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 BeH 2 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) = Ry 1s (H) = Ry 2s (Be) = Ry 2p (Be) = 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)?

BeH 2 MO Diagram Geometry of BeH 2 ?

BH 3 What orbital combinations are possible now?

BH 3 MO Diagram CH 4 - The third dimension…

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

CO 2 - Pi bonds 2p Orbital combinations: Order of energies:

Valence MO Diagram for CO 2 Ordering of orbitals:

An Introduction to FMO Theory BH 3 H-H- BH 3 + H - —> BH 4 - 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? In general, reactions take place via the interaction of the HOMO of one component with the LUMO of the other because

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.

Ethylene

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

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

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

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). The  -MOs of Benzene How many pi-electrons? Nodes…