Series09MolecularOrbitals

Series09MolecularOrbitals - Molecular Orbital Theory...

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Molecular Orbital Theory Reading: Gray: (2-5), (2-6), (3-1) to (3-6) OGC: (6.1) and (6.2) IX-1
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Molecular Orbital Theory MO–LCAO: M olecular O rbital– L inear C ombination of A tomic O rbitals Lewis Dot Structures don’t work for excited states; sometimes they don’t even predict the ground state accurately : Example: O 2 expected Lewis Dot Structure: O O Actually O 2 has two unpaired electrons; this cannot be predicted with Lewis Dot Structures, but it can be predicted with MO-LCAO IX-2
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MO-LCAO Lewis Dot Structures tell us: H 2 + H 2 - H H H H H H MO-LCAO will tell us: a. their local stabilities b. their bond orders and their trend in bond lengths c. their magnetic properties H 2 IX-3
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Nobel Prize, Chemistry, 1966 ”For his fundamental work concerning chemical bonds and the electronic structure of molecules by the molecular orbital method" Robert S. Mullikan IX-4
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Energy dependence on inter-atomic distance E 0 r r 0 region where nuclei repel most likely distance for a bond (lowest energy point) electrons start being pulled to the other atom’s nucleus at r = , E = 0; the atoms don’t interact indicates the direction of dominating forces IX-5
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H 2 Molecule electron density in a molecular orbital r 0 equilibrium bond distance IX-6
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H a Ψ a r a 0 H b Ψ b r b 0 Linear Combinations of Atomic Orbitals Molecular orbitals are built up from linear combinations of atomic orbitals: IX-7
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Ψ H a H b + + + + + + + + + + + + + + + + The Wavefunctions of two Separate Hydrogen Atoms IX-8
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+ + + + + + + + + + + + + + + + Orbitals Start to Overlap as Atoms Approach + + + + + + + + + + + + + + + + IX-9
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Ψ Bonding Orbital: Ψ (1s a + 1s b ) = 1 σ b Wavefunction of H 2 Molecule in Ground State + + + + + + + + + + + + + + + + IX-10
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H a Ψ a r a 0 H b Ψ b r b 0 Another Linear Combination IX-11
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Ψ H a H b - - - + + + + + + + - - - - - + Combined Wavefunction with the Two Atoms a Distance Apart Note: Subtracting wavefunctions is the same as adding the negative of the second IX-12
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Ψ Anti-Bonding Orbital: Ψ (1s a - 1s b ) = 1 σ * node: Wavefunction of H 2 in Excited State + + + + + + + + - - - - - - - - IX-13
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Another Perspective on the σ
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This note was uploaded on 10/27/2010 for the course CH 1a taught by Professor Lewis during the Fall '08 term at Caltech.

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Series09MolecularOrbitals - Molecular Orbital Theory...

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