ELECTRONIC SPECTRSCOPY AND VARIOUS TRANSITION PRINCIPLES

1. CENTRAL UNIVERSITY OF HIMACHAL PRADESH
Department of Chemistry and Chemical Sciences
PRESENTATION TOPIC:-SPIN-ORBITAL & VIBRATIONAL COUPLING.
PRESENTED TO:-Dr.Manish Kumar.
PRESENTED BY:-Kanupriya (M.Sc. Chemistry).
ROLLNO.:- CUHP19CCS11.

2. SPECTROSCOPY
 The branch of science that deals with the investigation and measurement of
spectra produced when matter interacts with or emits electromagnetic
radiations.

3. TYPES OF SPECTROSCOPY

4. ELECRONIC SPECTROSCOPY
 It relies on the quantized nature of energy states
observed in UV region(about 380nm)
 Electronic transitions - only that frequency is
absorbed which matches the frequency of the
energy gap.
 Electrons are excited from one principal quantum
state to another.
 Absorption bands - observed in electronic
spectrum.
IN WHICH
WHICH CAUSES

5. SELECTION RULES
 The absorption of the light radiations by a sample
follows the certain rules known as selection rules.
 These are –
1. Spin selection rules.
2. Orbital selection rules.
 Electronic transitions are allowed when these rules are
followed.
 Intense absorption bands are absorbed when these
rules are followed during the electronic transitions.

6. SPIN SELECTION RULE
 Electronic transition from a lower to
higher electronic energy level.
 When the two energy levels involved in
transition are of same spin multiplicity.
Allowed when

7. LEPORTE ELECTION RULE
Also known as the orbital selection rule.
 It applies to centrosymmetric molecules.
 It states-”electronic transitions that conserve parity either symmetry or antisymmetry with respect to
an inversion centre are forbidden.”
i.e.- g g (Forbidden).
u u (Forbidden).
And g u (ALLOWED) .
u g (ALLOWED). (Change in parity should be there.)

8. RELAXATION IN SELECTION RULES
 The electronic transitions that are forbidden also occurs in nature.
 Weak absorption bands i.e. a band of very low intensity are also absorbed
in spectrum.
 Such transitions in some way becomes partly allowed.
Due to which
CONCLUSION

9. REASONS FOR RELAXATION
 Spin selection rule- relaxation is due to spin
orbital coupling.
 Orbital selection rule-relaxation is due to
vibrational coupling.

10. SPIN-ORBIRAL COUPLING
 It occurs due to spin-orbital interaction.
 Spin orbital interaction occurs because the orbital motion is not completely quenched.
 Electronic configuration gets split into Energy Terms(2s+1)L due to Electrostatic Interactions(STARK
EFFECT).
 Energy Terms gets splitted into Spectroscopic states ( 2s+1Lj ) due to Magnetic Interactions
(because of incomplete orbital motion quenching)(ZEEMAN EFFCT).

12. This coupling increases with increase in the magnetic field & magnetic field increase due
to increased nuclear charge.
As the energy terms of any configuration splits the J states of the ground state energy
level come nearer to the J states of the excited state energy level.
Mixing of the excited state energy levels of ground state with the ground state energy
level of excited state occurs which causes mixing of the spin characters.
 The electronic transitions becomes partly allowed.
This is more effective in the heavier atoms because heavier atoms have great effective
nuclear charges.
Due To which
Under Such Conditions

14. VIBRATIONAL COUPLING
1) Derivation of Leporte selection rule ignores the
vibrational state of a molecule.
2) In actual the vibrations of an atom combines with
the electronic states of a molecule.
3) This is known as VIBRONIC COUPLING
4) This makes the electronic transitions partly
allowed.
5) e.g.- d-d transitions are partly allowed due to
vibrational coupling.

15. Loss Of Centre Of Symmetry(COS) due to
vibrations in an Octahedral Complex

17. Consequences Of Forbidden Transitions
 The absorption bands observed in electronic spectra are not
sharp (i.e. BROAD BANDS are absorbed).
 The bands in spectra are broad due to vibrational and spin
orbital coupling.

18. THANK-YOU