CCL: translational entrpy in solvent

 Sent to CCL by: Pierre Archirel [pierre.archirel+/-lcp.u-psud.fr]
 Dear colleagues,
1- I wonder that your disputation makes no reference to densities of states... Translation entropy in solution should be smaller than in the vacuum because due to caging effects the density of states decreases. The same should be true for rotation. Actually both vacuum translation and rotation become vibrations of the global system, it can be in principle evaluated with harmonic analysis of the solute + 1 solvent shell system. 
 2- I summit the following test to your commentaries:
 take the two reactions A+A->A2 and A2->A+A in the gas phase:
each of the three species has -TStrans=-0.5 eV at 300K (it only weakly depends on masses) Hence in the first case Delta(-TStrans) amounts to +0.5 eV, in the second case to -0.5 eV This means that in the gas phase translation entropy always tends to inhibate association 
 and to favor dissociation.
 THIS HAS BEEN EXPERIMENTALLY VERIFIED
 (see for example S. Le Caer et al, PCCP 4 1855-1865 (2002))
3- Is there any clue that this general law is true in solution? 0.5eV is huge! My current PCM calculations suggest that it is false, I best compare to experiment 
 when simply adding vacuum vibrational free energies to PCM electrostatic and
 non electrostatic solvation free energies.
This means that the translation entropy probably behaves like in the vacuum, but with 
 much smaller values, not 0.5 eV!!
 Any comment will be appreciated.
 Pierre Archirel
 LCP, bat 349
 Universite Paris-Sud
 Orsay, France
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 Pierre Archirel
 Groupe de Chimie Théorique
 Laboratoire de Chimie Physique      Tel: 01 69 15 63 86
 Bat 349                             Fax: 01 69 15 61 88
 91405 Orsay Cedex
 France                   pierre.archirel:-:lcp.u-psud.fr
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