CCL: Scaling of Raman Frequencies
- From: Cory Pye <cpye-x-crux.smu.ca>
- Subject: CCL: Scaling of Raman Frequencies
- Date: Wed, 13 Feb 2008 19:31:40 -0400 (AST)
Sent to CCL by: Cory Pye [cpye**crux.smu.ca]
On Wed, 13 Feb 2008, Mark Zottola mzottola]^[gmail.com wrote:
> I have a few questions concerning the calculation of raman frequencies.
> first is, should one consider the IR scaling factors to be valid for Raman?
> Raman is essentially an emission phenomenon while IR frequencies are an
> abosorbance phenomenon.
Yes. The Raman frequencies should be the same as the IR frequencies, within
the harmonic approximation. Any instances of non-coincidence point to some
other effect going on. Therefore, the same scaling factors should also apply.
The statement that Raman is essentially an emission phenomenon is true;
however, the frequency that is actually reported is the difference between the
frequency of light used (typically, a laser line, or in the older literature, a
mercury arc source with filters) versus that which is Raman scattered. It is
not the same thing as a absorption versus an emission spectra (much more
complicated), because the "virtual state" only sticks around for a ps
and thus the system doesn't have time to do anything fancy (IC,ISC,...).
> My second question is, if Raman frequencies should be scaled, should Raman
> frequencies determined by incorporation of solvent effects also be scaled?
It depends on the reason for scaling. Typically scale factors are derived for
gas-phase molecules (experiment and theory) and correct for both deficiencies
in level and deficiencies in the harmonic approximation. For something like an
OH or CH stretching, there is a very large anharmonic contribution.
Hartree-Fock scaling factors are also accounting for the neglect of electron
correlation, which is very important for motions which involve homolytic
cleavage (i.e. C-H). For M-O stretching modes in M(H2O)n m+ (aq), I have found
that the frequencies are underestimated, not overestimated. For these modes,
the stretching involves a heterolytic cleavage (electron pair remains intact,
at least near the equilibrium distance) so correlation is not a factor, and
anharmonicity is rather small (as determined by the T-dependence of frequencies
in solution). In this case, one can scale for the effect of the medium, or
include a second hydration sphere. We didn't find that scaling was that useful
for the modes that we were interested in.
For more details, you can check out some of my papers (esp. JPC 1998)
************* ! Dr. Cory C. Pye
***************** ! Associate Professor
*** ** ** ** ! Theoretical and Computational Chemistry
** * **** ! Department of Chemistry, Saint Mary's University
** * * ! 923 Robie Street, Halifax, NS B3H 3C3
** * * ! cpye~!~crux.stmarys.ca http://apwww.stmarys.ca/~cpye
*** * * ** ! Ph: (902)-420-5654 FAX:(902)-496-8104
************* ! Les Hartree-Focks (Apologies to Montreal Canadien Fans)