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14.8: Interpreting Ultraviolet Spectra- The Effect of Conjugation

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    31556
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    Objective

    After completing this section, you should be able to use data from ultraviolet spectra to assist in the elucidation of unknown molecular structures.

    Study Notes

    It is important that you recognize that the ultraviolet absorption maximum of a conjugated molecule is dependent upon the extent of conjugation in the molecule.

    The Importance of Conjugation

    A comparison of the UV/Vis absorption spectrum of 1-butene, λmax = 176 nm, with that of 1,3-butadiene, λmax = 292 nm, clearly demonstrates that the effect of increasing conjugation is to shift toward longer wavelength (lower frequency, lower energy) absorptions. Further evidence of this effect is shown below. The spectrum on the left illustrates that conjugation of double and triple bonds also shifts the absorption maximum to longer wavelengths. From the polyene spectra displayed in the right it is clear that each additional double bond in the conjugated pi-electron system increases the absorption maximum about 30 nm. Also, the molar absorptivity (ε) roughly doubles with each new conjugated double bond. Spectroscopists use the terms defined in the table below when describing shifts in absorption. Thus, extending conjugation generally results in bathochromic and hyperchromic shifts in absorption.

     

    conjenyn.gifpolyene.gif

    Terminology for Absorption Shifts

    Nature of Shift

    Descriptive Term

    To Longer Wavelength Bathochromic
    To Shorter Wavelength Hypsochromic
    To Greater Absorbance Hyperchromic
    To Lower Absorbance Hypochromic

     

    Many other kinds of conjugated pi-electron systems act as chromophores and absorb light in the 200 to 800 nm region. These include unsaturated aldehydes and ketones and aromatic ring compounds. A few examples are displayed below. The spectrum of the unsaturated ketone (on the left) illustrates the advantage of a logarithmic display of molar absorptivity. The \(\pi \rightarrow \pi^*\) absorption located at 242 nm is very strong, with an ε = 18,000. The weak \(n \rightarrow \pi^*\) absorption near 300 nm has an ε = 100.

     

    enone2a.gif thioestr0.gif

     

    Benzene exhibits very strong light absorption near 180 nm (ε > 65,000) , weaker absorption at 200 nm (ε = 8,000) and a group of much weaker bands at 254 nm (ε = 240). Only the last group of absorptions are completely displayed because of the 200 nm cut-off characteristic of most spectrophotometers. The added conjugation in naphthalene, anthracene and tetracene causes bathochromic shifts of these absorption bands, as displayed in the chart below. All the absorptions do not shift by the same amount, so for anthracene (green shaded box) and tetracene (blue shaded box) the weak absorption is obscured by stronger bands that have experienced a greater red shift. As might be expected from their spectra, naphthalene and anthracene are colorless (with their absorptions in the UV range), but tetracene is orange (since its absorptions move into the visible range).

    polyarom.gif

    Looking at UV-Vis SpectraEdit section

    Below is the absorbance spectrum of an important biological molecule called nicotinamide adenine dinucleotide, abbreviated NAD+ This compound absorbs light in the UV range due to the presence of conjugated pi-bonding systems.

    UV-vis spectra of NAD plus where the max wavelength is at 260 nanometers.

     

    Below is the absorbance spectrum of the common food coloring Red #3. The extended system of conjugated pi bonds causes the molecule to absorb light in the visible range.  Because the λmax of 524 nm falls within the green region of the spectrum, the compound appears red to our eyes (recalling the color wheel from Section 14.7).

    The max wavelength is at 524 nanometers

     

    Example 14.8.2

    How large is the π - π* transition in 4-methyl-3-penten-2-one?

    Solution
    Example 14.8.3

    Which of the following molecules would you expect absorb at a longer wavelength in the UV region of the electromagnetic spectrum? Explain your answer.

    image035.png

    Solution

     

    Exercise \(\PageIndex{1}\)

    Which of the following would show UV absorptions in the 200-300 nm range?

    14.8.png

    Answer

    B and D would be in that range.

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