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Different types of colour theory - Coggle Diagram
Different types of colour theory
Atomic spectroscopy
d-d transitions
Molecular orbital theory
Molecular orbital theory can be used to explain why organic molecules are colourless or coloured
Electrons fill bonding molecular orbitals, leaving higher energy anti bonding orbitals unfilled
The highest bonding molecular orbital containing electrons is called the highest occupied molecular orbital (HOMO)
The lowest anti bonding molecular orbital is called the lowest unoccupied molecular orbital (LUMO)
Energy level diagrams
Electrons fill the bonding molecular orbitals first as they have a lower energy - under normal conditions, the antibonding orbitals will be empty
The lowest energy orbital which is empty is called the Lowest Unoccupied Molecular Orbital (LUMO)
The highest energy orbital which contains electrons is called the Highest Occupied Molecular Orbital (HOMO)
Electron transitions
Absorption of electromagnetic energy can cause electrons to be promoted from HOMO to LUMO.
Energy from photons is used to promote electrons from bonding or non-bonding orbitals into the higher energy anti-bonding orbitals.
non bonding orbital=one unpaired electron
bonding orbital=two paired electrons
Organic molecules containing single bonds
In ethane (and other organic molecules containing only single bonds),the gap between HOMO and LUMO is very large and so the energy absorbed is not in the visible spectrum - hence they are colourless (UV absorbed)
Chromophores
some organic molecules contain
chromophores
A
chromophore
is a group of atoms within a molecule that is responsible for absorption of light in the visible region of the spectrum
Light can be absorbed when electrons in a chromophore are promoted from the HOMO to the LUMO
Conjugated systems
Chromophores exist in molecules containing a
conjugated system
A conjugated system is a a system of adjacent unhybridised p orbitals that overlap side-on to form a molecular orbital across a number of carbon atoms.
Electrons within this conjugated system are delocalised
Molecules with alternating single and double bonds, and aromatic molecules have conjugated systems
When identifying the chromophore it must start on a pi bond
Summary
The more atoms in the conjugated system the smaller the energy gap between HOMO and LUMO
A lower frequency of light (longer wavelength, lower energy) is absorbed by the compound
When the wavelength of light absorbed is in the visible region, the compound will exhibit the complementary colour.
Exciting the delocalised electrons will therefore require less energy
If this energy falls within the visible part of the electromagnetic spectrum this will result in the compounds appearing coloured
