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13.20 Ultraviolet-Visible (UV-VIS) Spectroscopy

13.20 Ultraviolet-Visible (UV-VIS) Spectroscopy. Gives information about conjugated p electron systems. Transitions between electron energy states. gaps between electron energy levels are greater than those between vibrational levels gap corresponds to wavelengths between 200 and 800 nm.

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13.20 Ultraviolet-Visible (UV-VIS) Spectroscopy

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  1. 13.20Ultraviolet-Visible (UV-VIS) Spectroscopy Gives information about conjugated p electron systems

  2. Transitions between electron energy states gaps between electron energy levels are greater than thosebetween vibrational levels gap corresponds to wavelengthsbetween 200 and 800 nm DE = hn

  3. Conventions in UV-VIS X-axis is wavelength in nm (high energy at left, low energy at right) lmax is the wavelength of maximum absorption and is related to electronic makeup of molecule— especially p electron system Y axis is a measure of absorption of electromagnetic radiation expressed as molar absorptivity (e)

  4. UV Spectrum of cis,trans-1,3-cyclooctadiene 2000 Molarabsorptivity (e) lmax 230 nm emax 2630 1000 200 220 240 260 280 Wavelength, nm

  5. y4* y3* y2 y1 pÆp* Transition in cis,trans-1,3-cyclooctadiene y4* y3* LUMO DE = hn y2 HOMO y1 Most stable p-electron configuration p-Electron configuration of excited state

  6. pÆp* Transition in Alkenes HOMO-LUMO energy gap is affected by substituents on double bond as HOMO-LUMO energy difference decreases (smaller DE), lmax shifts to longer wavelengths

  7. C C C C Table 13.5 (p 525) Methyl groups on double bond cause lmax to shift to longer wavelengths H H CH3 H H CH3 H H lmax 170 nm lmax 188 nm

  8. C C C C C C Table 13.5 (p 525) Extending conjugation has a larger effect on lmax; shift is again to longer wavelengths H H H H H H H H H H lmax 170 nm lmax 217 nm

  9. H H C C H H C C H H H3C H C C H H H C C H C C CH3 H Table 13.5 (p 525) lmax 217 nm(conjugated diene) lmax 263 nmconjugated triene plus two methyl groups

  10. Lycopene orange-red pigment in tomatoes lmax 505 nm

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