Subject: QM CHARGES

From: "E. Lewars" <[email-masked]>
Date: Thu, 22 Jun 1995 15:41:12 -0400

There was a question about QM charges and fluorocarbanions reccently.
Some of these species were said to show a positive C, and the question of the
reliability of quantum mechnically calc charges arose.
  There seems to be no one, right way to calc charges on atoms in molecules,
and these charges evidently can't be measured ("are not QM observables") --
but see Pearlman and Kim, J Mol Biol 211 p 171 (1990).
General refs to calculating charges:
Wiberg et al, J Comp Chem 14 p1504 (1993)
Basis set dependence:
Rodrigues et al, J Comp Chem 14 p922 (1993)
Other refs:

JACS 107 p1829 (1985)
JACS 113 p 5203 (1991)
JACS 115 p614 (1993)
JACS 115 p1084 (1993)
JACS 116 p6812 (1994)
J Comp Chem 14 p530 (1993)
J Comp Chem 14  799 (1993)
J Comp Chem 15  200 (1994)
Rev in Comp Chem II, 219 (1991)

I did a few calcs on some fluorocarbanions: the electrostatic potential charges
look very reasonable, and hardly change with the basis:

H-F level              CH3-              F-CH2-               F-CH-F-
charge on
C  3-21G/6-31G*     -1.53/-1.43       -1.07/-1.03           -0.70/-0.71
F    " / "            -----           -0.26/-0.27           -0.24/-0.22
H    " / "          0.18/0.14          0.17/0.15             0.17/0.15