2.7: The Carbonyl Group

Let’s look at the electron transitions of H₂CO, formaldyhyde, for a typical description of the MO of an organic molecule.

Pretty valence-bond theory

(first year description of formaldehyde): https://www.chemtube3d.com/orbitalsformaldehyde

View 1: Before (but after hybridization)

Carbon and the oxygen are both \(\mathrm{sp}^{2}\) hybridized.

(CC BY-SA-NC; Nick Graves via ChemTube3D)

View 2: Moved together to demonstrate overlap

When moved together, there is overlap of the \(\pi\) and \(\mathrm{sp}^{2}\) orbitals.

(CC BY-SA-NC; Nick Graves via ChemTube3D)

View 3: Generation of new bonds to make the carbonyl bond)

Formation of bonding interactions to form the sigma bond (from the \(\mathrm{sp}^{2}\) orbitals on \(\ce{C}\) and \(\ce{O}\) AND an \(\pi\) bond from the two non-hybridized \(p\) orbitals on \(\ce{C}\) and \(\ce{O}\).

(CC BY-SA-NC; Nick Graves via ChemTube3D)

H²CO is a planar molecule with \(C_{2v}\) symmetry. We ignore the n(1s) core electrons on C and O as well as the \(σ\) electrons in the C-H bonds. Very low energy excitations will not be involved in the UV-VIS region. The ground state valence electronic configuration of interest is:

\[n_{a}^{2} \sigma^{2} \pi^{2} n_{b}^{2}\left(\pi^{*}\right)^{0}\left(\sigma^{*}\right)^0 \nonumber \]

This is the lowest energy MO’s that originate from the valence atomic orbitals of the \(\ce{C}\) and \(\ce{O}\).

What do they look like? http://pubs.acs.org/doi/pdf/10.1021/ed050p400

Figure XX: http://pubs.acs.org/doi/pdf/10.1021/ed050p400. All Rights Reserved ACS

The MO energies are

\[n_{a}<\sigma<\pi<n_{b}<\pi^{*}<\sigma^{*} \nonumber \]

from a simple MO calculation. \(n < \sigma\) because \(2 s \ll 2p\). Let's ignore the lowest non-bodning state and focus on the "frontier orbitals" - the higher lying MOs.

Possible Electronic Transitions

There are six outer valence e- are involved in major electronic transitions. The lowest energy transition is a \(n \rightarrow \pi^{*}\) transition, while \(\pi \rightarrow \pi^{*}\) and \(n_a \rightarrow \sigma^{*}\) occur at higher \(ΔE\) and shorter \(λ\).

Energy levels of molecular orbitals in formaldehyde.

Molecular orbitals of formaldehyde emphasizing the transitions (after Guillet [19]).

Triplet States

In addition to change in the orbital type, the excited electron may keep the same \(m_s\), spin quantum number producing a singlet excited state. If \(m_s\) changes during the transition, the excited state had two ↑↑ electrons and is a triplet state. For instance, the \(n \rightarrow \pi^{*}\) transition can lead to a \(^1\left(n_{1} \pi^{*}\right)\) if the \(m_s\) does not change, but to \(^3\left(n_{1} \pi^{*}\right)\) if it does change (flip). Both the singlet and triplet states have the same configuration, but lies lower in energy because ↑↑ electrons are on average further apart than ↓↑ electrons (reducing e-e correlation).

This MO description is oversimplified. e-e interactions are different in an excited state than in the ground state. So A single configuration description is not very good; however this descriptions can be improved by allowing configurations to exit together to minimize the energy of the excited state (e.g. CI-doubles, CI-triples). We will discuss this later.