14-Molecular Orbital Theory

14-Molecular Orbital Theory - 5.111 Lecture Summary #14...

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5.111 Lecture Summary #14 October 13, 2009 14.1 MOLECULAR ORBITAL THEORY QM theory to describe chemical bonding. We want to see what wavefunctions look like on molecules, not just atoms! HOMONUCLEAR DIATOMICS + 1s a 1s b a b σ 1s 1s a + 1s b = σ 1s MOLECULAR ORBITAL atomic orbitals Linear combination of atomic orbitals (LCAO) creates a molecular orbital. σ symmetry: Molecular wavefunction or molecular orbital is symmetric around bond axis. σ 1s is a wavefunction. Physically significant quantity is a probability density. () ()() () 22 1s a a a bb b P σ = 1s +1s = 1s + 1s +2 1s 1s The cross-term represents constructive interference between the two wavefunctions. Result is higher probability density between the nuclei – “bonding” orbital WHOLE IS MORE THAN SUM OF ITS PARTS! What happens to energy of interaction between the 2 atoms? Goes down! 1 s b 1 s a σ 1s atomic orbital on atom a atomic orbital on atom b E bonding orbital: molecule more stable than individual atoms But since e s are “waves”, they can also destructively interfere.
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5.111 Lecture Summary #14 October 13, 2009 14.2 - 1s a 1s b a b σ 1s * 1s a – 1s b = σ 1s * MOLECULAR ORBITAL () ()() () 2 22 2 * 1s a a a bb b P σ = 1s -1s = 1s + 1s -2 1s 1s The cross-term represents destructive interference between the two wavefunctions.
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This note was uploaded on 02/08/2010 for the course CHEM 143 taught by Professor Mike during the Spring '10 term at École Normale Supérieure.

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14-Molecular Orbital Theory - 5.111 Lecture Summary #14...

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