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Why is the segment of the IR spectrum below 1500 wavenumbers (cm$^{-1}$) considered as the fingerprint region? It is said that "although the entire IR spectrum can be used as a fingerprint for the purposes of comparing molecules, the 600 - 1400 wavenumber range is called the fingerprint region. This is normally a complex area showing many bands, frequently overlapping each other". However, I don't understand why the range from 600 - 1400 wavenumbers can be particularly complex and is unique to the compounds, for instance, why is the fingerprint region of propan-1-ol and propan-2-ol so different from each other given roughly the same appearance for rest of the spectrum?

IR spectrum of propan-1-ol IR spectrum of propan-2-ol

Spectra taken from http://www.chemguide.co.uk/analysis/ir/fingerprint.html

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I guess the naming "fingerprint region" results from the fact that while I can immediately identify amine, alcohol, carbonyl groups, double and triple bonds and so on in the "upper" and "middle" part of the IR spectrum, the peaks observed in the fingerprint region are often arising from vibrations that are very specific to the very compound you are looking at, thus a "fingerprint" of your compound.

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