UV-Vis spectrophotometer measure absorption of UV radiation (200-400 nm) and visible light (400-800 nm) by molecule.

2. Ultraviolet spectroscopy (UV) :
UV-Vis spectrophotometer measure absorption
of UV radiation (200-400 nm) and visible light
(400-800 nm) by molecule.
Promotion of electrons to higher energy levels
through irradiation of the molecule with
ultraviolet light. Provides mostly information
about the presence of conjugated π systems
and the presence of double and triple bonds.

3. It is called electronic spectroscopy, because the
energy absorbed (ΔE) is used to induce electron
transition i.e. one electron is promoted from
one molecular orbital to higher energy one.
UV-Visible spectrometry commonly used
because of its simplicity, versatility, speed,
accuracy and cost-effectiveness.

5. Electronic spectrophotometer consist of the following
components:-
1-Radiation source: There are 2 different sources of
radiation;
A- tungsten lamp for scanning in the visible region.
B- hydrogen discharge tube for scanning in the UV
region.
2- Monochromator: which resolve polychromatic light
into a spectrum of monochromatic (i.e. of single
wavelength) light of the entire range.

6. 3- Sample and reference cells: the radiation
source is split into 2 beams;
The reference beam which passes through
reference cell containing solvent only.
Sample beam which passes through sample
solution.
4- Detector and recorder:
record absorbance against wavelength (λ) in nm.
This double beam technique is used to cancel out
solvent absorption between sample and reference
beams.

7. UV-Vis spectrophotometer

8. Solvent and Solution:-
Solvent must be transparent to UV-Vis radiation.
 Common solvents are;
1- Polar solvents: Methanol, ethanol and water.
2- Non polar solvents: Hexane, cyclohexane.
3- Intermediate polarity: Ether and dioxane.
The UV cell (cuvette) is made of quartz (glass
absorb UV light) and has length of 1 cm.

9. The energy of a photon absorbed or emitted during a transition
from one molecular energy level to another is given by the
equation;
therefore, E= hc/λ
The shorter the wavelength, the greater the energy of
the photon and vice versa.

10. The energy supplied by the light will promote
electrons from their ground state orbitals to
higher energy, excited state orbitals or
antibonding orbitals.
Potentially, three types of ground state orbitals
may be involved:
i) σ (bonding) molecular as in
These contain normal bonding pairs of electrons.
The lowest energy level (it is the most stable).

11. ii) π (bonding) molecular orbital as in
These contain normal bonding pairs of electrons.
They are of higher energy than sigma
electrons.
iii) n (non-bonding) atomic orbital as in
Atomic orbitals of hetero atoms (N, O, or S)
which do not participate in bonding, they usually
occupy the highest level of ground state.

12. In addition, two types of anti-bonding orbitals
may be involved in the transition:
i) σ* (sigma star) orbital
ii) π* (pi star) orbital
The following electronic transitions can occur
by the absorption of ultraviolet and visible light:
σ to σ*
n to σ*
n to π*
π to π*

14. In the exited state
 σ electrons occupy an anti-bonding energy level
denotes as σ* and the transition is termed σ to σ*
transition.
 π electrons occupy an anti-bonding energy level
denotes as π* and the transition is termed π to π*
transition.
 n electrons occupy either π* or σ*.

16. Both σ to σ* and n to σ* transitions require a great
deal of energy and therefore occur in the far
ultraviolet region (short λ) or weakly in the
region 180-240nm.
Consequently, saturated groups do not exhibit
strong absorption in the ordinary ultraviolet
region.
Transitions of the n to π* and π to π* type occur in
molecules with unsaturated centers; they require
less energy and occur at longer wavelengths than
transitions to σ* anti-bonding orbitals.

17. Table illustrates the type of transition and the resulting
maximum wavelength.

18. Some terms related to UV-Vis spectrophotometer:
1- Bathochromic shifts:
The shift of absorption to a longer wavelength due
to substitution or solvent effect (red shift)
2- Hypsochromic shifts:
The shift of absorption to a shorter wavelength due
to substitution or solvent effect (blue shift)
3- Hyperchromic effect:
It is increase in absorption intensity.
4- Hypochromic effect:
It is simply a decrease in the absorption intensity.

19. A
Wavelength λmax
Hypsochromic
shift
Bathochromic
shift
Hyperchromic
effect
Hypochromic
shift

20. 5- Chromophore:
It is unsaturated group responsible for
electronic absorption e.g. C = C, C = O, NO2.
6- Auxochrome:
It is a saturated group, which when attached to a
chromophore alter both of wavelength λ and the
intensity of the maximum absorption.
These groups are generally characterized by having
unshared electron pairs e.g. -OH, -NH2, -Cl.

21. Factors affecting on Wavelength λ :-
1- Conjugation:-
E.↓λ↑more conjugated compounds
2- Substitution:-
E.↓λ↑more substituted compounds
 Alkyl group substitution in benzene(λmax =256 nm)ring
cause slight bathochromic shift e.g. toluene has λmax at
261 nm.
Auxochrome e.g. –OH, -NH2, -OR … the interaction of
non-bounding electrons with π electrons of benzene
increase conjugation & cause bathochromic shift e.g.
aniline & phenol.

22. 3- Solvent:-
when the UV-Vis spectrum is recorded in different
solvents, The position and intensity of an absorption
band may shifted. For change to solvents of
increasing polarity the pattern of shifts will be as
follows;
1- Conjugated dienes & aromatic hydrocarbons display
very little solvent effects.
2- α,β- unsaturated carbonyl compounds show 2
different shifts;
a- The π π* band moves to longer wavelength
(bathochromic shift) red shift
b- The n π* band moves to shorter Wavelength
(hypsochromic shift) blue shift

23. Calculation of λmax
The first extensive set of rules was created by R. B.
Woodward and used extensively in the structure
elucidation of natural products.
Each type of diene or triene system is having a certain
fixed value at which absorption takes place; this
constitutes the Base value or Parent value.
The contribution made by various alkyl
substituents or ring residue, double bond
extending conjugation and polar groups such as –
Cl, -Br etc are added to the basic value to obtain
λmax for a particular compound

24. CONJUGATED DIENE CORRELATIONS:
1-Homoannular Diene:-
Cyclic diene having conjugated double bonds in
same ring.
2-Heteroannular Diene:-
Cyclic diene having conjugated double bonds in
different rings.
3-Endocyclic double bond:-
Double bond present in a ring.

25. 4- Exocyclic double bond: -
Double bond in which one of the doubly bonded
atoms is a part of a ring system.
Here Ring A has one exocyclic and endocyclic
double bond. Ring B has only one endocyclic
double bond.

26. i) Base value for homoannular diene = 253 nm
ii) Base value for heteroannular diene = 214 nm
iii) Alkyl substituent or Ring residue attached to the
parent diene = 5 nm
iv) Double bond extending conjugation = 30 nm
v) Exocyclic double bonds = 5 nm
vi) Polar groups:
1- OAc = 0 nm
2- OAlkyl = 6 nm
3- Cl, Br = 5 nm

27. e.g.
Base value = 214 nm
Ring residue = 3 x 5 = 15 nm
Exocyclic double bond = 1 x 5 = 5 nm
λmax = 214 + 15 +5 = 234 nm

28. α, β UNSATURATED CARBONYL COMPOUNDS OR
KETONES:
1. Base value:
a) Acyclic α, β unsaturated ketones = 214 nm
b) 6 membered cyclic α, β unsaturated ketones = 215 nm
c) 5 membered cyclic α, β unsaturated ketones = 202 nm
d) α, β unsaturated aldehydes = 210 nm
e) α, β unsaturated carboxylic acids & esters = 195 nm

29. 2. Alkyl substituent or Ring residue in α position = 10
nm
3. Alkyl substituent or Ring residue in β position = 12
nm
4. Alkyl substituent or Ring residue in γ and higher
positions = 18 nm
5. Double bond extending conjugation = 30 nm
6. Exocyclic double bonds = 5 nm
7. Homodiene compound = 39 nm

30. 8. Polar groups:
a) –OH in α position = 35 nm
–OH in β position = 30 nm
–OH in δ position = 50 nm
b) –OAc in α, β, γ, δ positions = 6 nm
c) –OMe in α position = 35 nm
–OMe in β position = 30 nm
–OMe in γ position = 17 nm
–OMe in δ position = 31 nm
d) –Cl in α position = 15 nm
–Cl in β position = 12 nm
e) –Br in α position = 25 nm
–Br in β position = 30 nm
f) –NR2 in β position = 95 nm

31. Base value = 215 nm
Double bond extending conjugation =1 x 30 = 30 nm
Exocyclic double bond = 5 nm
β- Substituents = 1 x 12 = 12 nm
δ- Substituents = 1 x 18 = 18 nm
λmax = 280 nm
α
β ɤ
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