Subject: CCL:Empirical solvation model

From: "George P. Ford" <[email-masked]>
Date: Fri, 15 Aug 97 12:18 CDT

Jian Wang wrote:
> 
> Could someone points to me some empirical methods to 
> estimate free solvation energy of a molecule in
> aqueous solution? 
> 
There are a couple of purely empirical methods for estimating solvation free 
energies. All use some kind of atom, group, or bond additivity. The scheme 
published by Cabani et al also predicts enthalpies and heat capacities of 
solution. Hine's method [2], based on Benson-like group contributions, is very 
accurate at the cost of a somewhat narrower range of applicability. We published
an extension of Hine's method to organic onium ions [3] that worked quite well.

[1] Cabani, S.; Gianni, P.; Mollica, V.; Lepori, L, J. Solution Chem.,
    1981, 10, 563-595.
[2] Hine, J.; Mookerjee, P. K., J. Org. Chem., 1975, 40, 292-298.
[3] Ford, G. P.; Dcribner, J. D., J. Org. Chem., 1983, 48, 2226-2233.
 
> There are many solvation models to calculate solvation 
> free energy such as QM SCRF methods, AMSOL, PB,
> GBSA, MD, MC and etc.  What we would like to know is
> the performance of empirical methods, i.e., what are
> their standard errors over a large set of experimental 
> data or over other theoretical results, where an empirical
> approach will break down.  

This is an interesting question. Most published comparisons of theoretical and 
experimental hydration energies are dominated by simple monofunctional 
compounds. Here the empirical methods are far more accurate than even highly 
parameterized methods like SM2. Unfortunately the most interesting chemistry 
invovles complex polyfunctional compounds where empirical methods should fail to
some degree because of direct and/or indirect interaction between the 
substituents. On the other hand QM-based methods should be partly immune to 
this. We've tried to look into this in more detail. Unfortunately there just do 
not seem to be enough accurate experimental hydration data for appropriate 
polyfunctional compounds available. 

============================================================================
George P. Ford                      |     email:  [email-masked]
Department of Chemistry             |       web:  http://www.smu.edu/~gford/
Southern Methodist University       | telephone:  (214)768-2479
Dallas, Texas 75275                 |       fax:  (214)768-4089
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