Additionally we can see that the and orbitals of the

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Unformatted text preview: ifferent electronegativities and orbital sizes, we would expect the C ­Br bond to be weak relative to the C ­H bond. Both of these observations are displayed in the above molecular orbital diagrams. Additionally we can see that the σ and σ* orbitals of the C ­Br bond have differential orbital coefficients on them. This tells us that the σ orbital has a majority of the density on the chlorine and the σ* would have the majority of the orbital density on the carbon. This is a key to understanding why a nucleophile attacks the carbon rather than the chlorine atom in an SN2 reaction. Dunbar, K and Petrik, I UIUC Chemistry 436 When two molecules interact with one another they do so with their molecular orbitals. As we have stated previously, atomic orbitals must be close in energy for a strong interaction to be formed; the same is true for interaction between molecular orbitals. This means that only a small set of the molecular orbitals play a role in any given interaction and is the basis for Frontier Molecular Orbital Theory. FMO theory states that interactions occur between the highest occupied molecular orbital (HOMO) of one molecule and the lowest occupied molecular orbital (LUMO) of a second molecule. In the case of an SN2 reaction, the HOMO is on the nucleophile and the LUMO is the σ* of the bond. Generally speaking the HOMO is usually a lone pair or another bonding orbital such as a π bond being broken (C ­Br in the example above). The FMO diagram of a general HOMO ­LUMO interaction is shown below. From the above diagram we can see that other possibilities exist, however the strongest interaction will occur between the HOMO of molecule 2 and the LUMO of molecule 1, and therefore, this interaction is the most significant contributor to reactivity. II. Using Molecular Orbital Theory to Rational Molecular Conformations: To begin studying the effects of orbital interactions on molecular conformations consider 1,2 ­difluoroethane. If we only consider steric interactions we w...
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This note was uploaded on 01/24/2014 for the course CHEM 436 taught by Professor White during the Spring '08 term at University of Illinois, Urbana Champaign.

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