3- Notes on Ultra Violet-Visible Spectroscopy.pdf

Notes on Ultra Violet-Visible Spectroscopy.pdf

1. UV-Vis Spectroscopy
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Dr. Jorphin Joseph
Assistant Professor
Department of Chemical oceanography
CUSAT

2. Electronic transitions in polyatomic molecules
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HOMO
LUMO
The energy differences between electronic levels in most molecules vary from 150 to
600 kJ/mole (kilojoules per mole).
Absorption of electromagnetic radiation in the
region 200-800 nm

3. Absorption of electromagnetic radiation in
UV-Vis region
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4. Nature of Electronic Excitations
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HOMO
LUMO

5. Nature of Electronic Excitations
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6. Nature of Electronic Excitations
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HOMO
LUMO

7. Transitions involving σ,πand n electrons

8. Transitions involving σ,πand n electrons

9. Selection rules for electronic transitions
The n to π* transition is the most common type of forbidden transition.
Selection Rules
∆S =0 and ∆l = ∓1

10. MO diagram of electronic transitions
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11. MO diagram of electronic transitions in >C=O group
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12. CHROMOPHORE
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13. CHROMOPHORE
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14. AUXOCHROME
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15. Presentation of UV-Vis spectra
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16. Solvents used in UV-Vis Spectra
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1. Solvents should be transparent to UV-Vis
2. Non-polar solvents will not form H-bonds
with the substrate and hence fine structure is
often observed3.

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Solvent Effect

18. Effect of conjugation
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Oneofthebestwaystobringaboutabathochromicshiftisto
increasetheextentofconjugationinadouble-bondedsystem.
Inthepresenceofconjugateddoublebonds,theelectronic
energylevelsofachromophoremoveclosertogether.Asa
result,theenergyrequiredtoproduceatransitionfroman
occupiedelectronicenergyleveltoanunoccupiedlevel
decreases,andthewavelengthofthelightabsorbedbecomes
longer.
Conjugationoftwochromophoresnotonlyresultsina
bathochromicshiftbutincreasestheintensityofthe
absorption.Thesetwoeffectsareofprimeimportanceinthe
useandinterpretationofelectronicspectraoforganic
moleculesbecauseconjugationshiftstheselectivelight
absorptionofisolatedchromophoresfromaregionofthe
spectrumthatisnotreadilyaccessibletoaregionthatis
accessible.

19. Effect of conjugation in alkenes
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CH
3-(CH=CH)
n-CH
3ultraviolet spectra of
dimethylpolyenes(a) n = 3; (b) n = 4; (c) n = 5

20. The effect of conjugation in alkenes
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21. The effect of conjugation in alkenes
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22. Woodward-Fieserempirical rule for dienes
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23. Woodward-Fieserempirical rule for dienes
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24. Woodward-Fieserempirical rule for dienes
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25. Woodward-Fieserempirical rule for dienes
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26. Woodward-Fieserempirical rule for dienes
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27. Problems
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28. The effect of conjugation in C=O absorption
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29. The effect of conjugation in C=O absorption
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30. WOODWARD’S RULES FOR ENONES
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31. Examples
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32. Examples
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33. Problems
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34. Problems
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35. Problems
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36. MO picture of Benzene
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37. Benzene
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46. Investigating photochemical reactions
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47. Conclusions
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Questions…!!