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Lecture

11.5 Molecular Orbit Theory

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School
McGill University
Department
Chemistry
Course
CHEM 110
Professor
Ariel Fenster
Semester
Fall

Description
Atomic Theory M.O. Theory 1. Derive a set of atomic orbitals 1. Derive a set of molecular orbitals 11.5 – Molecular Orbital Theory 2. Arrange the atomic orbitals in 2. Arrange the molecular orbitals in orderoffnccreasing energy ordeerof ncreaasnggennergy Paramagnetic Species: unpaired electrons, attracted to a magnetic field Diamagnetic Species: paired electrons, slightly repelled by a magnetic fieldmong the available atomic orbitals obeying available molecular orbitals Hund's rule and Pauli's exclusion obeying Hund's rule and Pauli's principle exclusion principle Valence Bond Theory vs. Molecular Orbital Theory: Valence Bond Theory Molecular Orbital Theory Bonding and Geometry (no magnetism) Bonding, Geometry, Magnetism Valence Bond Theory: Molecular Orbital Theory: 1. Electrons in atomic orbitals 1. Electrons in atomic orbitals 2. EDiatomic Molecules atomic 2. ElectrDiatomic Moleculesbitals orbitals Hydrogen H 2 Atomic Theory: M.O. Theory: Hydrogen 1. Oxyygena set of Ot2mic orbitals 1. Derive a set of molecular orbitals 2. Arrange the atomic orbitals in 2. Arrange the molecular orbitals inum Norder of increasing energy order of increasing energy 3. Distribute the electrons among the 3. Distribute the electrons among the etc• available atomic orbitals obeying available molecular orbitals Hund’s rule and Pauli’s exclusion obeying Hu1 period elementsli’s principle exclusion principle Apply the M.O. Theory to Diatomic Molecules: molecules that only contain two atoms (i.e. 2 ,2O and 2 ) Example: H 2 1. Derive a set of molecular orbitals Take the wave functions of one orbital and add it to the wave function of thels other orbital (and vice versa with subtraction; antibonding) 1. Derive a set of molecular orbitals (L.C.A.O. method) Ψ A Ψ B Ψ M.O.(bonding) Ψ A Ψ B Ψ M.O.(antibonding) Ψ 2 = Ψ2 + Ψ2 + 2Ψ Ψ M.O. A B A B 2 ~ In2rease2in electron density Ψ M.O.= Ψ A + Ψ B- 2ΨAΨ B ~ Decrease in electron density Atomic orbitals •6 •10/25/2011 11.5 – Molecular Orbital Theory The combination of two atomic orbitals M.O. Theory Electron density symmetrical with *M.O. respect to bond axis. of molecular orbitals 2. Arrange the molecular orbitals in Bonding orbital: maximum electron density between the two nucleirgy M.O.
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