Lecture 13 Molecular Orbital Theory

Lecture 13 Molecular Orbital Theory - Molecular Orbital...

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Molecular Orbital Theory B. A. Rowland 53750/53760 Molecular Orbital Theory Molecular Orbital (MO) theory is a theory of bonding which is more consistent with quantum mechanics, as it allows us to delocalize the electrons (rather than keeping them localized in a particular bond). QM tells us that electronic density is delocalized over the entire molecule, a fact that cannot be accounted for via our other methods. The basic idea —construct a set of molecular orbitals (analogous to atomic orbitals) from the atomic orbitals and apply an Aufbau type order of filling. MO solutions are indeed solutions to the Schrödinger equation, and they very naturally handle cases that were difficult for Lewis dot structures (i.e. odd electron molecules, resonance, magnetic properties). Still, it isn’t a physically accurate picture of bonding as the Configuration Interaction (i.e. electron-electron repulsions) still has not been fully taken into account. Mixing s-orbitals Two s-orbitals will either constructively (bonding) or destructively (anti-bonding) interfere to produce two molecular orbtials. When two atomic orbitals come together and constructively interfere, the orbital density gets smeared out over the entire nuclear framework of the molecule. This is how the electrons are delocalized in the molecule. This molecular orbital (known as the bonding orbital) is lower in energy than either of the original atomic orbitals, meaning it is advantageous for two atoms to form a chemical bond. They can also destructively interfere, yielding an anti-bonding orbital with a nodal plane. This orbital will be higher in energy than the free atomic orbitals, so it would be energetically advantageous for the electrons to remain on their respective atomic orbitals (hence the term anti-bonding). Remember that for every n atomic orbitals you put in you must recover n molecular orbitals. Mixing p-orbitals
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This note was uploaded on 02/02/2009 for the course CH 53750 taught by Professor Rowland during the Spring '09 term at University of Texas at Austin.

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Lecture 13 Molecular Orbital Theory - Molecular Orbital...

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