Ultraviolet-visible (UV-Vis) spectroscopy probes electronic transitions between molecular orbitals. The four principal transition types are σ→σ*, n→σ*, π→π*, and n→π*, listed in increasing order of wavelength. σ→σ* transitions require high energy and occur below 200 nm, making them observable only in vacuum UV. n→π* transitions (e.g., carbonyl compounds) occur at longer wavelengths (270-350 nm) with low molar absorptivity, while π→π* transitions are typically more intense and appear across the 160-700 nm range depending on conjugation length.
The UV-Vis spectrum is interpreted primarily in terms of chromophores — functional groups responsible for absorption (e.g., C=C, C=O, NO₂, aromatic rings) — and auxochromes — substituents that shift or intensify absorption without being chromophores themselves (e.g., -OH, -NH₂, -Cl). A bathochromic shift (red shift) moves absorption to longer wavelengths, often caused by increased conjugation or auxochrome substitution. A hypsochromic shift (blue shift) moves absorption to shorter wavelengths, typically due to loss of conjugation or solvent effects. Changes in absorption intensity are termed hyperchromic (increase) or hypochromic (decrease).
Solvent effects provide structural information. Increasing solvent polarity generally causes π→π* transitions to undergo a bathochromic shift (stabilization of the excited state) and n→π* transitions to undergo a hypsochromic shift (stabilization of the lone pair in the ground state). The Woodward-Fieser rules provide empirical methods for predicting λ_max of conjugated dienes and enones based on the parent diene system and substituent contributions. These rules are valuable for structural elucidation of unsaturated compounds.
Band shape and fine structure offer additional interpretation clues. Conjugated systems typically produce broad, featureless bands, while aromatic compounds often display vibrational fine structure (e.g., the benzenoid bands at 184, 202, and 255 nm). The presence of fine structure can help distinguish between similar chromophores in different molecular environments.
Quantitative analysis using UV-Vis relies on the Beer-Lambert law, A = εbc, where absorbance is measured at λ_max — the wavelength of maximum absorption. For multi-component mixtures, simultaneous equations based on absorbances at multiple wavelengths can resolve overlapping spectra, provided the components obey the law of additivity of absorbances. Modern instruments incorporate diode-array detectors for rapid full-spectrum acquisition, enabling derivative spectroscopy and multivariate calibration for complex samples.
