1. UV-VIS SpectroscopyUV-VIS Spectroscopy
 An obvious difference between certain compounds is their
color.
 The human eye is functioning as a spectrometer analyzing the
light reflected from the surface of a solid or passing through a
liquid.
 Although we see sunlight (or white light) as uniform or
homogeneous in color, it is actually composed of a broad
range of radiation wavelengths in the ultraviolet (UV), visible
and infrared (IR) portions of the spectrum.

2. • The component colors of the visible
portion can be separated by
passing sunlight through a prism,
which acts to bend the light in
differing degrees according to
wavelength.
• Visible wavelengths cover a range
from approximately 400 to 800 nm.
The longest visible wavelength is
red and the shortest is violet.
• The wavelengths of what we
perceive as particular colors in the
visible portion of the spectrum are
displayed
Violet: 400 - 420 nmViolet: 400 - 420 nm
Indigo: 420 - 440 nmIndigo: 420 - 440 nm
Blue: 440 - 490 nmBlue: 440 - 490 nm
Green: 490 - 570 nmGreen: 490 - 570 nm
Yellow: 570 - 585 nmYellow: 570 - 585 nm
Orange: 585 - 620 nmOrange: 585 - 620 nm
Red: 620 - 780 nmRed: 620 - 780 nm

3. Color wheel: 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.
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 wavelengths
absorbed.

4. • Electromagnetic radiation such as visible light is commonly
treated as a wave phenomenon, characterized by a
wavelength or frequency.
• Wavelength is the distance between adjacent peaks, and
may be designated in meters, centimeters or nanometers (10-
9 meters).
• Frequency is the number of wave cycles that travel past a
fixed point per unit of time, and is usually given in cycles per
second, or hertz (Hz).

5.  The visible spectrum constitutes but a small part of the total
radiation spectrum.
 This electromagnetic spectrum ranges from very short
wavelengths (including gamma and x-rays) to very long
wavelengths (including microwaves and broadcast radio
waves). The following chart displays many of the important
regions of this spectrum, and demonstrates the inverse
relationship between wavelength and frequency.
The Electromagnetic SpectrumThe Electromagnetic Spectrum

6. The energy associated with a given segment of the spectrum is
proportional to its frequency.

7. λ* ν = c
∆E = hν
ν = frequency
λ = wavelength
C = velocity of light ( 3 x 1010
cm / sec )
E = energy
h = Planck’s constant (6.626068 × 10-34
m2
kg /
s )
Wavelength * frequency =velocity

8. Interaction between light and matter:Interaction between light and matter:
When radiation interacts with matter a number of processes
can occur including:
 reflection
 scattering
 absorbance
 fluorescence/phosphorescence (absorption and re-
emission)
 photochemical reaction

9. When measuring UV-Visible spectra we want only the
absorbance process to occur. Any of the other processes
will adversely affect the precision of our measurements.

10. Electronic SpectraElectronic Spectra
Absorption occurs due to the fact that all molecules
possess electrons which can be exited or raised to
higher energy level. Many can be excited by radiation of
UV or visible wavelengths but others are only excited by
vacuum UV.

11. Electronic SpectraElectronic Spectra
 The amount of energy a molecule can have in each
form is not a continuum but a series of levels or
states.
 The photons of UV and Visible light have sufficient
energy to cause transitions between the different
electronic energy levels of bonds (double bonds) in
molecules, whereas photons of infra-red radiation
have sufficient energy to cause transitions between
vibration energy levels.

12. The Energy difference
between the electronic energy
levels is given by the equation:
∆E = h ν
The absorption of energy
excites the electron from the
ground state to an electronic
excited state
Any change in electronic
energy is accompanied by
vibration and rotational
energy levels.

13. Chemical Structure and UVChemical Structure and UV
AbsorbanceAbsorbance
The absorbance energy depends on the nature of the bonds within a
molecule. Electrons in organic molecules may be in :
Strong σ bond ,weaker π bond or non bonding n. When energy is
absorbed all these types of electrons can be elevated to excited anti-
bonding states , represented with an asterisk:
Energy
σ
π
n
π∗
σ∗

14. Conjugation promotes λmax.
Example of Ethylene and Butadiene
Comments on possible Sigma transitions
Comments on possible Pi-transitions
Comments on n-transitions

16. Basic Terminologies
Batho-chromic Shift
shifting to a longer wavelength
Hypo-chromic Shift
shifting to a shorter wavelength
Hyper-chromic effect
increase in intensity of absorption
Hypochromic effect
decrease in absorption intensity

17. Some Chromophores and the wavelengths of theirSome Chromophores and the wavelengths of their
absorbance maxima.absorbance maxima.
Chromophore Formula Example (nm)
Carbonyl (ketone) RR'C=O Acetone 271
Carbonyl (aldehyde) RHC=O Acetaldehyde 293
Carboxyl RCOOH Acetic Acid 204
Amide RCONH2 Acetamide 208
Ethylene RCH=CHR Ethylene 193
Acetylene RC=CR Acetylene 173
Nitrile RC=N Acetonitrile <160
Nitro RNO2 Nitromethane 271
*The position of the absorbance maximum of a chromophore is not fixed, as it
is to some extent dependent upon the molecular environment of the
chromophore.

18. Spectroscopy is the study of the absorbance and emission
of electromagnetic radiation (light) by matter.
The collection of frequencies absorbed by a sample is its
absorption spectrum.
SpectroscopySpectroscopy

19. AbsorbanceAbsorbance
I0 Incident radiation I Transmitted radiation
Absorbance = log (I0 / I)
path-length
sample
Transmittance = 100* ( I/ I 0 )

20. transmittance (T = I/I0
) or
absorbance (A= log I0
/I).
If the sample compound does not absorb light at a specific
wavelength, I(λ) = I0
(λ)
I=I0 T = 1 and A = 0
If the sample compound absorbed light, I(λ)< I0
(λ)
I<I0 T< 1 and A > 0

22. Beer's law :Beer's law :
The amount of light absorbed is proportional to the number of
absorbing molecules through which the light passes.
The result of plotting absorbance against concentration is
shown below.
A=A= εε b cb c

23. The extinction coefficientThe extinction coefficient
Is a characteristic of a given substance at a specific
wavelength.
A= absorbance,
C = sample concentration in
moles/liter
b = length of light path through the
cuvette in cm
A=A= εε b cb c ; ε; ε == Α/Α/ b Cb C

26. Limitations to Beer’s Law
Fundamental dilute concentration refractive index
Chemical Equilibrium
Instrumental Stray radiation

27. UV-Visible InstrumentationUV-Visible Instrumentation
• Several types of Spectrometer
Single Beam Spectrophotometer
Double Beam Spectrophotometer

28. InstrumentationInstrumentation
 Light source
– Deuterium and
hydrogen lamps
– W filament lamp
– Xe-arc lamps
 Sample containers
– Cuvettes
 Plastic
 Glass
 Quartz
 Filter:
– the monochromator
select a narrow
portion of the
spectrum of a given
source
 Detector
– Photovoltaic
– Phototube
– Photomultiplier

29. Components of a SpectrophotometerComponents of a Spectrophotometer
Light SourceLight Source
Deuterium Lamps － a truly continuous
spectrum in the ultraviolet region is produced
by electrical excitation of deuterium at low
pressure. (160nm~375nm)
Tungsten Filament Lamps － the most
common source of visible and near infrared
radiation.

30. Prepare the StandardsPrepare the Standards
The concentration and volume of the stock
solution
added should be chosen to increase the
concentration of the unknown by about 30% in
each succeeding flask.

31. Application of UV-VisibleApplication of UV-Visible
SpectroscopySpectroscopy
 In structure determination : UV-VIS spectroscopy is used
to detect the presence of chromophores like dienes,
aromatics, polyenes, and conjugated ketones, etc.
 Number and location of substituents
 Qualitative analysis and purification control in dye stuff
 Calculation of dissociation constant
 Study kinetics of fast reactions.
 Structure elucidation in vitamins