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UV SpectroscopyDr Harisadhan Ghosh

Department of Chemistry, Surendranath College

Introduction; types of electronic transitions, end absorption; transition dipolemoment and allowed/forbidden transitions; chromophores and auxochromes;Bathochromic and Hypsochromic shifts; intensity of absorptions (Hyper-/Hypochromic effects); application of Woodward’s Rules for calculation ofλmax for the following systems: conjugated diene, α,β-unsaturated aldehydesand ketones(alicyclic, homoannular and heteroannular); extended conjugatedsystems (dienes, aldehydes and ketones); relative positions of λmax

considering conjugative effect, steric effect, solvent effect, effect of pH;effective chromophore concentration: keto-enol systems; benzenoidtransitions.

University Syllabus

UV Spectroscopy

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UV Spectrophometer UV Spectrum

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UV spectroscopy involves absorption spectroscopy where molecules interact withUV radiation and produce absorption spectra in the range of 200nm to 400nm.

UV spectroscopy is a molecular spectroscopic method arising due to transition ofvalence electrons in a molecule from the ground state energy (Ee) to the higherexcited state (Ee).

Then the difference in change of energy is ΔE = Ee- Eg = hν

Fundamentals of UV absorption

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Types of Electronic Transition

Energy absorbed in the UV region by valence electrons causes transition from groundstate to excited state.

The valence electrons are exitedfrom bonding to an antibondingorbitals (or HOMO to LUMO)

The energy required for varioustransitions are in the following order.

σ → σ* > n → σ* c→ π* > n→ π*

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ABSORPTION CURVE

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UV Spectra

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Selection Rules for Electronic Transition

Spin Selection Rule-Change of Spin Quantum Number is FORBIDDEN

Transition between same spin states allowed: singlet -> singlet, triplet -> triplet, others are forbidden: singlet -> triplet, doublet -> singlet, etc.

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Selection rules*

1. Spin selection rule S = 0 or MS = 0

(Transition between same spin states allowed: singlet -> singlet, triplet -> triplet, others are

forbidden: singlet -> triplet, doublet -> singlet, etc.)

emax < 1 M‒1cm‒1

spin multiplicity MS = 2S+1S = Ss = n/2 (total spin quantum number)

[Mn(H2O)6]2+

h

Pauli-Principlenot obeyed

S = 5/2 S = 5/2 S = 3/2

S = 1, forbidden

* were developed for metal atoms and ions (where they are rigorously obeyed), not complexes

.. only one electron is involved in any transition

Selection rules*

1. Spin selection rule S = 0 or MS = 0

(Transition between same spin states allowed: singlet -> singlet, triplet -> triplet, others are

forbidden: singlet -> triplet, doublet -> singlet, etc.)

emax < 1 M‒1cm‒1

spin multiplicity MS = 2S+1S = Ss = n/2 (total spin quantum number)

[Mn(H2O)6]2+

h

Pauli-Principlenot obeyed

S = 5/2 S = 5/2 S = 3/2

S = 1, forbidden

* were developed for metal atoms and ions (where they are rigorously obeyed), not complexes

.. only one electron is involved in any transition

Selection rules*

1. Spin selection rule S = 0 or MS = 0

(Transition between same spin states allowed: singlet -> singlet, triplet -> triplet, others are

forbidden: singlet -> triplet, doublet -> singlet, etc.)

emax < 1 M‒1cm‒1

spin multiplicity MS = 2S+1S = Ss = n/2 (total spin quantum number)

[Mn(H2O)6]2+

h

Pauli-Principlenot obeyed

S = 5/2 S = 5/2 S = 3/2

S = 1, forbidden

* were developed for metal atoms and ions (where they are rigorously obeyed), not complexes

.. only one electron is involved in any transition

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If the molecule has a centre of symmetry, transitions within a given set of p or dorbitals (i.e. those which only involve a redistribution of electrons within a givensubshell) are forbidden.

Symmetry Selection Rule

Whenever electrons change molecular orbitals due to excitation, e.g. in UVspectroscopy, the symmetry of the two involved orbitals is the criterion for atransition. We speak of symmetry-allowed and symmetry-forbidden transitions.

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There is Laporte's rule which states that, for any molecule possessing onecentre of inversion, only transitions between g and u or vice versa are allowed.

Take the molecular orbitals of trans-butadiene as a first example. Figure 1 is aschematic representation of the energies of these orbitals.

http://www.pci.tu-bs.de/aggericke/PC4e/Kap_V/Lage_Dipolmoment.htmFor Details see

https://en.wikipedia.org/wiki/Transition_dipole_momentTransition Dipolemoment

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Absorption phenomenon for a,b-unsaturated carbonyl system

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