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 their 3D shape.

Improved MOs calculated today
in less than 1 minute on a laptop

contoured where e-density would be 0.001 e/3
relatively far from nuclei
(note scale of bond lengths)

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/3
much higher value of psi; closer to nuclei

σ*

Leftovers;
mostly 2pC
antibonding
with a little 2pF

This "LUMO"
makes C-Halogen
a functional group

 

 

Degenerate
π HOMOs

slightly
antibonding
between 2pF
and 
σCH

 

 

 

Unshared pair
2pF 
σ AO

(with some
bonding to
C or
σCH)

 

Degenerate
p MOs

bonding between
2pF and
σCH

 

 

 

σCH bonds
overlapping
favorably
with a little of 2pF

 

 

Mostly 2sF

distorted by a
modest bonding
contribution
from C

 

 

From MacSpartan Plus [3-21G(*) AOs]
From Salem and Jorgensen,
Organic Chemist's Book of Orbitals
Academic Press, 1973

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 you're missing something important, humans can't 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.


Return to Examples of MOs
Return to Chem 125 Homepage
Compare LUMOs for HF, CH3F, C2H5F

copyright J. M. McBride 2001