CCL:G: Gsolv - pKa - Frequency

 Sent to CCL by: Andreas Klamt [klamt,+,cosmologic.de]
  Hi all together,
I think that Chris is quite right in most of his arguments, and definitely Mike is right as well in saying that the partial molar entropy is thermodynamically well defined and that it can be considerably different in solution compared to the ideal gas.
But this difference is to first order already implicitly parameterized into every continuum solvation model that is parameterized versus exp. dG_solv. data. Since these data do include the entropy change, the latter is also implicitly included into the parameterized solvation models, not based on a frequency change, but via surface proportional corrections.
If you try to add the same via frequency based corrections (which are not well defined, as I mentioned earlier and as Chris mentioned in detail), you can only make things worse!
In a solvation model which takes into account temperature dependence, as in COSMO-RS, you can quantify the included partial molar entropy of solvation based on the temperature derivative of dG_solv.
If you would like to develop a solvation model which is open for the explicit addition of solvation entropy, you would have to do that based on the the enthalpy (heat) of solvation, but there are much less exp. data available for dH-solv than for dG_solv, and few people are interested in dH_solv. The only situation for which lots of well measure dG_solv ad dH_solv data are available, is self-solvation, which is nothing else than free energy of vaporization (i.e. ln(vapor pressure) and heat of vaporization). It should be noted that COSMO-RS describes dG_vap and dH_vap consistently as self-solvation. 
 Regarding your pKa problem (as for any reaction calculation) this means:
- Do the best gas phase calculation as you can afford, incl. gas-phase entropy change, and then add the solvation free energies of educts and products (with the right sign). - Or use a parameterized pKa model directly based on the solution phase energy difference of protonated and deprotonated species. 
 Hope this helps.
 Andreas
 Am 05.09.2010 14:57, schrieb W Flak williamflak- -yahoo.com:
 
 Sent to CCL by: "W  Flak" [williamflak]![yahoo.com]
 Dear CCL
 I got some questions about pKa calculation hoping to get a help here.
 * First of all, on G03 help page, it says Gsolvation=EPCMEgas, but if I
 compared delta Gsolv obtained by SCFVAC with those obtained by carrying out two
 separate jobs, I'd get difference. Why?
 Example:
 by SCFVAC
 DeltaG (solv) (kcal/mol) = -10.26
 By two separated jobs:
 deta Gsolv = Epcm - Egas = -15.49
 * Is the entropy of a molecule in gas phase different from that in a solution?
 if yes, please direct me to the undergraduate book I should read
 * I read many posts on pKa calculation and found some people (as Andreas
 Klamt,2006) recommended not to include frequency in pKa calculation, and the
 others include it. What do you recommend?
 * In G.A.A. Saracino et al. / Chemical Physics Letters 373 (2003) 411415
 Gibbs energies have been estimated by single point HF/6-31+G(d,p) calculations
 at geometries optimised in aqueous solution at the PBE0/6-31+G(d,p) level. How
 they calculated frequency at level differs from the optimization level?
 Any kind of help would be appreciated
 W. Flak>
 
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