The fingerprint region of an IR spectrum is the region below 1500 cm−1.This region contains many overlapping absorptions from the bending and stretching of the molecule as a whole, including C-C and C-O bonds. The pattern is complicated, and it is unique to each compound, like a human fingerprint. Even two compounds with the same functional group, such as propan-1-ol and butan-1-ol, have different fingerprint regions.The fingerprint region is not usually used to find functional groups, because the individual absorptions are hard to assign. Instead it is used to identify a specific compound.How it is used
1. Record the IR spectrum of the unknown compound.
2. Compare its fingerprint region with the spectra of known compounds in a computer database.
3. If the fingerprint regions match exactly, the compounds are the same.This can also show whether a sample is pure. Extra absorptions that are not in the reference spectrum show that impurities are present.
The region above 1500 cm−1 is where most functional group absorptions (O-H, C-H, C=O, C=C) are found. Chemists normally use that region first to find the functional groups, then use the fingerprint region to confirm the exact identity.Worked example: a chemist makes ethanoic acid and records its IR spectrum. Above 1500 cm−1 there is a very broad O-H absorption at 2500 to 3000 cm−1 and a C=O absorption at 1710 cm−1. The fingerprint region matches the database spectrum of ethanoic acid exactly, with no extra absorptions. So the product is ethanoic acid, and it is pure.
1. Record the IR spectrum of the unknown compound.
2. Compare its fingerprint region with the spectra of known compounds in a computer database.
3. If the fingerprint regions match exactly, the compounds are the same.This can also show whether a sample is pure. Extra absorptions that are not in the reference spectrum show that impurities are present.
The region above 1500 cm−1 is where most functional group absorptions (O-H, C-H, C=O, C=C) are found. Chemists normally use that region first to find the functional groups, then use the fingerprint region to confirm the exact identity.Worked example: a chemist makes ethanoic acid and records its IR spectrum. Above 1500 cm−1 there is a very broad O-H absorption at 2500 to 3000 cm−1 and a C=O absorption at 1710 cm−1. The fingerprint region matches the database spectrum of ethanoic acid exactly, with no extra absorptions. So the product is ethanoic acid, and it is pure.