Infrared spectrum

The outcome of an infrared analysis is a spectrum showing different peaks, with values of wavenumber on the X-axis and intensity on the Y-axis. Each peak represents a bond within the compound we are analyzing. These peaks could be caused by bonds in the compound stretching or bending, and these bond motions are depicted in figure 1.

A peak at a specific wavenumber could be caused by a few different bonds or functional groups. For example, in figure 1, a peak at the wavenumber 1300 cm-1 could indicate the presence of a cyano (C=N) or alkene (C=C) group. Multiple bonds or groups can have the same wavenumber because a peak's wavenumber is influenced by the size of the atoms within the bond and the strength of the bond. This effect can be seen in figure 1. You can see that the carbonyl bond (C=O) has a higher wavenumber than a carbon-oxygen single bond because the double bond is stronger than the single bond.

The intensity of the peaks is mainly influenced by the concentration of the sample but also by bond polarity, which is the difference in electronegativities between the atoms in the bond. For example, carbonyl groups (C=O) have a higher intensity than cyano groups (C=N) because the difference in electronegativities between carbon and oxygen is greater than the difference between carbon and nitrogen.

Within the spectrum, the region between 400 and 1500 cm-1 contains many fine peaks. It is called the fingerprint region because these peaks can identify a compound precisely.

Figure 1: An IR spectrum showing where the peaks for certain bonds and functional groups are found.