Subject: Re: CCL:pKa calculation (2)

From: Alexandre Hocquet <[email-masked]>
Date: Thu, 28 Nov 1996 16:14:36 -0800

Emadeddin Tajkhorshid wrote:
> Actually I am mainly interested in the change of pKa value with regard to 
>conformational changes. Therefore we may bypass the calculation of ABSOLUTE
> pKa and go to RELATIVE one.
>I may compare the gas phase proton affinity of teh same species in different
>conformations.

Per-Ola Norrby wrote:
>         I would still add a semi-empirical solvation term to .Delta.G, as
> the conformers may be solvated differently.

Yes, it seems that the problem is just right here. Solvation entropy may vary
 a lot between two conformations, and thus the error will not be constant.

Another problem is the way pKa was measured (electrochemistry, NMR...),
at which temperature, and at which ionic strength. The latter is not so 
frequently notified.

By the way, i would like to thank Alan Shusterman who reminded me of the 
reference i poorly quoted :
G. Caldwell, R. Renneboog and P. Kebarle, "Gas phase acidities of aliphatic
carboxylic acids, based on measurements of proton transfer equilibria", 
Can. J. Chem., 1989, 67, 611-618.

regards to all,
-- 
Alexandre HOCQUET

Laboratorio de Cristalografía
Facultad de Ciencias Físicas
Universidad de Chile
Blanco Encalada, 2008
Santiago
Chile
fax : 56 2 696 73 59
email : [email-masked]