Interpretation of IR spectra

The first useful step in interpreting spectra is the analysis of their general appearance:

  1. Identify the type of sample: powder, liquid or gas
  2. Observe the shape of the peaks: intense, wide, narrow…
Example of a spectrum, on the abscissa the absorbance, on the ordinate the wave number. We can see different peaks of various appearance: broad, fine, intense, etc.
A peak is wide if it corresponds to a chemical function involved in hydrogen bonds or if it is a combination or an overtone - in the latter case its intensity is very weak.
A peak is intense in IR if the corresponding chemical function exists several times in the molecule (additivity of Beer-Lambert’s law) or if the atoms involved have a large difference in electronegativity (e.g. CO, OH).
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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:

Equation to calculate the wave number as a function of the celerity, the reduced mass and the force constant

Strategy: start with high wave numbers and confirm the stretches by their associated deformations.

Example of spectrum, on which one notes in the various ranges, from left to right: OH and NH stretching, CH stretching, aromatic harmonics and C=O stretching, CH, NH, OH, CO deformations and C-0, C-N stretching
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