Molecular Orbitals of CH3F
(ordered by energy)
The F atom is slightly nearer the viewer
than the C atom.
Different types of approximate calculation
yield fundamentally similar shapes, especially with respect to
arrangement of nodes.
For the qualitative purposes of Chem 125 we are interested in the
similarities rather that in the differences of detail.
A dramatic difference is in the sophistication of the graphical
presentation. In some ways the older, simpler presentation may be
clearer, but rotating the new ones on the computer display gives a
quicker feeling for their location relative to the nuclei and their3D
shape.
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Improved MOs calculated today
in less than 1 minute on a laptop
contoured at a lowish value of psi
relatively far from nuclei
(note scale of bond lengths)
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Crude MOs calculated 25 years
ago,
when it was a real accomplishment
using room-filling computers
contoured where e-density would be 0.07
e/Å3
at a higher value of psi; closer to nuclei
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s*
Leftovers;
mostly 2pC
antibonding
with a little 2pF
This "LUMO"
makes C-Halogen
a functional group
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Degenerate
p HOMOs
slightly
antibonding
between 2pF
and
sCH
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Unshared pair
2pF s AO
(with some
bonding to
C or sCH)
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Degenerate
p MOs
bonding between
2pF and sCH
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sCH bonds
overlapping
favorably
with a little of 2pF
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Mostly 2sF
distorted by a
modest bonding
contribution
from C
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From MacSpartan Plus [3-21G(*) AOs]
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From Salem and Jorgensen,
Organic Chemist's Book of Orbitals
Academic Press, 1973
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It is fun to find the nodes in these MOs, and to
rationalize, in retrospect, what went into each of them and why. But
it is not so easy to predict their exact shape without a computer, so
if this look almost impossibly challenging, don't feel that your missing something important,
humans can do it reliably in their head.
Fortunately, we don't really care about the shape
of most MOs - only the LUMO (and/or HOMO), and even then only when it
is unusually low
(or high) in energy. These "frontier orbitals" are the only ones
whose energy-match with orbitals from other molecules is good enough
to make them relevant to reactivity.
High Unoccupied MOs are irrelevant, because
electrons will never go there. Low Occupied MOs are nearly
irrelevant, because their energy match with UMOs is too poor for them
to be involved in reaction - as a set they are just one way to carve
up the pie of molecular electron density into cognitively bite-sized
portions. There are other ways.