1.8: Interpreting Ultraviolet Spectra- The Effect of Conjugation
After completing this section, you should be able to use data from ultraviolet spectra to assist in the elucidation of unknown molecular structures.
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, λ max = 176 nm, with that of 1,3-butadiene, λ max = 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.
| 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.
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).
Looking at UV-Vis Spectra
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.
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 λ max of 524 nm falls within the green region of the spectrum, the compound appears red to our eyes.
Which of the following compounds would you expect to show ultraviolet absorptions in the 200 to 400 nm range?
(a) (b) (c) (d) (e) (f)
- Answer
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All except (a) have UV absorptions.
Which of the following molecules would you expect absorb at a longer wavelength in the UV region of the electromagnetic spectrum? Explain your answer.
- Answer
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Molecule A has a longer system of conjugated pi bonds, and thus will absorb at a longer wavelength. Notice that there is an sp 3 -hybridized carbon in molecule B which isolates two of the pi bonds from the other three.