Computational Chemistry

5.5.4.1 Dipole Moments

The dipole moment [ 250 ] of a system of two chargesQand#Qseparated by a
distanceris, by definition, the vector Qr; the direction of the vector is officially
from#QtowardþQ, but chemists usually assign a molecular or bond dipole (see
below) the direction from the positive end of the bond or molecule to the negative
(Fig.5.37a). The dipole moment of a collection of chargesQ 1 ,Q 2 , ...,Qn, with
corresponding position vectorsr 1 ,r 2 , ...,rn(Fig.5.37b) is

m¼

Xn

1

Qiri ð 5 : 205 Þ

and the so the dipole moment of a molecule is seen to arise from the charges and
positions of its component electrons and nuclei. For a neutral molecule the dipole
moment is an unambiguous experimental observable [ 251 ] (unlike some other
quantities based on electron distribution), with which we can compare calculated

80

60

40

20

4000 3000

3500 3000 2500

2000 1000

4000 2000 1500 1000 500

4000 3500 3000 2500 2000 1500 1000 500

(^37003673)
2981
2865, 2844
2826
1056
1016
1033
4117
3306
3795
MP2 / 6-31G
3223
3076
3144
3185
3232
HF / 6-31G
1508
11881164
1418
1083
0
0
47
50
93
1.4e+002
1e+002
1.5e+002
Methonol
Experimental
FREQ_VAL
FREQ_VAL
IR_INTENS
IR_INTENS
Fig. 5.36 Experimental (gas phase), HF/6–31G and MP2(fc)/6–31G calculated IR spectra of
methanol
340 5 Ab initio Calculations