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Introduction to Molecular Orbital Theory

Molecular Orbital (MO) Theory. Diagram of molecular energy levelsMagnetic and spectral properties Paramagnetic vs. DiamagneticElectronic transitionsSolid State - ConductancePredicts existence of moleculesBond Order . Molecular Orbital (MO) Theory. Two atomic orbitals combine to forma bonding

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Introduction to Molecular Orbital Theory

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    1. Introduction to Molecular Orbital Theory Allyn Ontko, Ph.D. University of Wyoming School of Pharmacy

    2. Molecular Orbital (MO) Theory Diagram of molecular energy levels Magnetic and spectral properties Paramagnetic vs. Diamagnetic Electronic transitions Solid State - Conductance Predicts existence of molecules Bond Order

    3. Molecular Orbital (MO) Theory Two atomic orbitals combine to form a bonding molecular orbital an anti-bonding molecular orbital e- in bonding MO’s = stability e- in anti-bonding MO’s = instability # atomic orbitals combined equals # of molecular orbitals formed

    4. Central Themes Quantum mechanical level Molecule viewed as a collection of nuclei surrounded by delocalized molecular orbitals Atomic wave functions are summed to obtain molecular wave functions. If wave functions reinforce each other, a bonding MO is formed (region of high electron density exists between the nuclei). If wave functions cancel each other, an antibonding MO is formed (a node of zero electron density occurs between the nuclei).

    5. An Analogy

    6. MO: Molecular Hydrogen

    7. Considerations… Bond Order =1/2( # bonding e- – # antibonding e- ) Higher bond order = stronger bond Molecular electron configurations Highest Occupied Molecular Orbital = HOMO Lowest Unoccupied Molecular Orbital = LUMO An Example: H2 (?1s)2

    8. MO: Molecular Hydrogen

    9. Predicting Stability: H2+ & H2-

    10. Helium: He2+ vs He2

    11. Bond Length vs. Bond Order

    12. Next Row: 2s & 2p orbitals

    13. Combinations for p-orbitals

    14. P orbital Complications Results in one ? & one ?* MO One pair from 2pz Results in two ? & two ?* MO’s One pair for 2px and one pair for 2py > half filled p orbitals (O, F, Ne) energy ?2p < 2?2p < 1?*2p < 2?*2p ? half filled p orbitals (B, C, N) energy 2?2p < ?2p < 2?*2p < 1?*2p

    15. MO – Now with S & P

    16. P orbital Complications Results in one ? & one ?* MO One pair from 2pz Results in two ? & two ?* MO’s One pair for 2px and one pair for 2py > half filled p orbitals (O, F, Ne) energy ?2p < 2?2p < 1?*2p < 2?*2p ? half filled p orbitals (B, C, N) energy 2?2p < ?2p < 2?*2p < 1?*2p

    17. S-P Energy Separation

    20. Relative Energy Levels for 2s & 2p

    22. Triumph for MO Theory?

    23. Can MO Theory Explain Bonding?

    24. Can MO Theory Explain Bonding?

    25. Real World Applications Most molecules are heteroatomic What needs to be considered? Orbitals involved Electronegativity (Orbital energies) Hybridization (Group Theory) Mixing

    26. Let’s Start Slowly: HF Valence electrons H – 1s1 F – 1s2 2s2 2p5 Focus on the valence interactions Accommodate for differences in electronegativity Allow mixing between symmetry-allowed states

    27. Let’s Start Slowly: HF

    29. For Next Lecture: Generate an MO Diagram for CO What is the bond order? What is the HOMO? What is the LUMO? Draw a corresponding Lewis dot structure Bonus: Based on your answers above, what can you envision for the bonding interaction of CO with a transition metal (like Fe)?

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