From owner-chemistry@ccl.net Mon Dec 8 06:22:00 2008 From: "Pierre Archirel pierre.archirel{=}lcp.u-psud.fr" To: CCL Subject: CCL: translational entrpy in solvent Message-Id: <-38261-081205121324-11149-I2y4EO06zp0fYAceLc81kg[*]server.ccl.net> X-Original-From: Pierre Archirel Content-Transfer-Encoding: 8bit Content-Type: text/plain; charset=ISO-8859-1; format=flowed Date: Fri, 05 Dec 2008 17:31:46 +0100 MIME-Version: 1.0 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 __________________________________________________________ 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 __________________________________________________________