Orbital Energies in UV/Vis Spectroscopy

Published

June 23, 2025

Visual light has wavelengths in the range of 400-700 nm, which corresponds to the energy range of 1.77-3.10 eV. Ultraviolet (UV) light, which is shorter than visible light, has wavelengths between 200 and 400 nm, corresponding to energies of approximately 3.10-6.20 eV.

This is the energy range that can excite electrons in molecules, leading to electronic transitions that are fundamental to UV/Vis spectroscopy.

In figure Figure 1, we see the molecular orbital energy diagram that illustrates the electronic transitions for Acetone.

Figure 1: Molecular orbital energy diagram showing bonding energies for Acetone. The diagram illustrates bonding π orbitals, non-bonding n orbitals (lone pairs), and anti-bonding π* orbitals, with arrows indicating the two main types of electronic transitions that occur in the UV/Vis energy range.

Three possible transitions

There are three main types of electronic transitions that can occur in molecules:

n → π* (non-bonding to pi-antibonding) π → π* (pi-bonding to pi-antibonding) σ → σ* (sigma-bonding to sigma-antibonding)

The $\sigma \to \sigma^*$ transition require much higher energy than the UV/Vis range, and are not visible.

The two main transitions that occur in UV/Vis spectroscopy are $\pi \to \pi^*$ and $n \to \pi^*$.

$\pi \to \pi^*$ Transition

Electrons move bonding to anti-bonding states. This transition is common in molecules with conjugated double bonds, such as alkenes and aromatic compounds.

This transition shows up as a strong absoption in the UV/Vis spectrum, typically in the range of 200-400 nm.

Intensity. Because the $pi$ and $pi*$ orbitals occupy the same region of space, the transition has a high probability of occurring, resulting in a strong absorption peak.

$n \to \pi^*$ transition

$n$-orbitals (lone pares) are spatially separated from $\pi$-orbitals, so the transition $n \to \pi^*$ is less probable than $\pi \to \pi^*$ transitions. This leads to a suppresioni of the intensity of the absorption peak; typically a factor 2-3.

Bonding - and anti-bonding transitions

The $\sigma \to \sigma^*$ and $\sigma \to \pi$ transitions involve too high energies to be observed in the UV/Vis range.

That is why we usually talk about bonding and anti-bonding transitions in the context of UV/Vis spectroscopy, as these are the transitions that occur within the energy range of visible and ultraviolet light.

The bonding transitions are the once that involve electrons moving from bonding orbitals ($\pi$), while the anti-bonding transitions are the once that involve electrons moving from non-bonding orbitals ($n$).

--- title: "Orbital Energies in UV/Vis Spectroscopy" date: 2025-06-23 categories: [] draft: false --- Visual light has wavelengths in the range of 400-700 nm, which corresponds to the energy range of 1.77-3.10 eV. Ultraviolet (UV) light, which is shorter than visible light, has wavelengths between 200 and 400 nm, corresponding to energies of approximately 3.10-6.20 eV. This is the energy range that can excite electrons in molecules, leading to electronic transitions that are fundamental to UV/Vis spectroscopy. In figure @fig-orbital-energies, we see the molecular orbital energy diagram that illustrates the electronic transitions for Acetone. ![Molecular orbital energy diagram showing bonding energies for Acetone. The diagram illustrates bonding π orbitals, non-bonding n orbitals (lone pairs), and anti-bonding π* orbitals, with arrows indicating the two main types of electronic transitions that occur in the UV/Vis energy range.](orbital-energies.svg){#fig-orbital-energies} ## Three possible transitions There are three main types of electronic transitions that can occur in molecules: n → π* (non-bonding to pi-antibonding) π → π* (pi-bonding to pi-antibonding) σ → σ* (sigma-bonding to sigma-antibonding) The $\sigma \to \sigma^*$ transition require much higher energy than the UV/Vis range, and are not visible. The two main transitions that occur in UV/Vis spectroscopy are $\pi \to \pi^*$ and $n \to \pi^*$. ## $\pi \to \pi^*$ Transition Electrons move bonding to anti-bonding states. This transition is common in molecules with conjugated double bonds, such as alkenes and aromatic compounds. This transition shows up as a strong absoption in the UV/Vis spectrum, typically in the range of 200-400 nm. Intensity. Because the $pi$ and $pi*$ orbitals occupy the same region of space, the transition has a high probability of occurring, resulting in a strong absorption peak. ## $n \to \pi^*$ transition $n$-orbitals (lone pares) are spatially separated from $\pi$-orbitals, so the transition $n \to \pi^*$ is less probable than $\pi \to \pi^*$ transitions. This leads to a suppresioni of the intensity of the absorption peak; typically a factor 2-3. ## Bonding - and anti-bonding transitions The $\sigma \to \sigma^*$ and $\sigma \to \pi$ transitions involve too high energies to be observed in the UV/Vis range. That is why we usually talk about bonding and anti-bonding transitions in the context of UV/Vis spectroscopy, as these are the transitions that occur within the energy range of visible and ultraviolet light. The bonding transitions are the once that involve electrons moving from bonding orbitals ($\pi$), while the anti-bonding transitions are the once that involve electrons moving from non-bonding orbitals ($n$).

Orbital Energies in UV/Vis Spectroscopy

Three possible transitions

\(\pi \to \pi^*\) Transition

\(n \to \pi^*\) transition

Bonding - and anti-bonding transitions