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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

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The cis-trans change in bond geometry accompanying vision

Conjugation, Color, and the Chemistry of Vision