Ultraviolet (UV) region 4 x m to m Region of greatest interest to organic chemists from 2 x m to 4 x meters 10.9 Ultraviolet Spectroscopy

Absorption usually measured in nanometers (nm), where 1 nm = 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 (  ), defined by the equation where A = Absorbance c = Concentration in mol/L l = Sample pathlength in cm Ultraviolet Spectroscopy

Interpreting Ultraviolet Spectra: The Effect of Conjugation

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

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

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

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