Fingerprint Region

In-depth explanation of Fingerprint Region

The fingerprint region of an infrared spectrum, roughly below 1500 cm-1, contains many overlapping bending, rocking and skeletal vibrations that involve the whole molecule. Because these vibrations depend on the entire molecular framework, the pattern of peaks is essentially unique to each compound. Although individual peaks are hard to assign in this region, the overall pattern is valuable for identification when compared with a reference library or with an authentic sample. Two isomers with identical functional groups can therefore be distinguished by their fingerprint regions. The region is also sensitive to small structural changes, which makes it useful for confirming that a synthesized material matches the expected compound.

Going deeper: Fingerprint Region

Key ideas: Use it as a final confirmation rather than for first assignments. Overlay the spectrum of an unknown with a reference from a library, checking positions and relative intensities of at least five peaks. Typical features: out-of-plane C-H bending of aromatic rings at 690 to 900 cm-1, which indicates substitution pattern, and C-O and C-N stretches at 1000 to 1300 cm-1.

Common mistakes and connections: Do not try to assign every peak, since coupling makes vibrations mixtures of motions. Students sometimes claim that two different compounds are identical because the functional-group region matches. A mismatch in the fingerprint region is strong evidence for different compounds, while a match is strong evidence for identity only if the sample preparation is the same. Related to IR, isomers and library searching.

Visualization of Fingerprint Region

Below about 1500 cm-1 many overlapping bending and skeletal bands form a pattern unique to each compound, so isomers with the same groups can still be told apart. The interactive lesson shows this as a live animation or chart.

Worked example and use case

Two positional isomers with the same functional groups show different peaks in the fingerprint region.

Where Fingerprint Region fits in Analytical, Lab & Spectroscopy

Fingerprint Region is topic 43 of 99 in Analytical, Lab & Spectroscopy.

Related topics

• Calibration CurveIntermediate
• Infrared SpectroscopyIntermediate
• Characteristic IR AbsorptionsAdvanced
• Raman SpectroscopyAdvanced
• NMR SpectroscopyAdvanced
• Chemical ShiftAdvanced

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