# Or o o normal lewis structure o o biradical lewis

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or O O Normal Lewis Structure O O ? Biradical Lewis Structure No They have different numbers of unpaired electrons.

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4 Oxygen: 12 valence electrons (6 from each oxygen) σ s σ s * σ p σ p * π π * Carbon Monoxide Carbon AOs CO MOs Oxygen AOs Note the energy levels for the AOs and MOs. CO has 10 valence electrons (4 from carbon and 6 from oxygen).
5 Carbon Monoxide : like N 2 , it has 10 valence electrons (but different π / σ p ordering) σ s σ s * σ p σ p * π π * Note the different lobe sizes in each orbital! Now we move to organic molecules: Ethylene (CH 2 =CH 2 ) Number of valence electrons: 2C = 2x4, 4H = 4 Total = 12 electrons Start with 12 AOs – make 12 MOs.

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6 Now we move to organic molecules: Ethylene (CH 2 =CH 2 ) Number of valence electrons: 2C = 2x4, 4H = 4 Total = 12 electrons Start with 12 AOs – make 12 MOs. A B C D E (First 8 MOs shown here.) Which orbital is the HOMO? Now we move to organic molecules: Ethylene (CH 2 =CH 2 ) Number of valence electrons: 2C = 2x4, 4H = 4 Total = 12 electrons Start with 12 AOs – make 12 MOs. A B C D E (First 8 MOs shown here.) Do you recognize the π -antibonding orbital?
7 Formaldehyde (H 2 C=O) How many electrons are involved in filling the lowest energy MOs? A: 2 B: 4 C: 12 D: 16 We now have a non-bonding orbital as our HOMO. What kind of orbital is the HOMO? A: σ B: σ * C: π D: π * E. none of the above Formaldehyde (H 2 C=O) How about the LUMO? What kind of orbital is the LUMO ? A: σ B: σ * C: π D: π * E. none of the above

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8 The MOs for 1,3-Butadiene Separate the σ and π orbitals: π orbitals only here – it’s where all the action is How many π electrons? Compare vs localized VB picture How about 1,3,5-Hexatriene? How many π electrons? Compare vs localized VB picture
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