Interpretation of IR spectra
The first useful step in interpreting spectra is the analysis of their general appearance:
- Identify the type of sample: powder, liquid or gas
- Observe the shape of the peaks: intense, wide, narrow…
When the incident energy of the photon is high enough, besides provoking a transition towards the first vibrational state ν0-1, a transition ν0-2 is also possible. This transition is the first overtone of the vibration and requires an energy twice as much energy.
Thus ν0-2 (in cm-1)= 2 x ν0-1 (in cm-1) ; ν0-3 (in cm-1)= 3 x ν0-1 (in cm-1) ; ...
When two vibrations are intense on a spectrum, they can generate a third band called combination. The wavenumber of this band is the sum of both vibrations involved.
On a spectrum, combinations and overtones are small and thick bands.
The second step is to compare the position of the vibrations with existing interpretation tables.
The position of the peaks depends on the strength of the bond between the two atoms and on their reduced mass:
Strategy: start with high wave numbers and confirm the stretches by their associated deformations.