Ultraviolet Spectroscopy
High-Yield Summary
- UV-Vis spectroscopy measures absorption of ultraviolet/visible light, which is high-energy enough to cause electronic transitions — electrons promoted from a lower orbital (HOMO) to a higher antibonding orbital (LUMO), not bond vibration.
- Two electron types absorb UV light: pi electrons (in double bonds, π→π* transition) and nonbonding electrons (lone pairs on O/N, n→π* transition). The pi-system/group responsible is a chromophore.
- A UV spectrum plots absorbance vs. wavelength; λmax is the wavelength of strongest absorbance (e.g., NAD⁺'s λmax ≈ 260 nm).
- Absorption wavelength depends on the HOMO-LUMO gap: small gap → longer wavelength (lower energy) absorbed; large gap → shorter wavelength (higher energy) needed.
- Conjugation (alternating single/double bonds, delocalized p-orbitals) narrows the HOMO-LUMO gap, letting conjugated systems absorb longer wavelengths — shifting λmax to longer wavelength is a red shift (bathochromic shift).
π→π* vs. n→π* Transitions
| Transition | What's promoted |
|---|---|
| π→π* (pi-to-pi-star) | A pi electron (from a double bond) promoted to an antibonding orbital |
| n→π* (n-to-pi-star) | A nonbonding electron (lone pair, e.g. on O or N) promoted to an antibonding orbital |
Key Terms
- λmax
- The wavelength at which a molecule absorbs UV/Vis light most strongly.
- Chromophore
- The pi-system or functional group in a molecule responsible for its UV absorption.
- HOMO
- Highest occupied molecular orbital — the lower-energy orbital an electron starts in before excitation.
- LUMO
- Lowest unoccupied molecular orbital — the higher-energy orbital an electron is promoted into.
- Red shift (bathochromic shift)
- A shift of λmax toward longer wavelength, caused by increased conjugation narrowing the HOMO-LUMO gap.
Conjugation → Red Shift Logic Chain
- 1More conjugation (more alternating single/double bonds) → more electron delocalization across the system.
- 2Delocalization lowers the energy difference between HOMO and LUMO.
- 3Smaller HOMO-LUMO gap → electron can be excited by lower-energy (longer-wavelength) light.
- 4λmax shifts to longer wavelength — a red shift (bathochromic shift).
Common MCAT Trap
- Don't confuse IR and UV-Vis: IR probes vibrational transitions (bond stretching/bending) via low-energy radiation; UV-Vis probes electronic transitions via higher-energy radiation. Different phenomenon, different spectrum axes entirely.
- A small HOMO-LUMO gap means longer-wavelength absorption, not shorter — small gap = less energy needed = lower-energy (longer-wavelength) photon suffices. Don't invert this relationship.
- More conjugation always shifts λmax longer (red shift), never shorter — non-conjugated molecules absorb at shorter wavelengths by comparison.
Quick Recall
What two types of electrons can absorb UV light, and what are the resulting transitions called?
Molecule A has more conjugation than Molecule B. Which has the larger HOMO-LUMO gap, and which absorbs at longer wavelength?
Why is NAD⁺'s λmax around 260 nm meaningful?