media, it undergoes
a change in speed that may result
in refraction.
•Diffraction is when light waves strike
an obstacle and new waves are
produced.
•Interference takesplace when two or
more waves overlap and combine as a
result of diffraction.

7. ELECTRONIC TRANSSITION
 Electronic transitions
 The absorption of UV or visible radiation corresponds to the excitation
of outer electrons.There are three types of electronic transition which
can be considered;
❑ Transitions involving p, s, and n electrons
❑ Transitions involving charge-transfer electrons
❑ Transitions involving d and f electrons
When an atom or molecule absorbs energy, electrons are promoted
from their ground state to an excited state. In a molecule, the atoms can
rotate and vibrate with respect to each other.These vibrations and
rotations also have discrete energy levels, which can be considered as
being packed on top of each electronic level.

8. •Absorbing species containing p, s, and n electrons
Absorption of ultraviolet and visibleradiation in organic molecules is restricted to certain
functional groups (chromophores)that contain valence electrons of low excitation energy.
•The spectrum of a molecule containing these chromophores is complex.
•Thisis becausethe superposition of rotational and vibrationaltransitions on the electronic
transitionsgives a combination of overlapping lines.
•Thisappears as a continuous absorption band.
•Possibleelectronic transitionsof p, s,and n electrons are;

9. An electron in a bonding s orbital is excited to the corresponding antibonding
orbital.The energy required is large.
For example, methane (which has only C-H bonds, and can only undergo σ-
σ* transitions) shows an absorbance maximum at 125 nm. Absorption maxima due
to σ-σ* transitions are not seen in typical UV-Vis.spectra (200 - 700 nm)
σ-σ*Transition
σ-σ*Transition
Saturated compounds containing atoms with lone pairs (non-bonding electrons)are
capable of n-σ* transitions.Thesetransitionsusually need less energy than n-
σ* transitions.Theycan be initiatedby light whose wavelengthis in the range 150 - 250
nm.The number of organic functional groups with n-σ* peaks in the UV region is
small.

10. η→π* and η→π*Transitions
Most absorption spectroscopy of organic compounds is based on transitions
of n or p electrons to the excited state.This isbecausethe absorption peaks for these
transitionsfall in an experimentally convenient region of the spectrum (200 - 700 nm).These
transitionsneed an unsaturated group in the molecule to provide the p electrons.
Charge -TransferAbsorption
Many inorganic speciesshow charge-transfer absorption and are called charge-transfer
complexes.For a complex to demonstrate charge-transfer behaviour,one of its components
must have electron donating properties and another component must be able to accept
electrons.Absorption of radiation then involvesthe transfer of an electron from the donor
to an orbital associatedwith the acceptor.

11. CHROMOPHORES
 CHROMOPHORE:Theterm chromophore was previously
used to denote a functional group of some other structural
feature of which gives a color to compound. For example-
Nitro group is a chromophore because its presence in a
compound gives yellow color to the compound. But these
days the term chromophore is used in a much broader sense
which may be defined as “any group which exhibit absorption
of electromagnetic radiation in a visible or ultra-visible
region“It may or may not impart any color to the compound.
Some of the important chromophores are: ethylene,
acetylene, carbonyls, acids, esters and nitrile groups etc.

12. AUXOCHROMOPHORES
 AUXOCHROMES: It is a group which itself does not act as
a chromophore but when attached to a chromophore, it shifts
the adsorption towards longer wavelength along with an
increase in the intensity of absorption.Some commonly
known auxochromic groups are: -OH, -NH2, -OR, -NHR,
and –NR2.
 For example:When the auxochrome –NH2 group is attached
to benzene ring. Its absorption change from λ max 225
(ɛmax 203) to λmax 280 (εmax1430)

13. ❑All auxochromes have one or more non-bonding pairs
of electrons.
❑If an auxochromes is attached to a chromophore, it
helps is extending the conjugation by sharing of non-
bonding pair of electrons as shown below.
CH2 = CH – NR2 ------------- > CH2-CH-NH2
❑
The extended conjugation has been responsible for
bathochromic effect of auxochromes.

14. Effect of substituent's on the
absorption spectra
 Bathochromic shift (red shift): a shift to lower energy
or longer wavelength.
 Hypsochromic shift (blue shift):a shift to higher
energy or shorter wavelength.
 Hyperchromic effect: An increase in intensity.
 Hypochromic effect: A decrease in intensity.

16. BEER AND LAMBERT’S LAW
 According to the Beer-Lambert Law, absorbance is
proportional to concentration, andAbsorbance versus
concentration plot is a straight line.

17. Cont…