[Oxbow-]

Hello everybody,
I will have a big exam soon about IR spectroscopy
My teacher want us to write the kind of vibrations, by exemple :

1687 cm-1 : ν C=O  (ν is for stretching)

And δ for the deformations bonds by exemple

We have seen in class only these cases :

- ν for stretching vibrations (ν sym and ν asym)
- δ for deformation (asym and sym) it was for CH3 at ~1375 and ~1475 cm-1

So my question : how do you know it is a deformation or a stretch vibration ? A friend says that when it is in the fingerprint it is always deformation, is he right ? Thanks !

[mjc123]

No he isn't. It could be  a deformation involving H, or a stretch involving non-H atoms. There is no absolute way to tell. The best you can do is familiarise yourself with a table of common IR vibrations and their characteristic frequencies, such as:
https://chem.libretexts.org/Reference/Reference_Tables/Spectroscopic_Parameters/Infrared_Spectroscopy_Absorption_Table
You perhaps don't need to remember e.g. all the different substituted benzene patterns, unless you're analysing such compounds, but "700-900: C-H deformations" will do.

[Babcock_Hall]

I am not sure exactly what you need to know, but I have found it helpful to bear in mind some simple concepts.  For example the higher the bond order, the higher the frequency, all else held equal.  So a carbon-carbon triple bond has a higher stretching frequency than a carbon-carbon double bond.  Also (although the relationship is not as simple as one might wish) the larger the mass, the lower the frequency, all else held equal.