Conjugation in UV/Vis spectroscopy
Conjugation
In Orbital Energies in UV/Vis Spectroscopy we learned that allmost all transitions in the UV/Vis range orinate from the \(\pi\) and \(n\) orbitals.
Conjugation occurs when you have alternating single and double bonds in a molecule, allowing π (pi) electrons to delocalize across multiple atoms.
Why Is It Called “Conjugation”?
The term comes from Latin “conjugatus” meaning “joined together” or “yoked together” (like oxen working as a team).
In chemistry, it describes how separate π electron systems become “yoked” or “coupled” together to work as a unified electronic system. Just as conjugated oxen are more effective working together than separately, conjugated π systems have properties (stability, reactivity, spectroscopic behavior) that differ dramatically from isolated double bonds.
The term was first used in organic chemistry in the early 20th century when chemists noticed that certain compounds with alternating bonds behaved very differently from their “unconjugated” analogs.
The Molecular Orbital Picture
Isolated double bonds:
- In ethylene (H₂C=CH₂), the π electrons are localized between just two carbon atoms
- You have one bonding π orbital and one antibonding π* orbital
- Large energy gap between them
Conjugated system:
- In butadiene (H₂C=CH-CH=CH₂), the four π electrons can spread across all four carbons
- Now you have multiple π orbitals of different energies
- The energy gap between the highest occupied (HOMO) and lowest unoccupied (LUMO) π orbitals gets smaller
Key Effects of Conjugation
1. Electron Delocalization
Instead of electrons being “stuck” between two atoms, they can move freely across the entire conjugated system. This is like the difference between being confined to one room versus having access to an entire building.
2. Energy Stabilization
The molecule becomes more stable because electrons can spread out over a larger volume, reducing electron-electron repulsion. It’s energetically favorable.
3. Smaller HOMO-LUMO Gap
As conjugation increases, the energy gap between occupied and unoccupied orbitals decreases. This is the key to understanding color!
The Spectroscopic Connection
Short conjugation (like ethylene):
- Large energy gap
- High-energy UV light needed for π→π* transition
- Appears colorless
Extended conjugation (like β-carotene):
- Smaller energy gap
- Lower-energy visible light can cause π→π* transition
- Molecule appears colored
Real-World Examples
Polyenes: H₂C=CH-(CH=CH)ₙ-CH=CH₂
- n=1 (butadiene): colorless, absorbs ~217 nm
- n=4: yellow
- n=8: red
- n=15: absorbs in near-infrared
Aromatic systems: Benzene rings can also participate in extended conjugation, which is why many dyes contain multiple connected aromatic rings.
Why This Matters
Understanding conjugation explains:
- Why most organic molecules are colorless (no extended conjugation)
- How to design colored compounds (extend the conjugation)
- Why cooking changes food color (heat can break or extend conjugated systems)
- How photosynthesis works (chlorophyll’s extended conjugated system captures specific wavelengths)
The general rule: more conjugation = longer wavelength absorption = potential for visible color.
Do You Need Alternating Single/Double Bonds?
Not necessarily! While alternating single and double bonds are the most common example, conjugation simply requires overlapping p orbitals. Here are different types of conjugation:
1. Classical Conjugation (Alternating bonds)
- Butadiene: C=C-C=C
- Benzene: Aromatic ring with delocalized π electrons
- Polyenes: Long chains like β-carotene
2. Conjugation Through Lone Pairs
- Aniline (C₆H₅-NH₂): The nitrogen lone pair conjugates with the benzene ring
- Phenol (C₆H₅-OH): Oxygen lone pair participates in conjugation
- Carbonyl compounds: C=O can conjugate with adjacent π systems
3. Conjugation Through Radicals
- Allyl radical: CH₂=CH-CH₂• (unpaired electron in p orbital)
- Benzyl radical: C₆H₅-CH₂•
4. Cross-Conjugation
- Systems where conjugation branches, like in dendralenes
The Key Requirement: Overlapping p Orbitals
What matters is having adjacent atoms with p orbitals that can overlap. This typically means:
- sp² hybridized carbons (flat geometry allows p orbital overlap)
- Coplanar arrangement (orbitals must be parallel to overlap effectively)
- Continuous pathway for electron delocalization
Examples of Non-Alternating Conjugation
Benzene: All C-C bonds are equivalent (1.5 bond order), not truly alternating single/double
Tropylium ion (C₇H₇⁺): Seven-membered aromatic ring
Cyclobutadiene: Four-membered ring (though antiaromatic and unstable)
The Bottom Line
Conjugation is really about electron delocalization through overlapping p orbitals. Alternating single and double bonds are just the most common way this happens, but the fundamental requirement is orbital geometry and overlap, not bond alternation patterns.
Think of it as creating an “electron superhighway” - the road can have different lanes and configurations, but what matters is that electrons can travel freely along the route!