What absorbs at 1550 cm⁻¹ in an FTIR spectrum?
A band near 1550 cm⁻¹ can point to several functional groups. Below are the most likely assignments, ranked by how much published evidence supports each — every one traceable to literature (DOI) and cross-validated against our 130,000+ reference spectra and knowledge graph.
Backed by 8 cited sources
Quick answer
A band near 1550 cm⁻¹ is usually interpreted by checking which functional groups repeatedly co-occur there in the literature, then confirming at least one or two additional peaks in the same sample. This page ranks those assignments by accumulated evidence rather than by a single fixed textbook rule.
Possible functional-group assignments
| Functional group | Supporting facts | Cited sources | Top confidence |
|---|---|---|---|
| Amide | 123 | 109 | 1.0 |
| N h | 39 | 36 | 1.0 |
| Secondary amine | 20 | 17 | 1.0 |
| Carboxyl (COOH) | 17 | 14 | 1.0 |
| C n single bond | 16 | 14 | 1.0 |
| Acetate | 12 | 10 | 1.0 |
| Protein | 12 | 10 | 1.0 |
| Methacrylate | 10 | 9 | 1.0 |
| Alkyl C-H | 8 | 7 | 1.0 |
| Carbonyl (C=O) | 8 | 6 | 1.0 |
| Hydroxyl (O-H) | 7 | 7 | 1.0 |
| Ester | 7 | 5 | 1.0 |
| Methoxy (OCH3) | 6 | 6 | 1.0 |
| C-O single bond | 6 | 6 | 1.0 |
| Alkene (C=C) | 6 | 6 | 1.0 |
| Aromatic ring | 5 | 5 | 1.0 |
| Amine primary | 5 | 5 | 1.0 |
| Protein alpha helix | 5 | 5 | 1.0 |
| Ketone | 5 | 4 | 1.0 |
| C c single bond | 4 | 4 | 1.0 |
| Ring structure | 3 | 3 | 1.0 |
| N-O bond | 3 | 3 | 1.0 |
| Protein beta sheet | 3 | 3 | 1.0 |
| Carbonate | 2 | 2 | 1.0 |
| Nitrogen heterocycle | 2 | 2 | 1.0 |
| Hydrogen bond | 2 | 2 | 1.0 |
| Water (H2O) | 2 | 2 | 1.0 |
| Carbohydrate | 2 | 2 | 1.0 |
| Lignin | 2 | 2 | 0.7 |
| Borate | 1 | 1 | 1.0 |
| Phosphorus | 1 | 1 | 1.0 |
| Phosphate (PO4) | 1 | 1 | 1.0 |
| S eq o | 1 | 1 | 1.0 |
| Metal sulfur | 1 | 1 | 1.0 |
| Acetyl | 1 | 1 | 1.0 |
| Protein random coil | 1 | 1 | 1.0 |
| Nucleic acid | 1 | 1 | 1.0 |
| Lipid | 1 | 1 | 1.0 |
| C=n | 1 | 1 | 1.0 |
| Carbon dioxide | 1 | 1 | 1.0 |
| N n bond | 1 | 1 | 1.0 |
| Bronsted acid site | 1 | 1 | 0.9 |
| Nitrate | 1 | 0 | 0.7 |
Ranking reflects accumulated literature evidence, not a single fixed rule. Always confirm against your sample context.
Possible materials
| Material | Supporting peaks | Overlapping groups | Cited sources |
|---|---|---|---|
| chitosan | 1550, 1650, 1655 | Amide, N h | 1 |
| chitin | 1550, 1650, 1655 | Amide, N h | 1 |
Materials are shown only when the same literature pool supports this band and at least one additional characteristic peak.
Spectrum logic
This band becomes meaningful only when read with its neighboring peaks. In practice, analysts first look at the assignments above, then check whether the same sample also shows other peaks expected for the same structural motif. A lone band near 1550 cm⁻¹ is usually not enough for material identification by itself.
Real-world usage
This type of query is common in polymer identification, unknown plastic screening, QC troubleshooting, recycled-material verification, and literature-backed peak assignment review.
Common mistakes
- Treating one isolated band as proof of a material without checking at least one or two supporting peaks.
- Ignoring overlap: multiple functional groups can contribute near the same wavenumber.
- Skipping validation when additives, blends, oxidation, or contamination may distort the spectrum.
Verification advice
When ambiguity remains, validate the hypothesis with DSC, GC-MS, or TGA, especially for blends, degraded samples, and filled polymers.
Literature behind these assignments
-
Alkyl C-H confidence 1.0
“and a weak D peak near 1320 FTIR plot of DLC films shows a peak 1550 cm-1 corresponding to C-H stretching mode and peaks below 2000 near 2900 corresponding to C-C modes and C-H bending modes.”
DEPOSITION OF WIDE BAND GAP DLC FILMS USING R.F. PECVD AT VERY LOW POWER DOI: 10.1142/S0217979211100102 -
confidence 1.0
“1550 due to bending vibration of N-H group (60%)”
Gene-Transformation-Induced Changes in Chemical Functional Group Features and Molecular Structure Conformation in Alfalfa Plants Co-Expressing Lc-bHLH and C1-MYB Transcriptive Flavanoid Regulatory Genes: Effects of Single-Gene and Two-Gene Insertion DOI: 10.3390/ijms18030664 -
Amide confidence 1.0
“LLM confirmed rule peak-group candidate”
Fabrication of pristine electrospun kafirin nanofiber mats loaded with thymol and carvacrol DOI: 10.1007/s10853-020-05663-7 -
Amide confidence 1.0
“LLM confirmed rule peak-group candidate”
Shedding light on sporopollenin chemistry, with reference to UV reconstructions DOI: 10.1016/j.revpalbo.2016.11.014 -
Amide confidence 1.0
“Additionally, the band at 1650 was cm-1 attributed to C=O secondary amide I stretching vibrations, while the band at 1550 was attributed to amide II, which was indicative of N-H bending.”
Antimicrobial Activity of Chemically and Biologically Treated Chitosan Prepared from Black Soldier Fly (Hermetia illucens) Pupal Shell Waste DOI: 10.3390/microorganisms9122417 -
Amide confidence 1.0
“The peak 1531cm-1 1550 and in amide II band also corresponded to”
Lin 等 - 2005 - PHand thermal-dependent conformational transitio DOI: 10.1016/j.peptides.2004.11.005 -
Amide confidence 1.0
“LLM confirmed rule peak-group candidate”
Lin 等 - 2011 - A Study on One-Step Immobilization of Horse Immuno DOI: 10.1149/1.3559465 -
Amide confidence 1.0
“The band at about The The 14,24 14,24 Theband Theband 1640 1640 cm1 cm1 is is a a complex complex band band due due to to the the absorption absorption ofwater ofwater bending bending and and amidelofproteins amidelofproteins at at about ab”
Liu 等 - 2006 - Characterization of the basidiomycetes Thelephora DOI: 10.1117/12.710905
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