From owner-chemistry@ccl.net Thu Dec 4 02:49:00 2008 From: "Andreas Klamt klamt!=!cosmologic.de" To: CCL Subject: CCL: translational entropy in solvent Message-Id: <-38239-081204024042-24559-Eb/uImp1s5tFeD/dUP9sFA|server.ccl.net> X-Original-From: Andreas Klamt Content-Transfer-Encoding: quoted-printable Content-Type: text/plain; charset=ISO-8859-15; format=flowed Date: Thu, 04 Dec 2008 08:40:25 +0100 MIME-Version: 1.0 Sent to CCL by: Andreas Klamt [klamt*cosmologic.de] Dear Luis, yes, the issue has been previously discussed, but it it important and=20 thus comes again and again: 1) We computational chemists are lucky to be able to have the rather=20 exact equations for the (translational, rotational, and vibrational)=20 free energy of molecules in the ideal gas phase (apart from=20 anharmonicity corrections and some problems with rotor modes). This ia a = clear and nice reference state. 2) There are solvation models which allow us to reasonable estimate=20 dG_solv for almost any compound in almost any solvent. We have just=20 recently proven on an external data set (originally collected by Cramer=20 and Truhlar in context of their SM8 model) of ~2400 exp. dG_solv values=20 for a wide variety of molecules in a wide variety of solvents that our=20 COSMO-RS (COSMOtherm implementation) can calculate dG_solv with an=20 average error of 0.48 kcal/mol. Most likely the method is even better=20 than that, because the experimentatl data set included a lot of=20 experimental data which appear to be more than questionable, but we did=20 not exclude any of them for the sake of comparability. (The study is=20 submitted for publication). 3) Now, let us assume that the error of a good solvation model is 0.4=20 kcal/mol at room temperature, which is just only a factor of 2 in the=20 partition function (and in the Henry's Law constant, partial pressure,=20 =2E..) In the calculation of a reaction free energy we will most likely = benefit from some compensation of errors on the educt and product side.=20 Then this is most likely accurate enough for not being responsible for=20 significant errors in calculations of reaction free energies. All other=20 errors arising from quantum chemistry, and even from the approximations=20 in the rotational, and vibrational free energy contributions are most=20 likely larger than the error coming from good solvation models. Conclusion: The translational, rotational, and vibrational free energy=20 conributions of solvation are apparently well parameterized into=20 quantitative solvation models. Hence a combination of as good as=20 possible gas phase thermodynamics (i.e. free energy calulations) with a=20 quantitative solvation model is the best you can do in the moment. Don't = start to try to calculate parts of the translational, rotational, and=20 vibrational free energy conributions in solution. This will lead to=20 double counting of effects already implicitly taken into account in=20 solvation models. Andreas P.S.: For most solvation models this is true at room temperature. To my=20 best knowledge my COSMO-RS method is the only imlicit solvation model=20 which derives the free energy of solavtion from a partition functions=20 and separates entropy and enthalpy, and which is validated=20 simultaneously on free enrgies and enthalpies of solvation, and on=20 solvation properties at varying temperature in the range of -100 =B0C to = ~=20 300=B0C. Luis M Sim n lsimon^usal.es schrieb: > Sent to CCL by: "Luis M Sim n" [lsimon|-|usal.es] > Many quantum chemistry packages include implicit solvation models, such= as PCM, CPCM, COSMO,=20 > etc. Nevertheless, even when the optimization was done using any of tho= se solvation models, I have=20 > observed that the thermal contributions to delta G are calculated assum= ing that the molecules are in=20 > the gas phase. Therefore, the translational entropy is overestimated (s= ee, for example, C. Hunter,=20 > ACIE, 43, 5310-5324, 2004).=20 > > There are models that can estimate this translational entropy in soluti= on (Warshell et al.,=20 > J.Phys.Chem.B, 104, 4578-4584, 2000), but unfortunately these are not s= traightforward to implement,=20 > and computational requirements might exceed our possibilities.=20 > > On the other side, I have seen that implicit solvent calculations many = times offers a "free solvation=20 > energy". I do not know if, in that free solvation energy, the translati= onal energy is, somehow,=20 > evaluated and included, or if it just accounts for cavitation and polar= ization contributions. > > My question is: is it correct to add up the translational estropy calcu= lated assuming gas phase=20 > behaviour to that free solvation energy? Should not it be more correct = if the vibrational or even=20 > rotational contributions are included in deltaG calculations but transl= ational contribution is excluded?=20 > Does anyone knows any easy method for, approximately, account for the t= ranslational entropy in=20 > solution, in case that have to be added to the vibrational and rotation= al contributions? > > Sorry if that issue have previously been discussed. > > > > -=3D This is automatically added to each message by the mailing script = =3D- > To recover the email address of the author of the message, please chang= e> > Subscribe/Unsubscribe:=20> > Job: http://www.ccl.net/jobs=20= > > Search Messages: http://www.ccl.net/htdig (login: ccl, Password: searc= h)> > > > > =20 --=20 -------------------------------------------------------------------------= - Dr. habil. Andreas Klamt COSMOlogic GmbH&CoKG Burscheider Str. 515 51381 Leverkusen, Germany Tel.: +49-2171-73168-1 Fax: +49-2171-73168-9 e-mail: klamt[-]cosmologic.de web: www.cosmologic.de -------------------------------------------------------------------------= - COSMOlogic Your Competent Partner for Computational Chemistry and Fluid Thermodynamics -------------------------------------------------------------------------= - Please note our COSMO-RS Symposium in 2009=20 (For details see: http://www.cosmologic.de/Symposium/symposium.html)