Re: CCL:Frequency job in solvent

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")