Subject: summary: J in AM1

From: Inge Muszynski <[email-masked]>
Date: Wed, 17 Jun 1998 12:51:51 +0200

Dear CCL-lers,

here is a summary of the responses I got in answer to the question: whether
the Jodine atom is well parametrized within Mopac 6 (AM1) because a
minimization results in a positive partial charge of 0,137 for this atom.

1. J is parameterized in AM1. You will find this in the file block.f in the
source code.

2. A positive partial charge of Iodine in the Iodophenole
molecule may be right since the electronegativity of chlorine is 
a bit less than that of a sp2-carbon (I: 2.57, C(sp2): 2.69, 
cf. Angew. Chem. 1996, 108, 162ff.).

3. since C and I have equal Pauling electronegativities, one could expect
some near zero charge. If +0.14 is a small charge compared to e.g. the
partial charges in a C=O-group, I would not worry.

The NBO analysis of high level ab-initio calculations
(QCISD/pVTZ+diffuse, J. Am. Chem. Soc. 1997, 119, 6648) on CH2I+ shows
I-->C sigma- and pi-Donations of 0.19 and 0.67 electrons and I and C
partial charges of 0.86 and -0.35. This shows that I acts as a sigma
donor to C even though C is much more negatively charged. Correspondig
calculations on CH2Br+, CH2Cl+ and CH2F+ show that the Br-->C sigma
donation is weaker (0.07), while Cl and F are sigma _acceptors_ (0.04,
0.43) in these compounds.

4. The ESP-Charges and the charge calculated with GAMESS AM1 are comparable. 

Once again many thanks.

Inge Muszynski