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3- Notes on Ultra Violet-Visible Spectroscopy.pdf
- 1. UV-Vis Spectroscopy 11-11-2024 1 Dr.Jorphin Joseph Assistant Professor Department of Chemical oceanography CUSAT
- 2. Electronic transitions inpolyatomic molecules 11-11-2024 COD 2 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 -sigma to sigma star: Involves electrons in single bonds -pi to pi star: Involves electrons in double/triple bonds -n to sigma star or pi star: Involves lone pairs (non-bonding electrons) -In charge-transfer complexes, an electron moves from a donor (e.g., ligand or metal) to an acceptor (often a metal or ligand). -intense and easily detectable in UV-Vis because the transition dipole moment is large. If the energy of this light matches the energy gap (delta E) between the molecule’s ground state (HOMO) and excited state (LUMO), electrons absorb the energy and jump to a higher energy level. Not all metals can be analyzed using UV-Vis spectroscopy. Only those with suitable electronic transitions in the UV-Visible range (like many transition and f-block metals) give useful spectra.
- 3. Absorption of electromagneticradiation in UV-Vis region 11-11-2024 COD 3
- 4. Nature of ElectronicExcitations 11-11-2024 COD 4 HOMO LUMO
- 5. Nature of ElectronicExcitations 11-11-2024 COD 5
- 6. Nature of ElectronicExcitations 11-11-2024 COD 6 HOMO LUMO
- 7. Transitions involving σ,πand n electrons
- 8. Transitions involving σ,πand n electrons
- 9. Selection rules forelectronic transitions The n to π* transition is the most common type of forbidden transition. Selection Rules ∆S =0 and ∆l = ∓1 -If an electron has spin +½ before excitation, it must stay +½ after excitation -ie,singlet - Singlet (allowed) Triplet -Triplet (allowed) Singlet - Triplet (not allowed - "spin forbidden" - s -p, p -d, or d -p are allowed (delta L = ±1) - But s - s or d - d are forbidden (delta L = 0)
- 10. MO diagram ofelectronic transitions COD 10 sigma orbital =a bonding orbital formed by the end-to-end overlap of atomic orbitals (like C-C or C-H). sigma star = antibonding orbital (higher in energy). Transition Electron gets excited from the sigma bonding orbital to the sigma star antibonding orbital. Characteristics: Requires very high energy (usually far UV region, less than 200 nm). pi orbital = bonding orbital formed by sideways overlap of p orbitals (e.g., in C=C or aromatic rings). pi star= antibonding orbital. Transition: Electron jumps from pi bonding orbital to pi star antibonding orbital. Characteristics: Occurs in unsaturated compounds like alkenes, aromatics. Strong absorption in UV-Vis region (~200–400 nm). Intense bands in spectra due to allowed transitions. n orbital = nonbonding orbital (lone pairs), e.g., on oxygen, nitrogen, or halogens. sigma star = antibonding sigma orbital. Transition: Lone pair electron on heteroatom (like O or N) is promoted to sigma star orbital. Characteristics: Found in saturated compounds with lone pairs like alcohols, amines. Occurs in the UV region (~150–250 nm). Usually weaker absorption than pi-pi star
- 11. MO diagram ofelectronic transitions in >C=O group COD 11 sigma bonding pi bonding n non bonding pi star anti bonding
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- 15. Presentation of UV-Visspectra COD 15
- 16. Solvents used inUV-Vis Spectra COD 16 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 observed 3.
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- 18. Effect of conjugation 18 Oneof the best ways to bring about a bathochromic shift is to increase the extent of conjugation in a double-bonded system. In the presence of conjugated double bonds, the electronic energy levels of a chromophore move closer together. As a result, the energy required to produce a transition from an occupied electronic energy level to an unoccupied level decreases, and the wavelength of the light absorbed becomes longer. Conjugation of two chromophores not only results in a bathochromic shift but increases the intensity of the absorption. These two effects are of prime importance in the use and interpretation of electronic spectra of organic molecules because conjugation shifts the selective light absorption of isolated chromophores from a region of the spectrum that is not readily accessible to a region that is accessible.
- 19. Effect of conjugationin alkenes 19 CH3-(CH=CH)n-CH3 ultraviolet spectra of dimethylpolyenes (a) n = 3; (b) n = 4; (c) n = 5
- 20. The effect ofconjugation in alkenes COD
- 21. The effect ofconjugation in alkenes COD
- 22. Woodward-Fieser empirical rulefor dienes COD
- 23. Woodward-Fieser empirical rulefor dienes COD
- 24. Woodward-Fieser empirical rulefor dienes COD
- 25. Woodward-Fieser empirical rulefor dienes COD
- 26. Woodward-Fieser empirical rulefor dienes COD
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- 28. The effect ofconjugation in C=O absorption COD
- 29. The effect ofconjugation in C=O absorption COD
- 30. WOODWARD’S RULES FORENONES 11-11-2024 COD 30
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- 36. MO picture ofBenzene 11-11-2024 COD 36
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