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.
The IR spectrum can be divided into two main regions: The diagnostic region and the fingerprint region.
Bands in the diagnostic region arise from stretching absorptions of the bonds connected to hydrogen, and different types of triple bonds and double bonds.
The fingerprint region is the complex region in the IR spectrum between 1500 and 450 cm−1, showcasing a large number of absorptions resulting from stretching and bending vibrations of various single bonds.
Each compound has a unique pattern of peaks in the fingerprint region which is useful for distinguishing molecules that have identical functional groups.
For instance, 2–butanol and 2–propanol exhibit identical C–H and O–H signals in the diagnostic region and are, therefore, indistinguishable. However, they have a different pattern of peaks in the fingerprint region, making them easily distinguishable.
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