1. Chapter 7.4-7.5, Molecular Orbital (MO) Theory Continued
CHM1111 Section 04
Instructor: Dr. Jules Carlson
Class Time: M/W/F 1:30-2:20
Wednesday, November 9th

2. Housekeeping Final Exam is Friday, December 9th
The fall session lectures finish Tuesday November 29th, but we have Lectures on Wednesday, November 30th and Thursday December 1st (1:30 – 2:20 PM) to make up for the holidays (Thanksgiving Day + Remembrance Day).
At least half the lecture on Thursday December 1st will be for review but I would like to schedule another review with you. We can talk more later about when to do this.

3. MO Diagram for Ne2 Bond Order = 0 Bond Energy ~ 0 kJ/mol σp* πx* πy*
Electron Configuration:
(σ1s)2 (σ1s*)2 (σ2s)2 (σ2s*)2 (σp)2
(πx)2 (πy)2 (σp*)2
σp*
πx*
πy*
2px
2py
2pz πx πy
2px
2py
2pz σp
σs*
Energy σs 2s 2s

4. Trends in Bond Energy MO Diagram for NO
Species
Bond Length (pm)
Bond Energy (kJ/mol)
Bond Order O2
121
495 2 NO
115
605
2.5 N2
110
945 3
A π orbital strengthens a bond by 250 kJ/mol and a π* orbital weakens a bond by 250 kJ/mol
MO Diagram for NO
Work through Example 7-10 on p 392 (answer in text).
We will now do Practice Exercise 7.10 on p. 393:
Use Electron configurations to predict which of the following is the most stable diatomic combination of Carbon and Nitrogen: CN-, CN, or CN+.

5. Three-Centre π Orbitals
Ozone has delocalized π bonds.
Ozone is bent with a bond angle of 118⁰.
Steric # = 3.
Geometry is trigonal planar.
Hybridization is sp2.
The remaining p orbital forms a π bond through side by side overlap with either outer oxygen.
However, the resonance structures in the Lewis Diagram are misleading – all three perpendicular p orbitals overlap simultaneously.

6. Three-Centre π Orbitals - Ozone
If three atoms combine, need 3 molecular orbitals, but need same number of bonding and antibonding orbitals.
So, what is the third molecular orbital?
The resonance structures indicate delocalized π orbitals.
The lone pair switches between 2 outer oxygens.
This lone pair occupies a Non-bonding
Molecular Orbital

7. Extended π Systems
Except in diatomic molecules, ground-state configurations rarely include occupied antibonding π orbitals.
The π electrons can extend over across a whole region where there is alternating single and double bonds – these delocalized π orbitals are in conjugated π systems.
Each additional conjugated π bond stabilizes the ground state and puts more orbitals in the system.
More orbitals means smaller energy differences for excitation with a photon, so molecules with extended π systems can absorb visible light.

8. Examples of Molecules with Extended π Systems
Retinal important in vision, β-carotene makes carrots orange.
Chlorophyll A

9. I clicker Question Which of the following statements are true:
N2+ has a stronger bond than N2
O2 has a longer bond than N2
σp is higher in energy than πx for N2
Both (a) and (b)
Both (b) and (c)