13.11 : IR Frequency Region: Fingerprint Region

IR spectra are divided into two main regions: the diagnostic region and the fingerprint region. The diagnostic region of the spectrum lies above 1500 cm⁻¹. The absorptions resulting from single-bond vibrations of the N–H, C–H, and O–H stretch at higher wavenumbers and appear on the left side of the spectrum. The stretching absorptions of the C≡C and C≡N occur between 2100–2300 cm⁻¹. In contrast, those arising from stretching absorptions of the C=O, C=N, and C=C occur between 1600–1850 cm⁻¹.

The absorptions below 1500 cm⁻¹ lie in the fingerprint region resulting from vibrations of the C–C, C–O, and C–N bonds. This region is also called a complex spectra region and is more difficult to analyze. Sometimes spectra of structurally close molecules overlap and are not distinguishable in the diagnostic region. However, they can be distinguished in the fingerprint region since they exhibit a unique pattern of peaks. For instance, O–H and C–H groups of the butanol and propanol show similar stretching absorptions in the diagnostic region but they exhibit a different pattern of peaks in the fingerprint region, which helps distinguish them.