2. Ultraviolet (UV) region :
The term ultraviolet means “beyond violet”
which is derived from a Latin word “Ultra”
meaning beyond. Here the radiation starts at the
blue end of the visible light from 400nm and
ends at 200nm.
Visible region :
The visible region of the electromagnetic
spectrum ranges between 400-780nm.The visible
light is otherwise called as white light or ordinary
light and is composed of different colors as seen
in a rainbow.
3. Spectroscopically, visible light behaves in a
similar way as UV light. Hence, the techniques
of UV spectroscopy and visible spectroscopy are
considered together.
For this reason commercial UV equipment
operates between 800nm and 200nm.
4. On passing EMR in UV -visible regions through a
compound with multiple bonds, a portion of
radiation is normally absorbed by the compound.
The amount of absorption deepens on the
wavelength of radiation and the structure of the
compound.
The absorption of radiation is due to subtraction
of energy from the radiation beam when electrons
in orbitals of lower energy are excited into
orbitals of higher energy and transitions happen
thereby.
Since this is an electron excitation phenomenon,
UV is also called as electronic spectroscopy.
5. Theory of UV Spectroscopy:
Energy absorbed in the UV-visible region
produces changes in the electronic energy of the
molecule resulting from transitions (ground state
to excited state) of valence electrons in the
molecule.
Three distinct types of electrons are involved in
organic molecules. They are as follows:
a) σ electrons
b) π electrons
c) n electrons
6. σ electrons : These electrons are involved in
saturated bonds, such as those between carbons
and hydrogens in paraffins. These bonds as
sigma (σ) bonds. They require much more
energy than UV light to excite the electrons
hence they are frequently used as solvents.
π electrons : These electrons are involved in
unsaturated hydrocarbon bonds. Eg: trienes ,
aromatic compounds.
n electrons : These are the electrons which are
not involved in the bonding between atoms in
molecules. Eg: nitrogen, oxygen, halogens.
7. As n(valence)-electrons can be excited by UV
radiation, any compound that contains atoms
like nitrogen, oxygen, sulphur, halogen.
compounds or unsaturated hydrocarbons may
absorb UV radiation.
Both π and n electrons are less firmly held than
σ electrons. Thus π and n electrons requires
lesser energy, generally valence(n) electrons
requiring less energy than that required by π
electrons.
Thus Energy needed for promoting an
electron follows the order: σ > π> n
8. Different Types of Molecular Orbitals
σ* Anti-bonding
π* Anti-bonding
n*
π
σ
Bonding
Bonding
Non-Bonding
9. Bonding Orbital:
They directly participate in bond formation
between atoms. Electrons of bonding orbital are
represented as σ and π.
Non-Bonding Orbital :
They do not participate in bond formation. Eg :
atoms such as oxygen, sulphur, nitrogen
(valence electrons).
Anti-Bonding Orbital :
They oppose bonding and electrons do not take
part in bonding. Eg : π* and σ*.
10. Absorption Spectroscopy in UV-visible region:
It deals with the separation, detection and
recording of energy changes in nuclei, atom,
molecule or ion due to absorption of certain
wavelengths of UV-visible light.
When absorption takes place it undergoes
electronic transitions.
This technique is employed in pharmaceutical
analysis for quantitative, qualitative and
structural analysis of solvent in solution.
11. Types of electron transitions:
Energy absorbed in the ultraviolet region by complex
organic molecules causes transitions in the valence
electrons in the molecules.
The transitions are :
a) σ---σ* c) n---σ*
b) n---π* d) π--- π*
When selection rules are applied, the σ---σ*, n---σ*,
π---π*,transitions are usually allowed whereas n---π*
transition is forbidden.
Allowed transition :
These transitions occur due to π---π* transition
where €max is 104 or more.
Forbidden transition :
These transitions occur due to n---π* transitions
where €max is less than 104.
12. Selection rules for transition:
1) For transition to take place, the energy of
photons(E) must be equal to the change in
energy(δE) between two electronic energy
levels, E=δE..
2) There should not be any change in the spin of
electron, δS=0.
3) In very rare cases, change in spin is allowed
i.e., in forbidden transition which occurs only
in intramolecular or intermolecular
perturbation.
4) During transition, there should not be any
significant change between inter-nuclear
distance.
13. The energy level scheme for a molecule is shown as :
The energy required for excitation of different
transitions are:
n---π*˂ π---π*˂ n---σ*˂ σ---σ*
14. References :
Instrumental Methods of Chemical
Analysis by G.R. Chatwal and S.K.
Anand, page no: 2.149 to 2.153
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