Ultraviolet Spectroscopy

Ultraviolet-visible (UV-Vis) spectroscopy studies how molecules absorb ultraviolet or visible light. This light carries more energy than infrared radiation, and instead of making bonds vibrate, it excites electrons — promoting them from a lower-energy orbital to a higher-energy one. That electron promotion is called an electronic transition, and it's the second of the three spectroscopic techniques this chapter covers, alongside infrared (vibrational transitions) and NMR (nuclear spin transitions).

Key Takeaways

• UV-Vis spectroscopy measures absorption of ultraviolet or visible light, which causes electronic transitions — electrons promoted from a lower-energy orbital to a higher-energy antibonding orbital.

• A UV spectrum plots absorbance vs. wavelength; λmax is the wavelength of maximum absorbance (e.g., NAD⁺'s λmax is around 260 nm).

• Pi electrons (in double bonds) and nonbonding electrons (lone pairs) can absorb UV light and undergo electronic transitions — named π→π* and n→π* transitions, respectively.

• Absorption wavelength depends on the HOMO-LUMO gap: a small gap allows absorption of longer, lower-energy wavelengths; a large gap requires shorter, higher-energy wavelengths.

• Conjugation (alternating single/double bonds, delocalized p-orbitals) lowers the HOMO-LUMO gap, letting conjugated systems absorb lower-energy, longer-wavelength light.

• More conjugation shifts λmax to longer wavelength — a red shift or bathochromic shift.

Collecting a UV Spectrum

To collect a UV spectrum, ultraviolet light is shone through a sample. The sample is usually dissolved in a clear solvent that doesn't absorb UV light itself, so that any absorbance measured comes only from the molecule being studied. As the light passes through the sample, the instrument records how much light is absorbed at each wavelength.

Reading a UV Spectrum: Absorbance and Lambda Max

The result is a UV spectrum — a graph of absorbance (y-axis) versus wavelength (x-axis). One key feature of this spectrum is lambda max (λmax): the wavelength at which a molecule absorbs the most light.

For example, NAD⁺ (nicotinamide adenine dinucleotide) has a λmax around 260 nm — meaning 260 nm is the wavelength where it absorbs most strongly.

Which Electrons Absorb UV Light

Absorption happens because of the electrons in a molecule. Two types of electrons can absorb UV light:

• Pi electrons — the electrons found in double bonds.

• Nonbonding electrons — lone pairs, such as those on oxygen or nitrogen.

When either type absorbs UV light, the electron is promoted to a higher-energy orbital called an antibonding orbital.

MCAT Callout — Naming the Transitions: Promotion of a pi electron to an antibonding orbital is called a π→π* transition (pi-to-pi-star); promotion of a nonbonding electron is called an n→π* transition (n-to-pi-star). The pi-system or functional group responsible for a molecule's UV absorption is called a chromophore.

The HOMO-LUMO Gap

Whether an electron is promoted at a shorter or longer wavelength depends on how much energy is needed to excite it — which depends on the energy gap between two molecular orbitals:

• The HOMO (highest occupied molecular orbital): the lower-energy orbital the electron starts in.

• The LUMO (lowest unoccupied molecular orbital): the higher-energy orbital it moves into.

If the HOMO-LUMO gap is small, only a small amount of energy is needed to excite the electron, so the molecule can absorb longer wavelengths of light (which carry less energy). If the gap is large, more energy is needed, so the molecule absorbs shorter wavelengths (which carry more energy).

Conjugation and the HOMO-LUMO Gap

One major factor that affects the size of the HOMO-LUMO gap is conjugation. Conjugated molecules have a series of alternating single and double bonds — or connected p-orbitals — that allow electrons to spread out, or delocalize, over several atoms.

This delocalization lowers the energy difference between the HOMO and LUMO. As a result, electrons in conjugated systems can be excited by lower-energy, longer-wavelength light than electrons in non-conjugated systems.

Red Shift (Bathochromic Shift)

When a molecule has a lot of conjugation, its λmax shifts toward longer wavelength. This shift is called a red shift, or bathochromic shift. Molecules without conjugation, by contrast, typically absorb at shorter wavelengths within the UV region.

Common MCAT Mistakes

• Confusing UV-Vis with IR spectroscopy. UV-Vis probes electronic transitions (electrons jumping orbitals); IR probes vibrational transitions (bonds stretching/bending). Higher-energy UV-Vis light excites electrons, not bond vibrations — don't mix up which technique measures which kind of transition.

• Thinking a larger HOMO-LUMO gap means longer-wavelength absorption. It's the opposite: a larger gap needs more energy, which means shorter-wavelength, higher-energy light. A small gap is what allows longer-wavelength absorption.

• Forgetting that only certain electrons absorb UV light. Sigma (σ) bonding electrons are too tightly held to be excited by typical UV-Vis wavelengths. Only pi electrons and nonbonding (lone pair) electrons are relevant for MCAT-level UV-Vis absorption.

• Assuming red shift means a molecule looks red. "Red shift" just means λmax moves toward longer wavelength (toward the red end of the spectrum), not that the compound is actually red-colored. The term describes the direction of the shift, not the observed color.

MCAT-Style Concept Check

Question: Compound X is a highly conjugated polyene with many alternating double bonds. Compound Y is a similar molecule with no conjugation. Which statement correctly compares their UV absorption?

• A) Compound X has a larger HOMO-LUMO gap and absorbs at a shorter λmax than Compound Y.

• B) Compound X has a smaller HOMO-LUMO gap and absorbs at a longer λmax than Compound Y.

• C) Compound X and Compound Y have identical HOMO-LUMO gaps since both contain pi electrons.

• D) Compound X absorbs only infrared radiation because conjugation eliminates electronic transitions.

Answer: B

Explanation: Conjugation delocalizes pi electrons across the molecule, which narrows the HOMO-LUMO gap. A smaller gap requires less energy to excite an electron, so the molecule absorbs lower-energy, longer-wavelength light — a red shift. Compound X, being highly conjugated, therefore has a smaller HOMO-LUMO gap and a longer λmax than the non-conjugated Compound Y, ruling out (A). Conjugation changes the size of the gap, so (C) is wrong. UV-Vis spectroscopy — not IR — is what probes electronic transitions, so (D) is wrong.

FAQ

What does UV-Vis spectroscopy actually measure?

It measures how much ultraviolet or visible light a molecule absorbs at each wavelength. Absorption occurs when a photon's energy matches the energy gap between two electronic states (the HOMO-LUMO gap), promoting an electron to a higher-energy antibonding orbital.

What is λmax?

Lambda max (λmax) is the wavelength at which a molecule absorbs the most light in its UV spectrum — the peak of the absorbance-vs.-wavelength graph. It's a molecule-specific value; NAD⁺, for example, has a λmax around 260 nm.

Why does conjugation cause a red shift?

Conjugation delocalizes pi electrons across multiple atoms, which lowers the energy difference between the HOMO and LUMO. A smaller gap means less energy — and therefore longer-wavelength light — is needed to promote an electron, shifting λmax toward longer wavelengths.

What's the difference between a π→π* and an n→π* transition?

Both are electronic transitions into an antibonding orbital, but they start from different electrons: a π→π* transition promotes a pi (double-bond) electron, while an n→π* transition promotes a nonbonding (lone pair) electron, such as one on oxygen or nitrogen.