Subject: relative stabilities in water: zwitterion vs anion

From: "Olga Dmitrenko" <odmitr.-at-.UDel.Edu>
Date: Thu, 04 Mar 2004 15:00:03 -0500

Dear CCLers, 

The molecule I am studying is bifunctional and can exist in aqueous
solutions as zwitterion or anion depending on pH.
 I am trying to compare the energies of zwitterionic
and anionic forms by optimizations of the following H-bonded systems put
into water media (using COSMO model):

1) zwitterion/OH-  and anion/H2O    - this system has net charge ³-1²
2) zwitterion/H2O and anion/H3O+  - this system is neutral, which is better
> from the computational point of view.

I¹ve got VERY different results. In case #1, the anionic form of my molecule
was more stable. In the second case, the zwitterion was the preferred form.
I feel that this is a wrong approach, but I can not compare directly the
energies of zwitterion and anion because of different number of atoms.

I would greatly appreciate if someone will point out the publications which
rise similar questions.

Best regards,
Olga