CCL: Thermodynamic Data & Solvation - Calculation Questions:



 Sent to CCL by: Uwe Huniar [huniar~!~cosmologic.de]
 Hello,
 
some time ago we have been asked similar questions, so I forwarded this issue to our own implicit solvation expert, Frank Eckert. Here his answer (probably not directly an answer to all your questions, though):
 The topic of reference states is sometimes a bit confusing.
 The molar reference state of 1 mol/L in both gas and solution is the
 one that typically is used to compare the computed DGsolv to
 "experimentally" determined values, while the bar/mol reference state
 is
 intended for the use in reaction free energy calculations using the
 thermodynamic cycle.
 I have put "experimentally" in quotes, because DGfus is not a direct
 observable in experiments. DGfus typically is extrapolated from
 solubility or VLE data via the Henry law constant and the definition of
 DGsolv depends on a given reference state. This by convention is the
 molar reference state of 1 mol/L in both gas and solution. The molar
 convention is also followed by Cramer and Truhlar in their SMx papers.
 They converted both experimental and calculated DGsolv value to the
 molar reference frame. The molar reference state is used in most
 databases and also by most experimentalists, but it is in no way a
 natural or self-explaining choice. It is just a convention. If you look
 into original papers from experimentalists, in particular older oner
 
 from Russia or China, you might as well also find other reference states.
 
 
Here is s link to a recent publication that tries to unravel some of the confusion:
 http://onlinelibrary.wiley.com/doi/10.1002/aic.15716/full
 Hope this helps a bit,
 Uwe
 
Am 06.04.2017 um 08:24 schrieb MIELCZAREK Detlev Conrad detlev-conrad.mielczarek-.-ifpen.fr:
 Sent to CCL by: MIELCZAREK Detlev Conrad [detlev-conrad.mielczarek]^[ifpen.fr]
 Dear CCL, a question on thermodynamic data & solvation from me, maybe you
 can help me.
 So, the basic problem for me is, that I am calculating/want to calculate
 thermodynamic data (Hf, S - hence also dG) in solvation, using implicit
 solvation models, SMD with a COSMO cavity to be specific. For my application,
 these should be accurate enough. (So no molecular dynamics simulations etc.)
 Solvation models are normally parametrised for dGsolv - so this value can be
 extracted from the quantum chemistry calculation as the difference in the
 calculated Gibbs Free Enthalpy.
 Hf can calculated easily in the gas phase, and a re-optimisation of the
 structure with solvation should capture the majority of the impact of solvation
 on the enthalpy. (Which is dominated by molecular structure.)
 (I guess there is the case of stabilisation and complexes, such as are reported
 for water. However this is currently beyond the scope of my work.)
 The topic of solvation has been discussed previously on the CCL here:
 http://www.ccl.net/chemistry/resources/messages/2011/12/01.001-dir/
 http://www.ccl.net/chemistry/resources/messages/2011/10/06.005-dir/
 http://www.ccl.net/chemistry/resources/messages/2014/05/01.004-dir/
 And there is the book "Essentials of Computational Chemistry Theories and
 Models" from Professor Cramer with a section on phase change (the source of
 my confusion).
 Specifically, the discussion concerning the energy change related to the state
 conversion causes me some grief.
 On the one hand, the CCL responses read as if this should be applied in the case
 of any phase change, but then others suggest this is applicable only if the
 process is a second order reaction and thus the total number of mols changes? -
 The latter view seems to agree with the book...
 So if I have compound A in both the gas and liquid phase (from a quantum
 chemistry calculation), do I need to account for the phase change/change of
 state or not? Or is it something that can be included in the parametrisation of
 the solvation model/the quantum chemistry code already?
 Just to add more confusion to the topic: I have trialled a commercial product
 which gives the Gibbs Enthalpy of Solvation in kcal/mol for mol/L concentrations
 and using a very low end/fast functional, it gives values similar to when a
 correction term is added... on the other hand, where available, the calculated
 values without correction agree with the published values in the SMD paper: http://pubs.acs.org/doi/abs/10.1021/jp810292n (Supplementary
 Data)
 In addition, a regular computational chemistry calculation sees very little
 (virtually no) difference in the entropy between the gaseous and solvated phase.
 This would agree with the CCL-linked paper here http://pubs.acs.org/doi/abs/10.1021/jp205508z . But this
 would clash with the common expectation that entropy in the liquid phase is
 reduced...
 Hence, I would highly appreciate if someone knowledgeable in the field of
 solvation could guide me onto the correct track.
 Detlev Conrad Mielczarek
 Scientific Visitor/Post Doctorant
 IFP Energies nouvelles
 France
 www.ifpenergiesnouvelles.fr
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 Dr. Uwe Huniar
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