Spectroscopy pyrazine molecule

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!




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](https://i.stack.imgur.com/fQSlBm.png)](https://i.stack.imgur.com/fQSlBm.png) [![d2h](https://i.stack.imgur.com/eVyt0m.png)](https://i.stack.imgur.com/eVyt0m.png)


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](https://chemistry.stackexchange.com/questions/58609/understanding-group-theory-easily-and-quickly/58613#58613for) more details on using point groups)


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