[earthnation112]

Below is a past paper question:

Identify the HOMO and LUMO in the molecular orbital diagram of CN- and describe the character of these orbitals (whether it is polarised towardseither of the atoms, and if so which one)?

I have attached an image of the molecular orbital diagram below.

When it says "describe the character of these orbitals (whether it is polarised towards either of the atoms, and if so which one", would it be correct to mention that the more electronegative atom which is Nitrogen its atomic orbitals are slightly lower than the carbons as the more electronegative atom is placed lower so the atomic orbitals are polarised towards it? Or is the question referring to something different? Thank You

[Corribus]

This is more or less what the question is after. Essentially, whichever atomic orbitals the molecular orbital is closer in energy to contribute proportionally more "character" to that molecular orbital. As a result, electrons in that molecular orbital spend more time in proximity of the atom whose atomic orbitals contribute the most character. In the extreme limit that the "molecular orbital" has the exact same energy as one of the atomic orbitals, then this means that the atomic orbital is essentially nonbonding.

In the case of cyanide, the bonding orbital has more nitrogen character than carbon character, for the reason you have stated, and so electrons spend more time close to nitrogen.

[Enthalpy]

[...] electrons [...] spend more time [...]

I know human languages lack good wording for QM, but any reference to time risks to suggest wrong interpretations so I dislike them. Orbitals are stationary, their absolute value doesn't evolve over time, and the electron does not hop from one possible position to an other.

"wavefunction's absolute value", "electron density" (but electrons come in pairs...), "probability of interaction in a small volume"... would avoid this misleading notion of time - but they're clumsy, agreed.

[Enthalpy]

CN^- is a special simple case that resembles N₂:

• Same available molecular orbitals
• Same number of shared electrons
• The next available orbital has a much higher energy too.

So maybe we can just infer that the filling of the molecular orbitals is the same, the main difference being one proton less at the carbon nucleus, which makes CN^- negative at the carbon side, but the electrons compensate it partially by being denser at the nitrogen.

[Corribus]

Yes you are technically right. Language is built from our experience with classical physics, not our abstract knowledge of quantum physics. It is difficult to be rigorous, especially when educating students, who require analogies to foster understanding difficult concepts.

[earthnation112]

Thank you for the useful input