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Applications of UV/VIS

Applications of UV/VIS. Yongsik Lee 2004. 6. 14B absorbing species. Excitation Formation of M* Lifetime 1-10 nsec Experience relaxation Relaxation Photochemical rxn Luminescence heat. Three types of electronic transition. Three types Involving p, s , and n electrons

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Applications of UV/VIS

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  1. Applications of UV/VIS Yongsik Lee 2004. 6

  2. 14B absorbing species • Excitation • Formation of M* • Lifetime 1-10 nsec • Experience relaxation • Relaxation • Photochemical rxn • Luminescence • heat

  3. Three types of electronic transition • Three types • Involving p, s, and n electrons • Involving d and f orbital electrons • Charge transfer electrons

  4. Molecular orbitals (MO) • Sigma orbital • Rotaionally symmetric charge density around the axis of the bond • Pi orbital • Parallel overlap of atomic p orbitals • Nodal plane along the axis of the bond • Maximum density in regions above and below the plane

  5. MO in formaldehyde • Nonbonding electrons • Unshared electrons between atoms • Not participate in chemical bonding

  6. Molecular energy levels

  7. Sigma-sigma* transitions • Requires large energy • Usually in VUV • Not used much in UV/VIS • For C-H bond • Methane = abs max at 125 nm • Ethane = abs max at 135 nm • For C-C bond • Less bonding energy, longer abs wavelength

  8. N-sigma* transitions • Region • 150-250 nm • Table 14-1 some examples of absorption • Bond itself dependent not chemical strucuture of the molecule • Solvent effect • Shift to shorter wavelength in the presence of polar solvents • Water or ethanol

  9. n – pi*, pi-pi* transitions • 200-700 nm • Unsaturated absorbing center required • Ideal for UV-Vis spectrometry of organic chromophore

  10. Spectrum lmax shift

  11. List of simple chromophores • chromophores • only molecular moieties likely to absorb light in the 200 to 800 nm region • pi-electron functions • hetero atoms having non-bonding valence-shell electron pairs. • The oxygen non-bonding electrons in alcohols and ethers do not give rise to absorption above 160 nm. Consequently, pure alcohol and ether solvents may be used for spectroscopic studies. • The presence of chromophores in a molecule is best documented by UV-Visible spectroscopy • but the failure of most instruments to provide absorption data for wavelengths below 200 nm makes the detection of isolated chromophores problematic.

  12. Natural organic pigments

  13. Terminology for Absorption Shifts • each additional double bond in the conjugated pi-electron system • shifts the absorption maximum about 30 nm in the same direction. • Also, the molar absorptivity (ε) roughly doubles with each new conjugated double bond. • extending conjugation generally results in bathochromic and hyperchromic shifts in absorption

  14. Conjugated dienes

  15. Unsaturated ketone • The spectrum of the unsaturated ketone illustrates the advantage of a logarithmic display of molar absorptivity. • The π__>π* absorption located at 242 nm is very strong, with an ε = 18,000. • The weak n__>π* absorption near 300 nm has an ε = 100.

  16. UV/VIS of Aromatoc compound • E2 band • Exhibits very strong light absorption near 180 nm (ε > 65,000) • weaker absorption at 200 nm (ε = 8,000) • B band • a group of much weaker bands at 254 nm (ε = 240) • Only this group of absorptions are completely displayed • because of the 200 nm cut-off characteristic of most spectrophotometers.

  17. Added conjugation of benzene • The added conjugation in naphthalene, anthracene and tetracene -> bathochromic shifts of absorption bands. • All the absorptions do not shift by the same amount • for anthracene and tetracene the weak absorption is obscured by stronger bands that have experienced a greater red shift. • As might be expected from their spectra, naphthalene and anthracene are colorless, but tetracene is orange.

  18. Woodward-Fieser Rules for Calculating the λmax of Conjugated Dienes and Polyenes

  19. UV data sheet

  20. Calculating the π->π* λmax of Conjugated Carbonyl Compounds

  21. Woodward-Fieser Rules

  22. UV data sheet

  23. Physical states & spectra

  24. Inorganic ions • Most transition metal ions are colored (absorb in UV-vis) due to d -> d electronic transitions

  25. Color of the sample • Remember: • Solution absorbs red appears blue-green • Solution absorbs blue-green appears red

  26. Five d orbitals • Electron density distribution of d orbitals • Xy, xz, yz are similar in space (between 3 axes) • X2-y2, z2 are along the axes

  27. Effect of ligand field on d-orbital energies • Ligands cause different interactions with d electrons • ligand field “splitting” theory

  28. Ligand field strength • Ligand field increase -> D increase • The lmax decrease

  29. Homework • 14-1, 14-6, 14-7

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