Computational Chemistry

propenal). Consider first Mulliken charges and bond orders of the enolate
(Fig.6.9a). The AM1 and PM3 charges, which are essentially the same, are a bit
surprising in that the carbon which shares charge with the oxygen in the alternative
resonance structure is given a bigger charge than the oxygen; intuitively, one
expects most of the negative charge to be on the more electronegative atom,
oxygen; this “defect” of AM1 and PM3 has been noted by Anh et al. [ 110 ]. The
HF/3-21G(*)method gives the oxygen the bigger charge ("0.80 versus"0.67). The
two semiempirical and the HF methods all give C/C and C/O bond orders of about
1.5; this, and the rough equality of O and C charges, suggests approximately equal
contributions from the O-anion and C-anion resonance structures.

–0.76 AM1, ESP

–0.80 PM3, ESP

• 0.82 HF / 3-21G, ESP


• 0.92


• 0.94
  –1.13

1.48 AM1, Mulliken

1.47 PM3, Mulliken

1.51 1.69 HF / 3-21G, Lowdin

1.50

1.61

H

• 0.37 AM1, ESP


• 0.35 PM3, ESP


• 0.63 HF / 3-21G, ESP

0.67

0.76

0.17 0.76

0.26

• 0.03

1.45 AM1, Mulliken

1.47 PM3, Mulliken

1.19 1.55 HF / 3-21G, Löwdin

1.18

1.29

1.69

1.71

1.76

O+

O+

H

1.45 AM1

1.47 PM3

1.19 1.18 HF / 3-21G

1.18

1.15

1.69

1.71

1.59

• 0.11 AM1


• 0.08 PM3


• 0.63 HF / 3-21G

0.30

0.42

0.09 0.45

0.23

• 0.25

O–

O–

1.48 AM1

1.47 PM3

1.51 1.49 HF / 3-21G

1.50

1.42

• 0.60 AM1


• 0.65 PM3


• 0.80 HF / 3-21G


• 0.66


• 0.72


• 0.67

ab

cd

All Mulliken All Mulliken

Fig. 6.9Atom charges and bond orders calculated using the AM1, PM3 and HF/3–21G()
methods. In a and b the charges and bond orders are all from the Mulliken approach. In c and d
the charges are all electrostatic potential charges, and the bond orders are Mulliken for AM1 and
PM3, and L€owdin for HF/3–21G()(L€owdin bond orders were not available for AM1 and PM3
from the Spartan program used). Note that charges and bond orders involving hydrogens have been
omitted

430 6 Semiempirical Calculations