Measures absorbance of ultraviolet and visible light by electronic transitions.
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UV-visible spectroscopy measures absorption of ultraviolet and visible light, typically between 200 and 800 nm, by molecules whose electrons are promoted to higher-energy orbitals. Common transitions include pi to pi star in alkenes and aromatic compounds, n to pi star in carbonyl compounds, and d-d and charge transfer transitions in transition metal complexes. Extended conjugation narrows the energy gap between orbitals, shifting the wavelength of maximum absorption, lambda max, to longer wavelengths. The absorbance spectrum is used both to identify conjugated systems and to quantify coloured or UV-absorbing compounds using the Beer-Lambert law. Solvents must be transparent in the wavelength range used.
Key ideas: Report lambda max and molar absorptivity. Woodward-Fieser rules estimate lambda max for dienes: base value 217 nm for a diene plus about 30 nm for each additional conjugated double bond, and about 5 nm for each alkyl substituent. Use quartz cells for UV and a solvent blank to zero the instrument. Colour seen is the complement of the colour absorbed.
Common mistakes and connections: Students think the wavelength seen equals the wavelength absorbed, when it is the complementary colour. UV spectra of organic molecules show broad bands, not sharp lines, because of vibrational and solvent broadening. Solvent polarity can shift peaks (pi to pi star shifts red, n to pi star shifts blue in polar solvents). Related to Beer-Lambert law, conjugation and molecular orbitals.
Beta-carotene's long conjugated chain absorbs strongly near 450 nm with vibrational shoulders, so it appears orange. The interactive lesson shows this as a live animation or chart.
Beta-carotene, with a long conjugated chain, absorbs strongly near 450 nm.
Measures absorbance of ultraviolet and visible light by electronic transitions.
UV-Visible Spectroscopy is a intermediate topic in Analytical, Lab & Spectroscopy. It builds on the vocabulary of beginner topics in this area.
Study these earlier topics in Analytical, Lab & Spectroscopy first: Mass Spectrometry Basics, Molecular Ion Peak, Fragmentation Patterns.
The next topics in Analytical, Lab & Spectroscopy are Beer-Lambert Law and Colorimetry.
Yes. The interactive lesson includes a self-check question on UV-Visible Spectroscopy, and the Analytical, Lab & Spectroscopy quiz draws on every topic in this area.
UV-Visible Spectroscopy is topic 37 of 99 in Analytical, Lab & Spectroscopy.