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Triatomics

Triatomics. • more complex, so we deal with simple symmetrical molecules • same principles apply to orbital combinations: compatible symmetry and energy (within 1 Rydberg, 1 Ry) • the number of valence AOs must equal the number of MOs. BeH 2.

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Triatomics

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  1. Triatomics • more complex, so we deal with simple symmetrical molecules • same principles apply to orbital combinations: compatible symmetry and energy (within 1 Rydberg, 1 Ry) • the number of valence AOs must equal the number of MOs

  2. BeH2 • BeH2 is the simplest triatomic molecule. In the gas phase, it is linear (as we’d expect from VSEPR): • The relative energies for the atomic orbitals of Be and H are: • 1s (Be) = -9.38 Ry 1s (H) = -0.99 Ry • 2s (Be) = -0.61 Ry • 2p (Be) = +0.14 Ry • Which atomic orbitals will combine to make σ MOs? • Which will combine to make π MOs? • Which will not combine at all (leaving them as either σ or π nonbonding MOs)?

  3. BeH2 MO Diagram Geometry of BeH2?

  4. BH3 What orbital combinations are possible now?

  5. BH3 MO Diagram CH4 - The third dimension…

  6. CO2 - The Orbitals… Valence atomic orbitals on C and O: 2s and 3 x 2p Sigma combinations: Order of energies:

  7. CO2 - Pi bonds 2p Orbital combinations: Order of energies:

  8. Valence MO Diagram for CO2 Ordering of orbitals:

  9. An Introduction to FMO Theory Reactions take place during collisions. Bonds are formed and/or broken. That must mean that there is some kind of orbital interaction. Which orbitals of these two species are most likely interact to form a new bond? BH3 H- In general, reactions take place via the interaction of the HOMO of one component with the LUMO of the other because BH3 + H- —> BH4-

  10. Electron delocalization (Resonance) and the FMOs of molecules with multiple bonds. In resonance structures, the only electrons that move are: • Delocalized electrons are always found in  orbitals. Whenever a lone pair is found on an atom adjacent to a multiple bond, it is delocalized in the pi system. • At the same time, because the  orbitals are usually found at higher energy than the  orbitals, the HOMO and LUMO of molecules with multiple bonds are usually  orbitals. As a result of this, we often look only at the  orbitals and construct  MO diagrams.

  11. Ethylene

  12. The -MO diagram of Ethylene Ozone Nodes… Pi-bond order… Total bond order When Ethylene reacts… Nodes… Pi-bond order… Total bond order Lewis BO Formal Charge: Lone pairs.

  13. Butadiene Ethylene 2 1.2 0.2 -7.3eV -9.5 -12 -11 Note that this is not two isolated double bonds but a single -system spread out over four carbons. Nodes… The importance of the HOMO/LUMO gap.

  14. Benzene The polygon method for determing -MOs of monocyclic unsaturated molecules: Works for any monocyclic molecule with contiguous atomic p orbitals.

  15. The -MOs of Benzene How many pi-electrons? Nodes… Benzene can’t be considered to have “three double bonds and three single bonds”. It has six bonds with bond order _____. Accordingly, all six C-C bonds in benzene are 140 pm (whereas pure C-C bonds are 154 pm and pure C=C bonds are 134 pm).

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