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4.5 Total Charges

Total charges are obtained by summation of the results of the calculations on - and -charges (see the two preceding sections).

Values calculated
qA,tot (QTOT (A)): Total charge on atom A

qAB,tot (DQTOT (AB)): Difference in the total charges on atoms A and B of a bond

Some systematic investigations have been made to compare the PEOE total charges with those from Mulliken population analyses (MPA). Because of the strong dependence of MPA on the basis set, only MPA values from the same basis set should be compared. The choice fell on a MPA of ab initio STO-3G wave functions as this basis set is known to give reasonable partial charges. Furthermore, a large variety of results is available in the literature. Altogether, STO-3G charge values were found in the literature for 63 compounds. This dataset was augmented with calculations on another 65 molecules performed for the purpose of this comparison. It is known that ab initio STO-3G calculations tend to overestimate the polarization of bonds. Therefore, comparison was made not with atomic charges but with group charges, obtained by adding the charges of terminal atoms to that of the atom to which they are bonded. Table 10 summarizes the results obtained for neutral species.

class of
compounds

no. of
cpds

no. of
groups

correlation
coefficient

hydrocarbons

9

31

0.82

halogen cpds

23

74

0.83

nitrogen cpds

19

55

0.76

oxygen cpds

16

56

0.97

boron cpds

9

31

0.98

phosphorous (III)-cpds

4

13

-0.92

sulphur (II)cpds

9

25

-0.18

Table 10. Comparison of PEOE charges with values from a Mulliken population analysis on ab initio (STO-3G) group charges

The hydrocarbons cover only a small range of charge values. Therefore small changes in the charge values strongly influence the correlation. In fact, deleting the point for methane that has the largest deviation, improves the correlation coefficient to 0.92.
The results for the nitrogen compounds are somehow disappointing. The largest derivations occur for compounds having nitrogen-oxygen bonds (nitrile oxides, oxaziridine etc.). Apparently, the separation of the electron density in an N-O bond is handled differently by the two methods.
The results of the oxygen and boron containing compounds show an extremely good agreement.
For the phosphorous-III and sulphur-II compounds (thiols, thioaldehyds, thioacids) no correspondence in the charge values was found. In fact, the charge values of the PEOE method run against those from the MPA. No decision can be made whether this is a deficiency of the PEOE or of the MPA charges.

Application
Several physical properties have been correlated or calculated from the charge values. Dipole moments can be calculated from the total charges when the atomic coordinates are available. 3D-coordinates can be obtained from the CORINA system. The following set of three compounds shows that the effect of through-conjugation in p-nitroaniline is taken into account by the procedure calculating -charges.

 

 

Experimental

1.15

4.16

6.91

[Debye]

Calculated

0.90

4.29

6.87

[Debye]

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