Ultraviolet-Visible Spectroscopy

1. Electronic Transitions

UV-Vis promotes electrons between molecular orbitals. The two most important transitions for organic molecules are:

• π→π\*: an electron moves from a filled pi bonding orbital to an empty pi antibonding orbital; common in alkenes, carbonyls, and conjugated systems, with high molar absorptivity.
• n→π\*: a nonbonding (lone-pair) electron is promoted to a pi antibonding orbital; common in carbonyls and much weaker (low ε) because the orbitals overlap poorly.

2. Chromophores and Auxochromes

A chromophore is the functional group responsible for absorption (for example, C=C, C=O, N=N). An auxochrome is a substituent with lone pairs (for example, –OH, –NH2, –Cl) that, when attached to a chromophore, shifts and intensifies its absorption. Conjugation extends the pi system, shrinking the HOMO-LUMO gap so absorption moves to longer wavelengths (a bathochromic or red shift) and becomes more intense (a hyperchromic effect).

3. The Beer-Lambert Law

Absorbance is directly proportional to concentration, path length, and the compound's intrinsic ability to absorb: A = εb c, where A is absorbance (unitless), ε is molar absorptivity (L·mol⁻¹·cm⁻¹), b is path length (cm), and c is concentration (mol·L⁻¹). This linear relationship is the basis for quantitative analysis.

How it works

1. Dissolve the sample and place it in a cuvette of known path length.
2. Scan the UV and visible regions and record absorbance versus wavelength.
3. Locate λmax and match it to likely chromophores and the extent of conjugation.
4. Use a calibration curve or A = εb c to find the concentration.
5. Interpret the results with awareness of the limitations (solvent effects, overlapping bands, and similar λmax values among different compounds).

Common confusions

Memory aids

"Beer Buys Concentration — Absorbance Equals ε B C." (A = εb c) The law ties absorbance to concentration, and the amount of conjugation sets the wavelength the molecule "buys" (absorbs).

Quick review

Topic Recap

UV-Vis spectroscopy measures electronic transitions (π→π* and n→π*) in chromophores, with auxochromes and conjugation shifting absorption to longer wavelengths. The Beer-Lambert law, A = εb c, converts absorbance into concentration, usually through a calibration curve. The technique is fast and quantitative but limited in structural detail, complementing rather than replacing MS, IR, and NMR. This topic closes Unit 3: the conjugated pi systems that give rise to UV-Vis absorption are the same systems that participate in the pericyclic reactions of Topic 08 and the conjugated diene chemistry of Topic 07.

Knowledge Check

1. Why does increasing conjugation shift λmax to longer wavelengths?
2. In A = εb c, what does each symbol represent?
3. Which transition, π→π* or n→π*, is usually stronger, and why?
4. If a compound absorbs blue light, what color does it appear?
5. What is one limitation of UV-Vis for structure determination?

Answers and Rationales

1. More conjugation spreads the pi electrons over more atoms, lowering the energy of the lowest unoccupied orbital and narrowing the HOMO-LUMO gap; a smaller gap requires lower-energy (longer-wavelength) photons.
2. A = absorbance (unitless), ε = molar absorptivity (L·mol⁻¹·cm⁻¹), b = path length (cm), c = concentration (mol·L⁻¹).
3. π→π* is usually much stronger because the pi and pi* orbitals overlap well, giving a large transition probability; n→π* involves a lone-pair orbital that overlaps the pi* orbital poorly.
4. Orange — the complementary color of blue on the color wheel.
5. It gives only limited structural information: many different compounds absorb at similar wavelengths, and the spectrum cannot reveal the full molecular skeleton, so UV-Vis must be combined with MS, IR, and NMR for structure proof.