Computational Chemistry

7.3.4.2 Charges and Bond Orders

The theory behind these was given in Section 5.5.4. Although it is sometimes
said that charges on atoms cannot be measured, i.e. are not observables, carefully
defined atom charges can apparently be measured [ 104 ]. However, such experi-
mental charges are not readily available, and there is no agreed-on standard for
judging the “correctness” of calculated charges (and bond orders). In practice,
electrostatic potential charges and L€owdin bond orders are often preferred to
Mulliken charges and bond orders. The effect of various computational levels on
atom charges has been examined [ 105 ].
Figure7.8shows charges and bond orders calculated for an enolate and a
protonated enone system (the same as in Fig. 6.9), using B3LYP/6-31G* and HF/
3-21G. The results are qualitatively similar regardless of whether one uses B3LYP
or HF, or Mulliken versus electrostatic potential/L€owdin. This is in contrast to the

80

60

40

20

IR_INTENS

0

43

87

1.3e+002

4000

4000

3000

3500 3000

2000

2500 2000

1000

1500 1000 500

Methanol

Experimental

(^37003673)
3759
3132
3041
B3LYP / 6-31G
2998
1399
3144
(^37953223)
MP2 / 6-31G
3076
1066
1418
1083
(^10561033)
1016
2826
2981
2865, 2844
FREQ_VAL
IR_INTENS
0
47
93
1.4e+002
4000 3500 3000 2500 2000 1500 1000 500
FREQ_VAL
Fig. 7.7 Experimental (gas phase), DFT (B3LYP/6-31G) and ab initio (MP2(fc)/6-31G)
calculated IR spectra of methanol
7.3 Applications of Density Functional Theory 489