Lesson11 - Energy Mismatched AOs Make Different Contributions to MOs Energy Matched Case AOs contribute equally to the MOs Energy Mismatched Case

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Energy Mismatched AOs Make Different Contributions to MOs Energy Matched Case Energy Mismatched Case AOs contribute equally to the MOs AOs make disproportionate contributions to the MOs
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Polarized Bonds: Unequal AO Contributions Compare the C-F Bond in CH 3 –F to the C-C bond in H 3 C-CH 3
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H 3 C–CH 3 H 3 C–F sp 3 - sp 3 sp 3 - sp 3 C C C C σ σ ± C F C F σ σ ± C-C vs. C-F Sigma Bond Energies are matched Energies are mismatched Calculated LUMO for CH 3 -F (WebMO)
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σ n π σ ± a π ± σ→σ ± σ→ a σ→π ± filled empty n →σ ± n a n →π ± π→σ ± π→ a π→π ± Filled Empty Orbital Interactions When it comes to molecular structure and reactivity, the only interactions that are important are interactions between filled and empty orbitals. In other words, filled-filled and empty-empty interactions are of no significance. Of all the possible filled-empty orbital combinations, one pairwise combination matters most - it’s the interaction between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) . The HOMO & LUMO orbitals are known as the frontier orbitals , and the HOMO-LUMO pairwise combination is called the frontier orbital interaction . We can systematically enumerate the possible HOMO-LUMO pairwise combinations of the commonly encountered filled ( σ , n , π ) and empty ( σ ± , a , π ± ) orbitals (note: a = an atom-centered empty orbital). The table below shows there are just 9 such combinations. Each combination can either be a sigma-type (coaxial or end-on) or a pi-type (side-by-side), making a total of 18 different frontier orbital interactions that will explain almost everything in chemistry! Whether an interaction is pi-type or sigma-type will depend on constraints imposed by molecular geometry (if there are no constraints, a sigma-type interaction is favored).
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Frontier Orbitals & Reactivity Consider the reaction: BF 3 + NH 3 F 3 B–NH 3 - + Empty a Filled n This is the meaning behind our use of curved arrow convention to illustrate __________ electron flow B F F F sp 2 -sp 3 p orbital is empty (a boron-centered empty orbital) N H H H sp 3 -s sp 3 a non-bonding electron pair An n a σ -type interaction The usual order of energy levels σ * π * a n π σ energy
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Curved Arrow Convention & Electron Flow
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This note was uploaded on 01/25/2012 for the course CHEM 232 taught by Professor Miller during the Spring '08 term at University of Illinois, Urbana Champaign.

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Lesson11 - Energy Mismatched AOs Make Different Contributions to MOs Energy Matched Case AOs contribute equally to the MOs Energy Mismatched Case

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