Electronic transitions and chromophores
UV–Visible Spectroscopy · Chemistry
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Study notes
Why is β-carotene orange but ethane colourless? Step 1: Ethane: only σ→σ* (high energy, UV ~150 nm) — no visible absorption. Step 2: β-carotene: 11 conjugated C=C — extended π→π* system. Step 3: Conjugation lowers the π→π* gap → absorbs blue-green (~450 nm). Step 4: Absorbs blue → appears orange (complementary). Conjugation paints!