Lecture 1 Monday January 5 2015 11 56 AM The resonance structures for 3 ally fragments cation radical and anion imply the 2 electrons of the double bond are shared by all three atoms in the fragment The key aspect of the structure of these three ally fragments is the presence of 3 p atomic orbitals on 3 consecutive carbons All 3 orbitals overlap to give a conjugated system in which electrons are shared by all 3 atoms The resonance structures also suggest that the charge is localized on terminal carbons carbons 1 and 3 and not on the middle carbon carbon 2 The same is true for the radical and the anion This is consistent with the shapes of molecular orbitals that we get from computations Shown below are 3 molecular orbitals that are obtained by linear combination of the 3 atomic orbitals The lowest energy one is one with no nodes and electrons in that orbital those from a double bond can find themselves on any of the carbons with equal probability The next orbital has one node it s not continuous has a break in which the probability of finding an electron there is 0 That node is on the second carbon So in the allyl cation we have 2 electrons and they go in the lowest energy orbital The next molecular orbital MO is empty and that s where the charge is terminal carbons Similar analysis tells us that the 2 electrons in the radical and the anion are shared by all 3 carbons those are the electrons in the lowest energy MO and that radical and the anion are localized at the termini LUMO Lowest unoccupied orbital tells us where the charge is in allylcation HOMO Highest occupied molecular orbital tells us where the charge is in allylanion SOMO Singly occupied molecular orbital tells us where the unpaired electron is Lectures 2016 Page 1
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