FTIR Fingerprint Region
The infrared spectroscopy fingerprint region spans 1500 to 400 cm−1 and contains a complex pattern of absorptions that is unique to every compound. Because no two molecules produce the same fingerprint pattern, this region is one of the most powerful tools for confirming the identity of an unknown sample against a reference spectrum table.
Key takeaway
Use the fingerprint region for confirmation, not discovery. Identify functional groups above 1500 cm⁻¹ first, then match the fingerprint pattern against reference spectra to confirm the exact compound.
What Is the Fingerprint Region?
In FTIR spectroscopy, the mid-infrared spectrum is conventionally divided into two halves. The functional group region (4000–1500 cm−1) contains stretching vibrations of bonds to hydrogen and multiple bonds (C=O, C=C, C≡N) that can often be assigned to specific functional groups. Below 1500 cm−1, the fingerprint region begins.
This region is dominated by C–C, C–O, and C–N single-bond stretches, along with a variety of bending, rocking, and wagging modes. These vibrations are highly coupled—each one is influenced by the entire molecular skeleton around it. The result is a dense, intricate pattern of peaks that acts like a molecular fingerprint: unique to each compound and virtually impossible to replicate by chance in a different molecule.
The term “fingerprint region” reflects this uniqueness. Just as no two people share the same fingerprints, no two compounds produce the same absorption pattern between 1500 and 400 cm−1.
Why the Fingerprint Region Matters
- Compound identification. The most direct use of the fingerprint region in FTIR is matching an unknown spectrum against a library of reference spectra. If the fingerprint patterns overlap, the compounds are the same.
- Distinguishing similar molecules. Structural isomers often share the same functional groups and therefore produce nearly identical peaks above 1500 cm−1. Their fingerprint regions, however, differ because the skeletal vibrations depend on the exact arrangement of atoms.
- Complementing functional group analysis. A thorough interpretation of an FTIR spectrum starts with the functional group region to identify which groups are present, then uses the fingerprint region to confirm the specific compound.
Key Absorptions in the Fingerprint Region
Showing 59 of 125 entries from the full FTIR spectrum table that fall at least partially within the 1500–400 cm−1 fingerprint region. For interactive sorting, filtering, and search, use the main absorption table.
| Functional Group | Wavenumber (cm−1) | Intensity | Category |
|---|---|---|---|
| C=C stretch (aromatic, ~1475 cm⁻¹) | 1450–1510 | variable | Aromatics |
| C-H bend (CH₃ asymmetric deformation) | 1440–1465 | medium | Alkanes |
| C-H bend (CH₃ symmetric umbrella) | 1370–1390 | medium | Alkanes |
| C-H bend (CH₂ scissors) | 1440–1475 | medium | Alkanes |
| NO₂ symmetric stretch | 1310–1370 | strong | Nitro Compounds |
| C-O stretch (1° alcohol) | 1040–1085 | strong | Alcohols |
| C-O stretch (2° alcohol) | 1085–1125 | strong | Alcohols |
| C-O stretch (3° alcohol) | 1125–1175 | strong | Alcohols |
| C-O-C stretch (ether) | 1060–1150 | strong | Ethers |
| C-O stretch (ester C-O-C) | 1150–1300 | strong | Esters |
| C-N stretch (aliphatic amine) | 1020–1250 | medium | Amines |
| C-N stretch (aromatic amine) | 1250–1360 | strong | Amines |
| S=O stretch (sulfoxide) | 1030–1070 | strong | Sulfoxides |
| S=O stretch (sulfone, asymmetric) | 1290–1350 | strong | Sulfones |
| S=O stretch (sulfone, symmetric) | 1120–1170 | strong | Sulfones |
| C-F stretch | 1000–1400 | strong | Haloalkanes |
| P=O stretch | 1150–1300 | strong | Phosphorus Compounds |
| Si-O stretch | 1000–1100 | strong | Organosilicon |
| O-H bend (in-plane, alcohol) | 1230–1320 | medium | Alcohols |
| O-H bend (carboxylic acid) | 1395–1440 | medium | Carboxylic Acids |
| C-O stretch (epoxide ring) | 1230–1280 | strong | Epoxides |
| C-O stretch (phenol) | 1170–1250 | strong | Phenols |
| C-H oop bend (monosubstituted aromatic) | 730–770 | strong | Aromatics |
| Ring oop bend (monosubstituted aromatic) | 690–710 | strong | Aromatics |
| C-H oop bend (1,2-disubstituted/ortho) | 735–770 | strong | Aromatics |
| C-H oop bend (1,3-disubstituted/meta) | 770–810 | strong | Aromatics |
| Ring oop bend (1,3-disubstituted/meta) | 680–720 | strong | Aromatics |
| C-H oop bend (1,4-disubstituted/para) | 800–860 | strong | Aromatics |
| C-Cl stretch | 550–800 | strong | Haloalkanes |
| C-Br stretch | 500–680 | strong | Haloalkanes |
| =C-H oop bend (trans alkene) | 960–975 | strong | Alkenes |
| =C-H oop bend (cis alkene) | 650–730 | medium | Alkenes |
| =C-H oop bend (vinyl/terminal) | 905–920 | strong | Alkenes |
| =C-H oop bend (vinylidene) | 880–900 | strong | Alkenes |
| N-H wag (1° amine) | 650–900 | variable | Amines |
| N-H wag (2° amine) | 700–750 | weak | Amines |
| C-S stretch | 570–710 | weak | Thioethers |
| Ring breathing (aromatic) | 990–1010 | variable | Aromatics |
| Si-CH₃ deformation | 1240–1280 | strong | Organosilicon |
| B-O stretch | 1310–1380 | strong | Boronic Acids |
| S=O stretch (sulfonamide, asymmetric) | 1335–1370 | strong | Sulfonamides |
| S=O stretch (sulfonamide, symmetric) | 1155–1170 | strong | Sulfonamides |
| S=O stretch (sulfonyl chloride, asymmetric) | 1380–1410 | strong | Sulfonyl Chlorides |
| S=O stretch (sulfonyl chloride, symmetric) | 1177–1204 | strong | Sulfonyl Chlorides |
| S=O stretch (sulfonic acid, asymmetric) | 1342–1350 | strong | Sulfonic Acids |
| S=O stretch (sulfonic acid, symmetric) | 1150–1165 | strong | Sulfonic Acids |
| S=O stretch (sulfate ester, asymmetric) | 1380–1415 | strong | Sulfate Esters |
| S=O stretch (sulfate ester, symmetric) | 1185–1200 | strong | Sulfate Esters |
| C=S stretch (thioamide/thione) | 1060–1200 | strong | Thiocarbonyls |
| C-I stretch | 500–600 | strong | Haloalkanes |
| P-O-C stretch (phosphate ester, asymmetric) | 990–1050 | strong | Phosphorus Compounds |
| P-O-C stretch (phosphate ester, symmetric) | 740–830 | medium | Phosphorus Compounds |
| N→O stretch (aromatic N-oxide) | 1250–1310 | strong | N-Oxides |
| N→O stretch (aliphatic N-oxide) | 950–970 | strong | N-Oxides |
| N-O stretch (organic nitrate, symmetric) | 1255–1285 | strong | Organic Nitrates |
| O-O stretch (peroxide) | 800–900 | weak | Peroxides |
| C-H rock (CH₂ rocking) | 715–740 | medium | Alkanes |
| COO⁻ stretch (carboxylate, symmetric) | 1390–1420 | strong | Carboxylate Salts |
| C-O-C stretch (vinyl ether) | 1200–1260 | strong | Ethers |
How to Use the Fingerprint Region
Don’t try to assign every peak. Unlike the functional group region, where each major band often corresponds to a specific bond type, the fingerprint region contains heavily coupled vibrations. Attempting to assign each peak individually is usually impractical and unnecessary.
Use it for confirmation, not discovery. First identify the functional groups present using peaks above 1500 cm−1. Then compare the full fingerprint pattern against a known reference spectrum to confirm the exact compound. A visual overlay or spectral database search is the most reliable approach.
Compare against reference databases. Commercial and open spectral libraries contain thousands of reference spectra. Matching the fingerprint pattern of your unknown against these libraries is the standard method for compound identification in analytical chemistry.
Watch for key diagnostic bands. While the full pattern is what matters, certain individual bands in the fingerprint region are still highly diagnostic—for example, aromatic C–H out-of-plane bending modes between 650 and 900 cm−1 reveal the substitution pattern of benzene rings.
Fingerprint Region vs. Functional Group Region
The two halves of the mid-IR spectrum serve complementary roles. The table below summarizes the key differences and how to use them together for complete FTIR analysis.
| Functional Group Region | Fingerprint Region | |
|---|---|---|
| Range | 4000–1500 cm−1 | 1500–400 cm−1 |
| Dominant vibrations | X–H stretches (O-H, N-H, C-H), triple bonds, double bonds (C=O, C=C) | Single-bond stretches (C-O, C-N, C-C), bending, rocking, and wagging modes |
| What it tells you | Which functional groups are present in the molecule | Which specific compound you have (molecular identity) |
| Interpretation approach | Assign individual peaks to specific bond types using an absorption reference | Match the overall pattern against reference spectra; do not assign peaks individually |
| Best used for | Determining functional group composition of an unknown | Confirming identity by comparison with known spectra |