Subject: Re: CCL:Visualization of COSMO polarization charge densities and logP

From: Jacco van de Streek <[email-masked]>
Date: Mon, 18 Dec 2000 14:04:45 +0100

Dr. Andreas Klamt wrote:
> As I pointed out in my COSMO-RS papers (for references see
> http://www.cosmologic.de) the polarization charge density sigma of a
> molecule embedded in a conductor is a very good, local descriptor for
> molecular interactions in solution. Indeed it is an even
> better descriptor than electrostatic potential (ESP), because ESP is rather
> longe-ranging while interactions are more local.
> As an example consider a surfactant with a positively charged head group.
> ESP shows a positive sign on the entire alkane chain of the surfactant,
> although this alkane chain behaves like a normal alkane chain.

Dear Dr Klamt,

I myself take an interest in the electrostatic properties of n-alkyl
fragments and find your concrete example very interesting.

Even though I'm interested in modelling crystal structures rather than
solutions, my interpretation of my own results on n-decylammonium seems
to contradict your statement, and I'd be interested in your comments.

When calculating the ESP derived charges of n-decylammonium using the
ChelpG option in Gaussian94 on a geometry-optimised molecule, SCF/HF
6-31G**, constraining the dipole moment of the point charges to its
quantum-mechanical value, my results are as follows (see the attached
Decylammonium.gif file):

- The ammonium group and the first methylene unit attached to it carry a
total charge of +0.9e.
- The remaining +0.1e is shared by the remaining 9 methylene units.

In my opinion, this seems reasonable for an n-alkyl chain. 

Where do our interpretations differ?

> Best regards
> Jacco van de Streek
-- 
Jacco van de Streek (mailto:[email-masked])
Dept. of Solid State Chemistry
University of Nijmegen

---

From: "Dr. Andreas Klamt" <[email-masked]>
Date: Mon, 18 Dec 2000 16:30:26 +0100

Dear Dr. de Streek,

I think we do not differ at all. You are talking about ESP fitted charge,
I am talking about ESP visualized on the surface. Even
though the ESP on such a chain is rather long-ranging,
i.e. the potential will have the positive signe of the head-group quite
far along the chain, the backfitted point-charges will be rather
localized. My point is, that using the ESP itself as a
descriptor of polarity on the molecular surface would be misleading.

Best regards

Andreas

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e-mail: [email-masked]
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