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Electronic transitions & Types of Electronic Transitions

Electronic transitions, in the context of instrumental methods of analysis, refer to the movement of electrons within a molecule from one energy level to another in response to the absorption or emission of electromagnetic radiation.
In essence, when a molecule absorbs a specific quantum of energy (like a photon of light), electrons can be promoted from their stable, or "ground," state to a higher-energy, or "excited," state.
The various types of electronic transitions can be understood better when we break them down:

Types of Electronic Transitions

These are observed for compounds that have conjugated double bonds (e.g., ethene).
Here, an electron transitions from a bonding π orbital to an antibonding π* orbital.
Such transitions are commonly observed in UV-Visible spectroscopy.

These are observed in molecules that have non-bonding electrons (n-electrons) and a π* orbital.
A classic example would be the carbonyl group (C=O).
An electron from a non-bonding orbital transitions to an antibonding π* orbital.
This transition typically requires less energy than π - π* transitions, so these absorptions often appear at longer wavelengths (closer to the visible region) compared to π - π* transitions.

These transitions involve promotion of an electron from a sigma (σ) bonding orbital to an antibonding sigma (σ*) orbital.
These transitions require a larger amount of energy, so they typically occur in the far UV region.

Here, an electron transitions from a non-bonding orbital to a sigma* antibonding orbital.
Like σ - σ* transitions, these also typically occur in the deep UV region.

This transition happens when an electron transfers from one location to another within a molecule, often resulting in the formation of a positive and negative site within the molecule.
This can be seen in complex ions or molecules.
Charge transfer bands are typically very intense and can occur anywhere in the UV-Visible spectrum, depending on the molecules involved.