What is UV-Visible Spectroscopy?
UV-visible (UV-Vis) spectroscopy measures the absorption of ultraviolet (200–400 nm) and visible (400–700 nm) light by molecules. Compounds with conjugated π systems absorb these wavelengths, allowing identification of aromatic rings, conjugated double bonds and organic dyes.
UV-Vis spectroscopy detects electronic transitions in conjugated systems: electrons jump from bonding (π) to antibonding (π*) orbitals when absorbing UV/visible photons.
Step-by-step worked examples
Benzene absorbs UV light at ~254 nm. Why does a longer conjugated system like naphthalene absorb at ~280 nm?
Extended conjugation lowers the π to π* orbital energy gap. A smaller energy gap means lower frequency (longer wavelength) light is absorbed. Naphthalene's two fused rings provide more conjugation than benzene's single ring.
A dye shows λmax = 500 nm (green light absorption). Why does it appear red?
The dye absorbs green light (500 nm, ~2.5 eV photons). The colour we see is the complement: red light is not absorbed and is reflected/transmitted. This is why dyes absorb their complementary colour.
Increasing the number of conjugated double bonds shifts λmax to longer wavelengths. Explain.
More conjugation = more π orbitals interacting. The HOMO-LUMO gap shrinks as conjugation extends. Smaller gap = lower energy photons needed = longer wavelengths absorbed. This is why carotenoids (long conjugation) absorb visible light.
Flashcards
Quick quiz
Q1.A compound absorbs at 350 nm. Is this UV or visible region?
Q2.Which has the longest λmax (most conjugation)?
Q3.A compound appears blue. What colour light does it absorb?
Q4.What does a larger extinction coefficient (ε) mean?
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Common mistakes
UV-visible spectroscopy directly shows the molecular structure. — Correct: It shows the presence of conjugation and aromatic systems, but full structure requires NMR or MS.
All conjugated systems absorb at the same wavelength. — Correct: λmax depends on the degree and type of conjugation — different compounds have different λmax values.
A red dye absorbs red light. — Correct: A red dye absorbs its complementary colour (cyan/green) and reflects/transmits red.
Longer wavelengths always mean more conjugation. — Correct: Yes — more conjugation → smaller gap → lower energy photons → longer wavelengths.
FAQ
What is the relationship between λmax and the HOMO-LUMO gap?
E = hc/λ, so a smaller HOMO-LUMO gap (lower E) means a longer wavelength λmax.
Why do carotenoids (orange/red pigments) absorb in the visible region?
They have long conjugated polyene chains (11+ C=C bonds), creating a small HOMO-LUMO gap in the visible range.
Can saturated compounds (no conjugation) be detected by UV-Vis?
No — they have no π to π* transitions. UV-Vis requires conjugated systems or aromatic rings.
What does the Beer-Lambert Law relate?
A = ε·c·l, where absorbance is proportional to molar extinction coefficient, concentration and path length.