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CHEMISTRY 2000

CHEMISTRY 2000. Topic #1: Bonding – What Holds Atoms Together? Spring 2010 Dr. Susan Lait. Molecular Orbitals of Heteronuclear Diatomics.

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CHEMISTRY 2000

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  1. CHEMISTRY 2000 Topic #1: Bonding – What Holds Atoms Together? Spring 2010 Dr. Susan Lait

  2. Molecular Orbitals of Heteronuclear Diatomics • The molecular orbitals of heteronuclear diatomics (HF, CO, CN-, etc.) can be predicted using the same principles that we used to construct the molecular orbitals of homonuclear diatomics: • Ignore the core electrons • Total number of MOs = Total number of AOs • Only AOs of similar energy combine to make LCAO-MOs • Only AOs of compatible symmetry combine to make LCAO-MOs: • -type AOs (s and pz orbitals) make  MOs • -type AOs (px and py orbitals) make  MOs

  3. Molecular Orbitals for HF • Consider the valence atomic orbitals of hydrogen and fluorine: • Which AOs will combine to make MOs? • Which AOs will not mix (and therefore still look like an AO)?

  4. Molecular Orbitals for HF • Using symmetry and energy as our guide, we predict that we will make LCAO-MOs that look something like: • There can be no  bonding in HF. Why not? • There will still be orbitals with  symmetry in HF. Which ones?

  5. Molecular Orbitals for HF • Using HyperChem, we generate a valence MO diagram for HF:

  6. Molecular Orbitals for HF • Write the valence orbital occupancy for HF. • Draw a Lewis structure for HF, and compare the bond order obtained by the Lewis method to the bond order obtained by MO. • How is the MO diagram on the previous page consistent with other features of your Lewis structure?

  7. Molecular Orbitals for CO • Consider the valence atomic orbitals of carbon and oxygen: • Which AOs will combine to make MOs?

  8. Molecular Orbitals for CO • Using symmetry and energy as our guide, we predict that we will make LCAO-MOs that look something like: • We rank these MOs from lowest to highest energy to the best of our ability; however, at this point, we are estimating for a few of the orbitals. We can use quantum mechanical software such as HyperChem to show us the exact geometry of the MOs formed as well as their exact energies.

  9. Molecular Orbitals for CO • Using HyperChem, we generate a valence MO diagram for CO: Contrary to what Petrucci states, the MO diagram for CO has the orbitals in the same order as N2 does. This has been verified experimentally. Why does this make sense? Can you identify each valence molecular orbital in CO as bonding , nonbonding or antibonding (with respect to the C-O bond)?

  10. Molecular Orbitals for CO • Write the valence orbital occupancy for CO. • Draw a Lewis structure for CO, and compare the bond order obtained by the Lewis method to the bond order obtained by MO.

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