14.8 Interpreting Ultraviolet Spectra: The Effect of Conjugation

Effects of Conjugation on UV Absorption

Conjugation effects on UV absorption

Conjugated systems have alternating single and multiple bonds, and this arrangement directly affects how molecules absorb UV light. The key idea: conjugation lowers the energy gap between the HOMO (highest occupied molecular orbital) and the LUMO (lowest unoccupied molecular orbital). A smaller HOMO-LUMO gap means the molecule absorbs longer-wavelength, lower-energy light.

• 1,3-butadiene ($CH_2=CH-CH=CH_2$) is a classic conjugated system. It absorbs at a longer wavelength than ethene because the four p orbitals overlap to create a more delocalized $\pi$ system, shrinking the HOMO-LUMO gap.
• As you add more conjugated double bonds, the absorption wavelength keeps shifting longer. This is called a bathochromic shift (or red shift).
• Extended conjugation can push absorption all the way into the visible region. $\beta$-carotene has 11 conjugated double bonds and absorbs blue-violet light (~450 nm), which is why it appears orange.
• The specific group of atoms responsible for light absorption is called a chromophore. When you're reading a UV spectrum, you're really looking at the electronic transitions within the chromophore.

UV absorptions of conjugated systems

Different classes of conjugated molecules have characteristic absorption ranges, mostly due to $\pi \rightarrow \pi^*$ transitions:

• Conjugated enones (conjugated ketones) typically absorb around 215–250 nm. For example, methyl vinyl ketone (3-buten-2-one) absorbs near 224 nm. The carbonyl extends the conjugated system beyond a simple diene, which shifts the absorption.
• Aromatic rings show characteristic absorption patterns. Benzene absorbs near 254 nm (sometimes called the B band). This absorption arises from the cyclic conjugated $\pi$ system.
• Substituents on aromatic rings shift the absorption wavelength:
  • Electron-donating groups ($-OH$, $-NH_2$) cause a red shift (longer wavelength) because they extend electron density into the ring's $\pi$ system
  • Electron-withdrawing groups ($-NO_2$, $-COOH$) can cause a blue shift (shorter wavelength, also called a hypsochromic shift) for certain bands
• Solvent polarity also influences absorption. Polar solvents tend to stabilize excited states differently than ground states, which can shift both the wavelength and intensity of absorption bands.

Predicting UV-active compounds

A practical skill for interpreting UV spectra is recognizing which structural features will produce absorption in the 200–400 nm range:

• Conjugated dienes and trienes: 1,3-butadiene, 1,3,5-hexatriene. More double bonds in conjugation means longer absorption wavelength.
• Aromatic compounds: Benzene, naphthalene, anthracene. Polycyclic aromatic hydrocarbons (PAHs) like anthracene absorb at longer wavelengths than benzene because of their more extended $\pi$ systems.
• Conjugated carbonyl compounds: Enones like methyl vinyl ketone, and conjugated aldehydes like cinnamaldehyde ($PhCH=CHCHO$). The carbonyl group in conjugation with a $\pi$ system creates a chromophore that absorbs in this range.
• Molecules without conjugation generally do not absorb between 200–400 nm. Ethene absorbs around 171 nm, and acetone's weak $n \rightarrow \pi^*$ transition is near 280 nm, but its stronger $\pi \rightarrow \pi^*$ band is below 200 nm. If you see no conjugation in a structure, don't expect a significant UV absorption in the standard measurement range.

Quantitative aspects of UV spectroscopy

UV spectroscopy isn't just qualitative. You can use it to measure concentrations through the Beer-Lambert law:

$A = \varepsilon \cdot c \cdot l$

• $A$ = absorbance (unitless, read from the spectrum)
• $\varepsilon$ = molar absorptivity (also called the extinction coefficient), in units of $L \cdot mol^{-1} \cdot cm^{-1}$
• $c$ = concentration of the sample in $mol/L$
• $l$ = path length of the sample cell in $cm$

Molar absorptivity ($\varepsilon$) tells you how strongly a compound absorbs at a particular wavelength. A large $\varepsilon$ value (e.g., $\varepsilon > 10{,}000$) indicates a strong absorber, which is typical of $\pi \rightarrow \pi^*$ transitions in conjugated systems. Weak absorbers like $n \rightarrow \pi^*$ transitions often have $\varepsilon$ values below 100.

To use this in practice: measure the absorbance at $\lambda_{max}$, look up (or calculate) $\varepsilon$ for that compound at that wavelength, and solve for concentration. This makes UV spectroscopy a straightforward tool for quantifying known compounds in solution.