Subject: Re: CCL:Frequency job in solvent

From: Andreas Klamt <klamt=at=cosmologic.de>
Date: Wed, 28 Apr 2004 07:36:45 +0200

Liviu,

I do not know what the reason for your technical problems are. But I 
definitely know that calculation of the frequencies will not lead you to 
the free energy in a solvent. In solvents the free energy is domonated 
by the interactions with the solvent molecules, while the changes in 
frequencies are of negligible importance.

Presently probably the most reliable method for prediction of the free 
energy of molecules in arbitrary solvents and even mixtures, at variable 
temperature, is my COSMO-RS method. We are winner of the first 
industrial fluid property prediction challenge organized by NIST in 
2002.  In Gaussian03  you have the option to choose the settings to 
produc e a COSMO file according to our standards. This file can be read 
by our COMSOtherm program and it gives you the free energy (together 
with lots of other thermodynamic infiormation).

Read more about COSMO-RS on our homepage: www.cosmologic.de.

Good luck

Andreas

Liviu Mihail Mirica wrote:

>Hi all,
>I have a question regarding a Gaussian98 frequency job in a solvent. The
>input section is given below:
>
>#P gfinput iop(6/7=3)
>#P RB3LYP/6-31G* scfcyc=400
>#P guess=read geom=check scrf=(pcm,solvent=thf) freq
>
>for some reason, the job seems to do the SP calculation in a solvent
>over and over again. i let the job to run for a week, and all i got as
>about 30 sp jobs. and it keeps going. it never starts calculating the
>force constants or so. does anyone know what could be the problem? is the
>solvent method i'm uasing the cause for this? is in any way possible to
>get the free energy in solvent (that's the reason i'm doing a frequency
>job).
>Thank you very much.
>Sincerely,
>Liviu Mirica
>
>
>
>
>**************************
>Liviu Mihail Mirica			tel: 650-497-3188
>Escondido Village, 35 B			email: liviu=at=leland.stanford.edu
>Stanford, CA 94305, USA
>
>
>
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-- 
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COSMOlogic GmbH&CoKG
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51381 Leverkusen, Germany

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---

From: Christopher Cramer <cramer*at*pollux.chem.umn.edu>
Date: Wed, 28 Apr 2004 11:39:10 -0500 (CDT)

David,
> 
> Question for other CCL-ers: 
> Are solvent effects on vibrational frequencies primarily of electrostatic origin?  That is what the SCRF method assumes, isn't it?  But I would think that van der Waals forces (otherwise known as bumping into neighboring solvent molecules) would be the main factor. 
> 

   There are several levels of response to your question.

   Ben-Naim has first codified the intuitively reasonable relationship

delta-G(solvation) = delta-G(solute-solvent coupling) + delta-G(delta-Q)

where the first term on the right-hand-side is associated with interactions
between the solute and the solvent (and changes in interactions between
solvent molecules with each other) and the second term is associated with
changes in the solute partition function. The most obvious of these would
be, for example, if you were considering a change in standard-state volume
(e.g., using 1 mol per 24.5 L for your gas-phase standard state and 1 mol
per 1 L for your solution standard state). Such a change in standard-state
volume leads to a change in the translational partition function that
affects the translational entropy and hence affects the free energy of
transfer. Of course, there could also be a change in the rotational and
vibrational partition functions too! However, this is much trickier than
the trivial correction for the standard-state volume. A solute presumably
does not HAVE a rotational partition function -- those degrees of freedom
are converted into solute/solvent librational modes, the natures of which
are quite difficult to determine either experimentally OR theoretically.
Hence, essentially all continuum models proceed from the assumption that
there is NO delta-Q affect on the electronic, rotational, or vibrational
partition functions, and transform experimental data as appropriate to make
the standard-state volume a constant, and parameterize against the
remaining coupling free energy (thus, by parameterization, any error in
the assumption of zero delta-Q gets buried in the fit).

   Second, as for the coupling free energy itself, it is divided up into
electrostatic and non-electrostatic components. The electrostatic part is
nearly always determined either from solution of the Poisson or generalized
Born equations (using a self-consistent reaction field formalism if the
solute is being treated at a QM level) and the non-electrostatic part is
typically (but not always) added at the end in a more or less parametric
fashion. To the extent that "bumping into other molecules" is associated
with a cavitation term that becomes more positive with increasing
solvent-accessible surface area along a vibrational mode (as one example of
how such a term is often calculated), this affect will appear in a
continuum model, as will the electrostatics.

   However, let us say that you really ARE interested in how solvation
affects the vibrational frequencies. It is not necessarily formally correct
to recompute the harmonic-oscillator force constants with the dielectric
continuum turned on, because the time scale of the vibration may be
considerably faster than the bulk dielectric response time of the solvent.
That is, vibration is subject to a certain degree of non-equilibrium
solvation, but continuum solvent models are equilibrium in nature unless a
frequency-dependent dielectric constant is employed. A nice article
addressing the precise issue of IR in solution is Rivail, J.-L.;
Rinaldi, D.; Dillet, V., "Solvation Effects on Infrared Spectroscopy:  A
Computational Approach" Mol. Phys. 1996, 89, 1521-1529.

   A more general discussion of many of the above issues that is, I hope,
still very useful, is the review that Don Truhlar and I wrote 5 years ago
Cramer, C. J.; Truhlar, D. G., "Implicit Solvation Models:
Equilibria, Structure, Spectra, and Dynamics" Chem. Rev. 1999, 99,
2161-2200.

Best regards,

Chris 
-- 

Christopher J. Cramer
University of Minnesota
Department of Chemistry
207 Pleasant St. SE
Minneapolis, MN 55455-0431
--------------------------
Phone:  (612) 624-0859 || FAX:  (612) 626-2006
Mobile: (612) 597-5275
cramer*at*pollux.chem.umn.edu
http://pollux.chem.umn.edu/~cramer
(website includes information about the textbook "Essentials
    of Computational Chemistry:  Theories and Models")