Subject: RE: CCL:Atomic charges of halogen containing molecules

From: <[email-masked]>
Date: Thu, 19 May 1994 19:54:26 +0200 (WET-DST)

(Regarding to the question from Jaikwang Shin)
 
Jai,
    You are right. The MEP fitted atomic charges of halogen
atoms should be relatively small. The reason is that the
electrostatic potentials around these atoms are much less
negative than those of Oxygen and Nitrogen atoms in molecules.
Examples may be found in J.Comp.Chem.14(1993)1101 for your 
interest. The charges, which might be too small in your
calculation, are those of the atoms in the carbonyl group.
    Below is a test I did on SGI with the method in the paper
mentioned above (also see JCC 15(1994)200). The MEP charge 
of the chloride is small in my calculation, too. The method 
has been coded and become available in MOPAC93.
 
> AM1 1SCF QPMEP CONNOLLY
> CH3COCH2CL, H=-55.036128 KCAL(am1,gas)
>
> (......)
>              NET ATOMIC CHARGES AND DIPOLE CONTRIBUTIONS
>
>         ATOM NO.   TYPE          CHARGE        ATOM  ELECTRON DENSITY
>           1          O          -0.2716          6.2716
>           2          C           0.2234          3.7766
>           3          C          -0.2697          4.2697
>           4          C          -0.1836          4.1836
>           5          H           0.1112          0.8888
>           6          H           0.1177          0.8823
>           7          H           0.1026          0.8974
>           8          H           0.1340          0.8660
>           9          H           0.1230          0.8770
>          10         Cl          -0.0869          7.0869
> DIPOLE           X         Y         Z       TOTAL
> POINT-CHG.     1.613     0.461    -1.036     1.972
> HYBRID         0.317    -0.078    -0.036     0.329
> SUM            1.930     0.383    -1.072     2.241
>
> (......)
>
> ATOMIC CHARGES DERIVED FROM MOLECULAR ELECTROSTATIC POTENTIAL
>        (CONNOLLY SURFACES)
>
>    ATOM NO.    TYPE        CHARGE
>         1         O       -0.4291
>         2         C        0.5904
>         3         C1      -0.3703
>         4         C2      -0.4495
>         5         H(C1)    0.1203
>         6         H(C1)    0.1152
>         7         H(C1)    0.1089
>         8         H        0.1975
>         9         H        0.1782
>        10        CL       -0.0615
>
>  NUMBER OF POINTS        695
>  RMS DEVIATION        1.1522
>  RRMS DEVIATION       0.1400
>
>
> DIPOLE MOMENT EVALUATED FROM THE MEP CHARGES
>       D(X)     D(Y)     D(Z)     TOTAL
>      1.8287   0.4348  -1.1277   2.1920
>
> TIME TO CALCULATE MEP     0.36 SECONDS
>

Ciao,

Bingze

========================================================
Bingze Wang, Ph.D.
IRBM     
Via Pontina Km 30,600
00040 Pomezia (Roma)
Italy
Tel +39-6-91093635
fax +39-6-91093225