Interpreting Ultraviolet Spectra The Effect of Conjugation

The wavelength necessary to effect the transition in a conjugated mole- 

Other kinds of conjugated systems, such as conjugated enones and aromatic rings, also have characteristic UV absorptions that are useful in structure determination. The UV absorption maxima of some representative conjugated molecules are given in Table 14.2. 

Problem 14.15 Which of the following compounds would you expect to show ultraviolet absorp- 

The wavelength necessary to effect the tt tt transition in a conjugated molecule depends on the energy gap between HOMO and LUMO, which in turn depends on the nature of the conjugated system. Thus, by measuring the UV spectrum of an unknown, we can derive structural information about the nature of any conjugated tt electron system present in a molecule. 

Why are some organic compounds colored while others aren t j8-Carotene, the pigment in carrots, is purple-orange, for instance, while cholesterol is colorless. The answer involves both the chemical structure of colored molecules and the way we perceive light. 

FIGURE 10.21 Ultraviolet spectrum of/3-carotene, a conjugated molecule with ii double bonds. The absorption occurs in the visible region. 

Conjugation is crucial not only for the colors we see in organic molecules but also for the light-sensitive molecules on which our visual system is based. The key substance for vision is dietary /3-carotene, which is converted to vitamin A by enzymes in the liver, oxidized to an aldehyde called 11-frans-retinal, and then isomerized by a change in geometry of the C11-C12 double bond to produce 11-cis-retinal. 

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The following are data in ethanol [4] which illustrate the effect of lengthening the conjugated system in aliphatic unsaturated compounds Table (13). Kochany and Piotrowska [10] examined the ultraviolet-spectra of a number of nitroalkenes. Their w — tt were observed between 360 and 420 run. The ultraviolet absorption spectra of nitroethylene, nitropropenes and nitro methane have been taken and interpreted with the Pariser-Parr-Pople self-consistent field — MO calculation. The absorption bands it it are in good agreement with calculated values [11]. 

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