1
$\begingroup$

enter image description here

This is the raman spectra of Toluene I obtained while working at my lab using a raman spectrometer with laser of 532 nm wavelength. Well, I am a physics student, but I want to know how information about vibrational modes and functional groups are inferred from the raman spectra, let's say from this spectra of Toluene. It would be of great help, since I am very curious about this spectroscopy method. I know physics part of raman spectroscopy, but really struggling with its application part.

$\endgroup$
2
$\begingroup$

Chemists have tables and tables and tables that correlate functional groups to numbers.

For example: http://www2.ups.edu/faculty/hanson/Spectroscopy/IR/IRfrequencies.html

If we see a number on the graph, a certain type of functional group may be present. The sharp one at 3056 tells me aromatic hydrogen. The 2919 is the benzylic hyrodgens. Since I'm a synthesis guy, knowing the exact types of vibrations occurring there was never important to me.

Be warned, chasing every peak is normally quite frustrating. Vastly more important is normally the overall look for comparison with a known compound, which is sort of a molecular finger print.

| improve this answer | |
$\endgroup$
2
$\begingroup$

You have asked question about vibrational modes and functional groups with respect to Raman spectra.

General information: Specific vibrational modes are deduced by normal-coordinate analysis. This gives most probable observable frequency for certain vibration (or vice-versa). Normal coordinate analysis is core idea in spectral interpretation and tables ( one such table ) for quick interpretation were made decades back using such procedures. All those peaks you see in the spectra are coming from different vibrations.

Other Clues: One clue is presence of combinations and overtones. These are observed when vibrational quantum number changes by two or more.

Some vibrational modes may have closer frequencies which can cause resonance and result in splitting of peaks. This is another clue to deduce more details.

Isotopic substitution gives ultimately large amount of information of the atoms involved, since the peak position changes considerably.

Important is to understand that a peak here refers to a vibration (and there are different peaks and so different kinds of vibrations) Each vibration tells about the atoms involved and symmetry of vibration (depending upon experiment).

For toluene, we can simply correlate to benzene first. Some interpretation is following:

~3056 cm-1 = CH stretch

~1000 cm-1 = ring stretch (symmetric mode)

~1606 and 1580 cm-1 = splitting by Fermi -resonance

~785 cm-1 = CH bend and so on.

| improve this answer | |
$\endgroup$
  • $\begingroup$ @Roshan Shrestha Do you need more details ?? $\endgroup$ – ankit7540 May 15 '16 at 5:32

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.