Transcription of Simple Molecular Orbital Theory
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Simple Molecular Orbital Theory
Simple Molecular Orbital Theory Chapter 5 Wednesday, October 7, 2015 Using Symmetry: Molecular OrbitalsOne approach to understanding the electronic structure of molecules is called Molecular Orbital Theory . MO Theory assumes that the valence electrons of the atoms within a molecule become the valence electrons of the entire molecule. Molecular orbitals are constructed by taking linear combinationsof the valence orbitals of atoms within the molecule. For example, consider H2: Symmetry will allow us to treat more complex molecules by helping us to determine which AOs combine to make MOs++1s1sLCAO MO TheoryMO Math for Diatomic Molecules1 A ------ B2 11 22cc Each MO may be written as an LCAO:Since the probability density is given by the square of the wavefunction:probability of finding the electron close to atom Aditto atom Boverlap term, important between the atoms1MO Math for Diatomic MoleculesThe individual AOs are normalized:100% probability of finding electron somewherefor each free atom1 SMO Math for Homonuclear Diatomic MoleculesFor two identical AOs on identical atoms, the electrons are equally shared, so:2212cc 12cc In other words:121/21()[2(1 )]S 121/21()[2()]
• If an orbital has S = 0 with all other orbitals in the molecule, then it is a 100% non-bonding orbital. • S = 0 if orbitals have different irreducible representations. • If S ≠ 0, then bonding and antibonding MOs result. Rule #3: Orbitals must have the same symmetry (same irreducible representation) to have non-zero overlap.
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