What absorbs at 2921 cm⁻¹ in an FTIR spectrum?
A band near 2921 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 2921 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 |
|---|---|---|---|
| Alkyl C-H | 59 | 52 | 1.0 |
| Methacrylate | 10 | 9 | 1.0 |
| Acetate | 10 | 9 | 1.0 |
| Amide | 9 | 8 | 1.0 |
| Methyl | 7 | 7 | 1.0 |
| Hydroxyl (O-H) | 6 | 6 | 1.0 |
| N h | 6 | 5 | 1.0 |
| Methoxy (OCH3) | 5 | 5 | 1.0 |
| Ketone | 5 | 4 | 1.0 |
| Ester | 5 | 4 | 1.0 |
| Carboxyl (COOH) | 5 | 4 | 1.0 |
| Carbonyl (C=O) | 5 | 4 | 1.0 |
| Carbohydrate | 4 | 3 | 1.0 |
| Secondary amine | 3 | 3 | 1.0 |
| Aromatic ring | 2 | 2 | 1.0 |
| C-O single bond | 1 | 1 | 1.0 |
| Chlorine | 1 | 1 | 1.0 |
| C=n | 1 | 1 | 1.0 |
| Water (H2O) | 1 | 1 | 1.0 |
| Silicon hydride | 1 | 1 | 1.0 |
| C n single bond | 1 | 1 | 1.0 |
| Urea | 1 | 1 | 1.0 |
| Lipid | 1 | 1 | 1.0 |
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 |
|---|---|---|---|
| cellulose | 2921, 1735, 1650 | Alkyl C-H, Acetate, Methacrylate | 1 |
| lignin | 2921, 1035, 1510 | Alkyl C-H, Methacrylate, Acetate | 1 |
| PLA | 2921, 1650, 1073 | Alkyl C-H, Methacrylate, Acetate | 1 |
| nanocomposite | 2921, 3300, 1540 | Alkyl C-H, Methacrylate, Acetate | 1 |
| silver nanoparticles | 2921, 1636, 1631 | Alkyl C-H, Methacrylate, Acetate | 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 2921 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
-
Acetate confidence 1.0
“Haque et al., have reported could be attributed to a combination of NH and OH groups cm1 on the capping and stabilization of ZnO NPs by the terpewhereas the peak at 2921 could be attributed to CH 3 cm-1 noids contained in the neem extract u”
Green Synthesis, Structural Characterization and Photocatalytic Activities of Chitosan-ZnO Nano‐composite DOI: 10.1007/s10904-021-01988-1 -
Carbohydrate confidence 1.0
“cm-1 cm-1 cm-1 vibrations, and the peaks at 2921 and 2853 arise due to at 2921 shifts toward lower value indicating the removal of C hemicelluloses, H stretching vibrations in cellulose and hemicellulose.”
Kasyapi 等 - 2013 - Bionanowhiskers from jute Preparation and charact DOI: 10.1016/j.carbpol.2012.10.021 -
Alkyl C-H confidence 1.0
“(cid:2) as, CH band further shifted to ther increased to 1.25 CEC, the the band center shifted to while the peak absorbance 2 2921cm-1,whilethe(cid:2) 2bandremainedat2850cm-1(Table5).”
Li 等 - 2008 - An FTIR investigation of hexadecyltrimethylammoniu DOI: 10.1016/j.saa.2008.05.015 -
Acetate confidence 1.0
“Moreover, a band around 1735 is cm-1 the characteristic of C=O stretching of unconjugated hemicellulose while the peak at 2921 is due to and CH which denoted the characteristics of cellulose.”
Lun 等 - 2017 - Fourier Transform Infrared Spectroscopy (FTIR) ana DOI: 10.1063/1.4981871 -
confidence 1.0
“compare film samples for neat PLA, composites of PLA and Consequently, there is the evolution of a band associcm-1 N-H WS ated with stretching at 2921 in the FTIR spectra that NTs, and composites of PLA and APTES modified WS 8:25:24 2 2”
Magee 等 - 2023 - Silane functionalization of WS2 nanotubes for inte DOI: 10.1039/d3nr00583f -
Alkyl C-H confidence 1.0
“LLM confirmed rule peak-group candidate”
Study on the Effect of Fe3+ on Zircon Flotation Separation from Cassiterite Using Sodium Oleate as Collector DOI: 10.3390/min7070108 -
Aromatic ring confidence 1.0
“have caused the O-H-extending vibration peak at 3422 The phytochemical's presence cm-1, of an aromatic component is indicated by the tiny peak at 2921 which represents the vibration of C-H stretching shown in Table 1.”
Biosynthesis and Characterization of Silver Nanoparticles Using Tribulus terrestris Seeds: Revealed Promising Antidiabetic Potentials DOI: 10.3390/molecules28104203 -
Acetate confidence 1.0
“cm-1), cm-1), cm-1), CH (2920.99 C=O stretching (1735.10 amide I (1642.12 CH scissoring (1465.40 2 2”
Encapsulation of olive leaf phenolics within electrosprayed whey protein nanoparticles; production and characterization DOI: 10.1016/j.foodhyd.2019.105572
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