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10.9 Ultraviolet Spectroscopy

10.9 Ultraviolet Spectroscopy

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10.9 Ultraviolet Spectroscopy

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  1. 10.9 Ultraviolet Spectroscopy Ultraviolet (UV) region • 4 x 10-7 m to 10-8 m • Region of greatest interest to organic chemists from 2 x 10-7 m to 4 x 10-7 meters

  2. Ultraviolet Spectroscopy Absorption usually measured in nanometers (nm), where 1 nm = 10-9 m • Energy absorbed from UV radiation promotes an electron from highest occupied molecular orbital (HOMO) to the lowest unoccupied molecular orbital (LUMO) MO diagram for buta-1,3-diene

  3. Ultraviolet Spectroscopy Ultraviolet spectrum of buta-1,3-diene

  4. Ultraviolet Spectroscopy Amount of UV light absorbed is expressed as the sample’s molar absorptivity (e), defined by the equation where A = Absorbance c= Concentration in mol/L l= Sample pathlength in cm

  5. Interpreting Ultraviolet Spectra: The Effect of Conjugation

  6. 10.11 Conjugation, Color, and the Chemistry of Vision Colored organic compounds have extended conjugated systems • “UV” absorptions extend into the visible region • b-Carotene has lmax = 455 nm • When white light strikes b-carotene wavelengths in the blue region are absorbed while the yellow-orange colors are transmitted to our eyes

  7. Conjugation, Color, and the Chemistry of Vision Ultraviolet spectrum of b-carotene, a conjugated molecule with 11 double bonds • Absorption occurs in the visible region

  8. Conjugation, Color, and the Chemistry of Vision b-carotene is converted in the human body to 11-cis-retinal, an essential molecule for vision

  9. Conjugation, Color, and the Chemistry of Vision The cis-trans change in bond geometry accompanying vision