The Mulliken charges of the protonated enone system (Fig.6.9b) make the oxy-
gen negative, which may seem surprising. However, this is normal for protonated
oxygen and nitrogen (though not protonated sulfur and phosphorus): the hetero
atom in H 3 Oþand in NH 4 þis calculated to be negative (i.e. the positive charge is
on the hydrogens) and the hetero atom is also negative in H 2 C¼OHþand
H 2 C¼NH 2 þ. On the oxygen and the carbon furthest from the oxygen (C 3 ) the
HF/3-21G()charges differ considerably from the semiempirical ones: the HF
calculations make the O much more negative, and make C 3 negative, suggesting
that they place more positive charge on the hydrogens than do the semiempirical
calculations. The three methods do not differ as greatly in their bond orders
(that bond orders are less fickle than charges has been noticed before [ 111 ]),
although the HF method makes the formal C/O double bond essentially a single
bond (bond order 1.18).
Finally, electrostatic potential (ESP) charges and, for the HF/3-21G()calcula-
tions, L€owdin bond orders, are shown (Fig.6.9c and d). For the enolate, all three
methods make the ESP charge on carbon more negative than that on oxygen, but the
bond orders are not greatly altered. For the protonated enone system, AM1 and PM3
suggest more polarization of electrons toward the O in the C/O bond than is shown
by the Mulliken charges, but while the HF ESP charge on this carbon is greater than
the Mulliken (0.76 versus 0.45), the charge on oxygen is unchanged. The Hartree-
Fock L€owdin bond orders for all three bonds of the CCO framework (1.55, 1.29,
1.76) are all somewhat bigger than the Mulliken bond orders (1.18, 1.15, 1.59).
These results indicate that charges are more dependent than are bond orders on
the method used to calculate them, and that charges are also harder to interpret than
are bond orders. As with ab initio charges and bond orders, the semiempirically
calculated parameters may be useful in revealing trends in a series of compounds or
changes as a reaction proceeds. For example, ab initio bond order changes along a
reaction coordinate have been shown to be useful [ 112 ], but presumably semiem-
pirically calculated bond orders would also yield similar information, at least if the
species being studied were not too exotic. Clearly, one must use the same semiem-
pirical method (e.g. AM1) and the same procedure (e.g. the Mulliken procedure) in
studying a series.
6.3.5 Miscellaneous Properties – UV Spectra, Ionization
Energies, and Electron Affinities
All the properties that can be calculated by ab initio methods can in principle also
be calculated semiempirically, bearing in mind that the more the molecule of
interest differs from the training set used to parameterize the semiempirical pro-
gram, the less reliable the results will be. For example, a program parameterized
to predict the UV spectra of aromatic hydrocarbons may not give good predictions
for the UV spectra of heterocyclic compounds. NMR spectra are usually calculated
with ab initio (Section5.5.5) or density functional (Chapter 7) methods. UV
6.3 Applications of Semiempirical Methods 431