10.9 Ultraviolet Spectroscopy Ultraviolet · PDF fileUltraviolet Spectroscopy. 2 Interpreting...
Transcript of 10.9 Ultraviolet Spectroscopy Ultraviolet · PDF fileUltraviolet Spectroscopy. 2 Interpreting...
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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
10.9 Ultraviolet SpectroscopyAbsorption 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
Ultraviolet Spectroscopy
Ultraviolet spectrum of buta-1,3-diene
Ultraviolet Spectroscopy
Amount of UV light absorbed is expressed as the sample’s molar absorptivity (ε), defined by the equation
where A = Absorbancec = Concentration in mol/Ll = Sample pathlength in cm
Ac l
ε×
=
Ultraviolet Spectroscopy
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Interpreting Ultraviolet Spectra: The Effect of Conjugation
Colored organic compounds have extended conjugated systems
• “UV” absorptions extend into the visible region• β-Carotene has λmax = 455 nm
• When white light strikes β-carotene wavelengths in the blue region are absorbed while the yellow-orange colors are transmitted to our eyes
10.11 Conjugation, Color, and the Chemistry of Vision
Ultraviolet spectrum of β-carotene, a conjugated molecule with 11 double bonds
• Absorption occurs in the visible region
Conjugation, Color, and the Chemistry of Vision
β-carotene is converted in the human body to 11-cis-retinal, an essential molecule for vision
Conjugation, Color, and the Chemistry of Vision