CHEM 3300 / Topic 2: Molecular Orbital Theory

What is a bonding molecular orbital? To what degree of probability can I find an electron in this map?

Molecular orbital stabilizing molecules, provided by the constructive interference of waves of same phase. High probability of finding an electron between the nuclei as they interact strongly with both nuclei.

What is an anti-bonding orbital?

Molecular orbital destabilizing molecules, provided by the destructive interference of individual wave functions in opposite phases.

Anti-bonding orbitals are slightly MORE “anti-bonding” (i.e., destabilized relative to parent AOs) than bonding MOs are “bonding” (i.e., stabilized relative to parent AOs). Why?

Repulsion between two positively charged nuclei being brought together with no electrons in between to mitigate.

What does the Pauli exclusion principle state?

Two electrons in an atom cannot have the same four quantum numbers.

What does the Aufbau principle state?

Electrons fill orbitals in order of increasing principal levels and sublevels.

What does Hund’s rule state? What does it ensure?

Electrons that spin up will singly fill up orbitals within the same sublevel before pairing up with electrons that spin down. It ensures that the atom will minimize electron-electron repulsion.

What are the five steps to making homonuclear and heteronuclear diatomic molecular orbitals?

1. Input the parent atomic orbitals according to their relative energy levels on both sides of the diagram, and the names of each atomic orbital.

2. Input the child molecular orbitals, according to how they bond via symmetry and energy.

3. Assign 1º parentage, then 2º parentage, and so on, if ever needed.

4. Ensure that the same amount of children are the same amount of parents in total. If missing child, find missing child by going back to step 2.

5. Input the electrons using Hund’s, Pauli, and Aufbau.

What are the 10 steps to making polyatomic molecular orbitals?

1. Assign the correct point group to the molecule.

2. Identify the irreducible representations of the point group.

3. Identify the symmetry species of atomic orbitals of the central atom.

4. Input the atomic orbitals in increasing order of energy.

5. Identify the symmetry species of all group orbitals of the ligand groups.

6. Input the group orbitals of the ligand groups in increasing order of energy. First noting whether they stem from s, p, or d orbitals, and then their number of internuclear nodes.

7. Combine atomic orbitals and ligand-group orbitals of the same symmetry to give molecule orbitals, estimating the relative energies of these molecular orbitals by considering the overlap and relative energies of the parent orbitals.

8. Assign 1º parentage, then 2º parentage, and so on, if ever needed.

9. Ensure that the same amount of children are the same amount of parents in total. If missing child, find missing child by going back to step 8.

10. Input the electrons using Hund’s, Pauli, and Aufbau.

There are three conditions for orbital overlap leading to bonding. What are they?

• Symmetry must match.

• Energies must match.

• Distance must mach.

What happens if any of these conditions are not met?

A non-bonding molecular orbital is formed from the corresponding atomic orbital. These MOs have almost the same energy as their parent AOs, but population of this orbital doesn’t promote stability nor instability.