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

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.

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

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+.

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.

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

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.

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

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)