Conjugation in UV/Vis spectroscopy

uv-vis

spectroscopy

chemistry

Published

June 24, 2025

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

Figure 1: Conjugation allows π electorns to spread accross multiple carbon atoms.

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

Figure 2: Extended conjugation reduces HOMO-LUMO gap.

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!

--- title: "Conjugation in UV/Vis spectroscopy" date: 2025-06-24 categories: [uv-vis, spectroscopy, chemistry] draft: false --- # Conjugation In [Orbital Energies in UV/Vis Spectroscopy](../2025-06-23-2/index.qmd) 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 ![Conjugation allows π electorns to spread accross multiple carbon atoms.](molecular_structures.svg){#fig-molecular-structures} ## 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 ![Extended conjugation reduces HOMO-LUMO gap.](conjugation_mo_diagram.svg){#fig-conjugation-energy} ## 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!