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Considering pyridazine and pyrazine molecules, I am asked to say if I can distinguish one from the other in a sample.

I supposed that is possible because pyrazine belongs to D2h and so it has the inversion center. Thus, in IR spectroscopy I can appreciate the antisymmetric stretching while in Raman I can identify the total-symmetric ones.

Then I am asked to say if this spectroscopic technique allows me to predict quantitive results. and how?

This question floored me because I have never done spectroscopic experience!

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  • $\begingroup$ You'd use relative peak intensities somehow. Very common technique in spectroscopy. Pyridazine has peak about 1300 webbook.nist.gov/cgi/cbook.cgi?ID=C289805&Mask=80#IR-Spec and pyrazine has peak about 900 for example webbook.nist.gov/cgi/cbook.cgi?ID=C290379&Mask=80#IR-Spec $\endgroup$
    – MaxW
    Feb 10 '17 at 22:12
  • $\begingroup$ How can identify which oscillation frequency is related to nth normal mode? For example in the pyrazine there are two total symmetric CH stretching: it is expected to find them in 3000cm^-1 region but I don't know how can I relate a particular frequency to a particular mode $\endgroup$
    – mickeymar
    Feb 10 '17 at 22:13
  • $\begingroup$ Assigning particular peaks to particular modes is the subject for a book. You have to use not only the position of the peaks but their relative intensity as well. Also you have to assign all the peaks to be sure what is going on. $\endgroup$
    – MaxW
    Feb 10 '17 at 22:19
  • $\begingroup$ Can you use any kind of spectroscopy? What about proton or carbon NMR? $\endgroup$
    – Zhe
    May 12 '17 at 22:08
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You need to understand some details of using point group tables. The pyrazine belongs to $\ce{D_{2h}}$ point group and pyridazine belongs to $\ce{C_{2v}}$

In the third column of the point group are found x, y, z which are used to identify dipole transitions, e.g. vibrational symmetries that can exhibit IR transitions and in the fourth $x^2, z^2, xz$ etc. which are used to identify Raman transitions. If there is a squared term in column 4 and in the top line of the table (totally symmetric representation) labelled $\ce{A_g, A_1}$ then a Raman transition is totally polarised. Raman transitions from other symmetry species vibrations are not fully polarised. (Ignore the $\ce{R_x, R_y,R_z}$ they do not refer to IR or Raman transitions)

The point groups are shown below

c2v d2h

In $\ce{C_{2v}}$ all vibrational species $\ce{A_1}$ etc. can have Raman transitions but in the IR the $A_2$ does not have an IR transition, no x, x, z in column 3. The other three types of vibrations could have both IR and Raman transitions. In the $\ce{D_{2h}}$ you can see that, because of the centre of inversion, Raman and IR vibrational transitions are mutually exclusive (compare symmetry species of $x^2, xy$ etc. with x, y, z ) because there is no vibration with a symmetry species that can generate both IR and Raman transitions.

Thus, with your compounds, the one with $\ce{C_{2v}}$ symmetry will have IR and Raman transitions with the same frequency, but for the $\ce{D_{2h}}$ compound there should be no common IR and Raman frequencies.

(See the answer to this question Understanding group theory easily and quickly more details on using point groups)

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  • $\begingroup$ Thank you very much! But I'd like principally to know if I can associate a particular frequency to a particular mode just looking the spectrum (without the help of the tables in book). $\endgroup$
    – mickeymar
    Feb 11 '17 at 12:07
  • $\begingroup$ Not really, but as a rule of thumb, the asymmetric stretches have highest frequency then symmetric stretches, then bends at lower frequency. There are rules also for particular groups such as C=O, CN, OH, NH etc. that I'm sure you will be familiar with and that are very useful when identifying compounds. $\endgroup$
    – porphyrin
    Feb 13 '17 at 9:22

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