5.5.6.3 Molecular Orbitals
Visualization of molecular orbitals shows the location of those regions where the
highest-energy electrons are concentrated (the highest occupied MO, the HOMO),
and those regions which offer the lowest-energy accommodation to any donated
electrons (the lowest unoccupied MO, the LUMO). Electrophiles should bond to
the atom where the HOMO is “strongest” (where the electron density due to the
highest-energy electron pair is greatest) and nucleophiles to the atom where the
LUMO is strongest, at least as seen on the van der Waals surface by an approaching
reagent. The information provided by inspection of the HOMO and LUMO (the
frontier orbitals) is thus somewhat akin to that given by visualizing the ESP
(electrophiles should tend to go to regions of negative ESP, nucleophiles to regions
of positive ESP). Figure5.49shows the LUMOs of the ketones norcamphor and
camphor, mapped onto their van der Waals surfaces. For norcamphor (Fig.5.49a),
the prominence of its LUMO at the carbonyl carbon as seen from the “top” orexo
face (the face with the CH 2 bridge) rather than the bottom (endo) face, suggests that
nucleophiles should attack from theexodirection. In accord with this, hydride
donors, for example, approach from theexoface to give mainly theendoalcohol.
For camphor, where the bridge is C(CH 3 ) 2 instead of CH 2 , theexoface is shielded
by a CH 3 group which sterically thwarts the electronically preferred attack from this
direction, and so nucleophiles tend to approach rather theendoface, a fact nicely
rationalized by visualizing simultaneously the LUMO and the van der Waals
surface (Fig.5.49b)[ 325 ].
Fig. 5.49 (a) Norcamphor, with the LUMO mapped onto the van der Waals surface. The LUMO
as seen on the surface is most prominent at the carbonyl carbon, on the "top" of the molecule (the
exoface), as shown by the blue area. Viewed from the bottom of the molecule (not shown here),
the LUMO still lies at the C¼O carbon, but is less prominent (the blue is less intense). We can thus
predict that nucleophiles will attack the C¼O carbon, from theexodirection. (b) Camphor
(norcamphor with three methyl groups): the carbonyl carbon is shielded fromexoattack by a
methyl group, so for steric reasons nucleophiles tend to attack this carbon from theendodirection,
despiteexoattack being electronically favored
370 5 Ab initio Calculations