Ultraviolet-visible Spectroscopy Principle Types of Electronic Transitions MCQs with answers

1. Submitted By:- Kritika Chandel
ELECTRONIC spectrum

2. INTRODUCTION
ULTRAVIOLET – VISIBLE SPECTROSCOPY
 It is the measurement of absorption of a beam of
light after it passes through a sample or after
reflection from a sample surface.
 Absorption measurements can be at a single
wavelength or over an extended spectral range.
 Spectrum which results due to energy changes in
electronic, vibrational, rotational levels is called
absorption spectrum.

3. PRINCIPLE
 It is based on the absorption of ultraviolet light or visible
light by chemical compounds, which results in the
production of distinct spectra.
 When the matter absorbs the light, it undergoes
excitation and de-excitation, resulting in the production of
a spectrum.

4. BEER LAMBERT LAW
 Beer’s Law :- The intensity of a beam of
monochromatic light decreases exponentially with
the increase in concentration of the absorbing
substance.
 Lambert’s Law :- When a beam of light is allowed to
pass through a medium, the rate of decrease of
intensity with the thickness of medium is directly
proportional to the intensity of the light.

5. As per the Beer-Lambert law, the greater the number
of absorbing molecules (that have the ability to
absorb light of a specific wavelength), the greater the
extent of absorption of the radiation.

6. PRESENTATION OF THE
SPECTRUM
 In UV spectrum, energy required for electronic transition
is higher than Infrared and Microwave.
 Electronic transitions tend to accompany both rotational
and vibrational transitions and that’s why its spectrum
has broad band.

8. POSSIBLE ELECTRONIC TRANSITIONS

9. SELECTION RULES
In electronic spectroscopy there are two selection rules
which determine whether or not transitions are formally
allowed.
1) Spin selection rule: ∆S = 0
Allowed transitions : Singlet —› Singlet or Triplet —›
Triplet
Forbidden transitions: Singlet —› Triplet or Triplet —›
Singlet

10. 2) Symmetry selection rule: It says that transitions
between states of the same symmetry with respect to
inversion are forbidden.
Symmetry allowed transitions : g —› u or u —› g
Symmetry forbidden transitions : g —› g or u —›
u
 g stands for gerade – Compound with a centre of
symmetry
 u stands for ungerade – Compound without a centre of
symmetry

11. ACTUAL ELECTRONIC TRANSITIONS

12. pi bonding and pi antibonding

13. Sigma bonding and
sigma antibonding

14. TYPES OF ELECTRONIC TRANSITIONS
 σ ⇾ σ* Transition :An electron in a bonding σ orbital of
a molecule is excited to the corresponding anti-bonding
orbital by the absorption of radiation. To induce a σ ⇾ σ*
transition it required LARGE ENERGY.
 Ex: Methane contain only single C-H bonds, it undergo
only σ ⇾ σ* transition only, it gives absorption maximum
at 125nm.

15.  n⇾ σ* transition : In this type, compounds containing
atoms with unshared electron pairs are undergo n ⇾ σ*
transition. It require less energy than the σ ⇾ σ* type.
 Ex: Methyl chloride, Oxygen, Nitrogen. Most of the
absorption peaks appearing below 200nm.

16.  π ⇾ π* transitions : The transitions in which a pi
electron is excited to antibonding pi orbital and occur in
unsaturated compounds and aromatic rings.
 Ex : Ethylene, Propene, Propyne, Carbonyl.
 n ⇾ π* transitions : In this transition, unsaturated
compounds containing unshared pair of electrons
undergo n to π∗ transitions. Out of all transitions, this
transition requires minimum energy and hence occurs
at longer wavelength.
 Ex: C=O, C=S and N=O
 The energy requirement order for excitation for different
transitions is as follows.
n→π*< π→π*< n→σ*< σ→σ*

17. Related Questions
1) The types of transitions possible in UV-visible region
for compound with molecular formula C 2H 4O are
 σ ⇾ σ*
 n⇾ σ*
 π ⇾ π*
 n ⇾ π*
2) The units for absorbance are
(A) L mol -1 cm -1
(B) g mol -1 cm -1
(C) Mol. L -1.cm =1
(D) None of the above

18. 3) Energies required for the following transitions in
increasing order
P. σ ⇾ σ* Q. n⇾ σ* R. π ⇾ π* S. n
⇾ π*
 P<Q<R<S
 Q<S<R<P
 S<R<Q<P
 P<S<R<Q
4) The possible transitions in UV-visible region for this
compound will be
 σ →σ *
 n→π *, π→π *
 σ→σ *, n→π *

19. 5) Which of the following statement is not correct ?
(A) Absorptivity changes with the intensity of the light
(B) Absorbance is independent of intensity of the light
(C) Absorbance has no units
(D) Absorptivity is a constant and depends on the nature
of the molecule

20. ANSWERS
1. π ⇾ π* and n ⇾ π*
2. Absorbance depends on the nature of the solution and
thus has no unit.
3. The energy requirement order for excitation for
different transitions is as follows.
n→π*< π→π*< n→σ*< σ→σ*
4. n→π *, π→π *
5. Absorbance is an intrinsic property.

21. THANK YOU