CCL: Scaling of Raman Frequencies



 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.
 The
 > 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
 or so,
 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)
 http://www.ap.smu.ca/~cpye/CV/index.html
 > Mark
 >
    *************    !  Dr. Cory C. Pye
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