Molecular Orbitals of O=CH2
(ordered by energy)

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.

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)

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

 

p* LUMO

Leftovers;
mostly 2p C
antibonding
with less 2pO

This LUMO
makes C=O
a functional group

The difference in relative "size" of 2pC and 2pO is somewhat exaggerated, because the O function increases in density much more rapidly as it approaches the nucleus. If they made equal contributions to the MO, the O would look smaller, but not this much smaller. Compare with their relative sizes in the p MO, two rows below, where the O/C ratio is reversed.

Unshared pair HOMO
in-plane 2pO

slightly
antibonding
with lesser
amount of
2 sCH

contrast with
third row below

 

pCO

Mostly 2pO
bonding
with less 2pC

contrast with
second row above

s unshared pair
2pO

with minor
contribution
from two sCH

contrast with
second row below

Combination
of two sCH

p bonding
with lesser
amount of
in-plane 2pO

contrast with
third row above

 

Combination
of two sCH

with minor
contribution
from 2pO

contrast with
second row above

 

sOC Bond

mostly 2sO
overlapping
favorably
with a lesser
amount from C

From MacSpartan Plus [3-21G(*) AOs]

From Salem and Jorgensen,
Organic Chemist's Book of Orbitals
Academic Press, 1973

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