I wanted
to make a small resume and add commentaries on this entropy of solvation topic.
Some people (Purisima, Gilson) accurately state that the volume of the box is
the same regardless of having or not a solvent in it, so ideally there should be
no loss of translational entropy. I agree, although some claim that the
accesible volume is reduced by the solvent. Others (Ribeiro) say that the
partition function in a solvent is way to different to the Sackur-Tetrode gas
phase translational entropy. I also agree. My physical chemistry university
teachers told us that the solvent is reorganized around a solute molecule, and
this means a loss of entropy. This effect should be considered when calculating
the solvation free energy. Acording to this we should use gas phase free energy
and add the solvation free
energy.
Sadly in most programs this doesn't work. As already posted the
best values I could find are between gas phase energy and free energy. The
solvent models are not good enough to account for this loss of entropy. This is
clear when trying solvation at different temperatures, where the solvation free
energy remains unchanged, something contrary to the free energy concept. Only
molecular dynamic calculations take this into account, but this is impossible to
achieve for most of quantum chemistry mortals like me.
Klamt argues that
COSMO-RS resolves this problem, but I have never worked with this model so I
cannot comment. I'd love to see referencees or accurate data on this, but I got
lost on the COSMO web page and couldn't find this information. If someone knows
a paper that use this program and clearly details the solvation, please write
down the cite.
I want to add some referencees I could find on this, in
case someone is
interested.
Theoretical works:
(a)
Tomasi, J; Persico, M. Chem. Rev. 1994, 94, 2027.
(b)
Tomasi, J.
Theor. Chem. Acc. 2004,
112, 184. (c) Ben-Naim,
A.
J. Phys. Chem. 1978,
82, 792.
Practical works:
(a) Tamura, H.;
Yamazaki, H.; Sato, H.; Sakaki, S.
J. Am. Chem. Soc. 2003, 125, 16114. (b) Sakaki, S.;
Takayama, T.; Sumimoto, M.;
Sugimoto, M. J. Am. Chem. Soc. 2004, 126, 3332. (c)
Sumimoto, M.; Iwane, N.; Takahama, T.; Sakaki, S. J. Am. Chem. Soc.
2004,
126, 10457. (d) Braga,
A.; Ujaque, G.; Maseras, F. Organometallics 2006, 25,
3647.
I hope I'm not making mistakes on all this issues. I'm not a specialist,
I just want to be practical and as accurate as possible on my
calculations.
Finally I want to thank Luis Simon for starting this
discussion. I feel I'm not alone in my doubts.
xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx..........Sebastian
Kozuch...........xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx...The Lise
Meitner-Minerva Center....for Computational
Quantum Chemistry....Hebrew University of
Jerusalem.........kozuchs/./yfaat.ch.huji.ac.il.....http://yfaat.ch.huji.ac.il/kozuch.htmxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx