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I'm not a physics grad, but I'm curious: can one predict the Raman response of a molecule? For example: suppose I'm interested in detecting molecule X. I have a

I'm not a physics grad, but I'm curious: can one predict the Raman response of a molecule? For example: suppose I'm interested in detecting molecule X. I have a laser of frequency f. Given these two, can I predict that the reflected light (after Raman effect) will definitely be of frequency f' ?

gipp 12y ago
In principle, yes. The relevant equations can't be solved exactly for any non-trivial molecules. But there are a large number of different approximation methods, which boil down to a numerical optimization problem that converges on a bounding value. These methods generally don't scale well, however, so as the size of the system grows, the approximations that can be successfully solved quickly grow too coarse to be useful. There are also a lot of simple heuristic methods that a chemist can employ that don't require collaboration with a theorist, e.g. particlar chemical "motifs"/"functional groups" generally have a Raman resonance at a consistent frequency that is affected only slightly by the surrounding parts of the molecule (and the general direction and order of magnitude of that shift can also be approximated heuristically). Finally, there are enormous catalogs of recorded spectra for a huge range of molecules at e.g. the NIST webbook (http://webbook.nist.gov/chemistry/ http://webbook.nist.gov/chemistry/).
- discardorama 12y ago
  Thanks. Followup question: does the light source have to be a laser? I read on the Wiki that CV Raman demonstrated the phenomenon with sunlight. So would it be possible to just observe the reflected light from an object and determine the molecules on it that are lighting up? If so, can (someday) someone come with a Star Trek-style tricorder that you look through and figure out the molecular composition of an object? Maybe at a distance of 100 meters?
jarvist 12y ago
More or less, you start with a quantum chemical DFT calculation, the Raman frequencies are associated with the vibrations of the molecule (second derivatives of the energy, once you're at the relaxed potential energy state). Raman frequencies are shifts (Stokes and anti-Stokes) relative to the laser frequency. For medium sized molecule, a hybrid DFT calculation (which scales in computational time with N_eletrons^3 ) would cost a few CPU days of time, giving pretty accurate frequencies (there are known correction factors of ~0.95x to compensate for systematic failings in the theories). What we are very bad at is predicting the Raman intensity from theory. But you get the frequencies in the correct order, and can use this to assign observed peaks to particular vibrations within the molecule. http://www.gaussian.com/g_tech/g_ur/k_freq.htm http://www.gaussian.com/g_tech/g_ur/k_freq.htm

gipp 12y ago

In principle, yes. The relevant equations can't be solved exactly for any non-trivial molecules. But there are a large number of different approximation methods, which boil down to a numerical optimization problem that converges on a bounding value. These methods generally don't scale well, however, so as the size of the system grows, the approximations that can be successfully solved quickly grow too coarse to be useful. There are also a lot of simple heuristic methods that a chemist can employ that don't require collaboration with a theorist, e.g. particlar chemical "motifs"/"functional groups" generally have a Raman resonance at a consistent frequency that is affected only slightly by the surrounding parts of the molecule (and the general direction and order of magnitude of that shift can also be approximated heuristically). Finally, there are enormous catalogs of recorded spectra for a huge range of molecules at e.g. the NIST webbook (http://webbook.nist.gov/chemistry/ http://webbook.nist.gov/chemistry/).

discardorama 12y ago
Thanks. Followup question: does the light source have to be a laser? I read on the Wiki that CV Raman demonstrated the phenomenon with sunlight. So would it be possible to just observe the reflected light from an object and determine the molecules on it that are lighting up? If so, can (someday) someone come with a Star Trek-style tricorder that you look through and figure out the molecular composition of an object? Maybe at a distance of 100 meters?

jarvist 12y ago

More or less, you start with a quantum chemical DFT calculation, the Raman frequencies are associated with the vibrations of the molecule (second derivatives of the energy, once you're at the relaxed potential energy state). Raman frequencies are shifts (Stokes and anti-Stokes) relative to the laser frequency. For medium sized molecule, a hybrid DFT calculation (which scales in computational time with N_eletrons^3 ) would cost a few CPU days of time, giving pretty accurate frequencies (there are known correction factors of ~0.95x to compensate for systematic failings in the theories). What we are very bad at is predicting the Raman intensity from theory. But you get the frequencies in the correct order, and can use this to assign observed peaks to particular vibrations within the molecule. http://www.gaussian.com/g_tech/g_ur/k_freq.htm http://www.gaussian.com/g_tech/g_ur/k_freq.htm