Introduction to Infrared Spectroscopy

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Infrared or IR spectroscopy measures molecular vibrations by recording what wavelengths of IR radiation get absorbed. The chemical bonds within molecules take different amounts of energy to “move”. This is linked to the vibrational mode of the chemical bonds. Stronger bonds absorb more energy than weaker bonds. In other words, stronger chemical bonds will absorb light of a higher frequency and lower wavelength compared to weaker bonds. This happens because the energy of light is directly proportional to its frequency and inversely proportional to its wavelength. This also means that the wavelength of light that gets absorbed varies based on the type of molecule it passes through.

Through this process an IR spectrometer produces an IR spectrum, where the Y-axis is the intensity of absorption and the X-axis is the wave number (a high wavenumber means more energy). Each “dip”, also referred to ask peak, in the spectrum corresponds to a wavenumber at which light has been absorbed by the sample molecule as the light has not reached the detector which in turn causes this dip in the spectrum. The “depth of the dip” or peak intensity as it is also called is dependent on Beer’s Law: “*Absorbance = Absorptivity x Pathlength x Concentration”.* Different functional groups absorb light at different wave numbers and we can use this information to determine the presence and concentration of these different functional groups.

The region between 600-1400 cm-1 is called the fingerprint region, and tends to be complex with multiple bands that overlap each other. Above this region is where one can manually identify the presence of most functional groups.

Spectra can also be compared digitally to digital spectra libraries, that contain a large number of reference spectra, in order to find a “match”.

Sources:

Research Gate: Valencia, Cesar & Valencia, Yamid & Grande Tovar, Carlos. (2020). Synthesis and Application of a Cationic Polyamine as Yankee Dryer Coating Agent for the Tissue Paper-Making Process. Polymers. 12. 10.3390/polym12010173.