What absorbs at 2900 cm⁻¹ in an FTIR spectrum?
A band near 2900 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 2900 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 | 167 | 149 | 1.0 |
| Hydroxyl (O-H) | 34 | 31 | 1.0 |
| Acetate | 16 | 14 | 1.0 |
| Carbohydrate | 14 | 14 | 1.0 |
| Methacrylate | 14 | 13 | 1.0 |
| Methoxy (OCH3) | 12 | 12 | 1.0 |
| Hydrogen bond | 11 | 10 | 1.0 |
| C-O single bond | 10 | 10 | 1.0 |
| Amide | 9 | 8 | 1.0 |
| Carboxyl (COOH) | 7 | 6 | 1.0 |
| Alkene (C=C) | 6 | 5 | 1.0 |
| Ketone | 6 | 5 | 1.0 |
| Ester | 6 | 5 | 1.0 |
| Carbonyl (C=O) | 6 | 5 | 1.0 |
| N h | 5 | 5 | 1.0 |
| Aromatic ring | 4 | 4 | 1.0 |
| Lipid | 3 | 3 | 1.0 |
| Water (H2O) | 3 | 3 | 1.0 |
| C c single bond | 3 | 2 | 1.0 |
| Methyl | 3 | 1 | 1.0 |
| Silicon carbon | 2 | 2 | 1.0 |
| Secondary amine | 1 | 1 | 1.0 |
| C n single bond | 1 | 1 | 1.0 |
| Fatty acid | 1 | 1 | 1.0 |
| Silicon (Si) | 1 | 1 | 1.0 |
| Chlorine | 1 | 1 | 1.0 |
| Phosphate (PO4) | 1 | 1 | 1.0 |
| Siloxane (Si-O-Si) | 1 | 1 | 1.0 |
| Silicon-oxygen (Si-O) | 1 | 1 | 1.0 |
| Chitosan | 1 | 1 | 1.0 |
| Lignin | 1 | 1 | 1.0 |
| Borate | 1 | 1 | 1.0 |
| Ring structure | 1 | 1 | 1.0 |
| Metal oxygen | 1 | 1 | 1.0 |
| Protein | 1 | 1 | 1.0 |
| Fluorine (C-F) | 1 | 0 | 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 |
|---|---|---|---|
| lignin | 2900, 1035, 1510 | Alkyl C-H, Acetate, Hydroxyl (O-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 2900 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
“The existence of the C-H stretching modes seen in Figure 4 is indicated by the formacm-1 tion of a significant peak at roughly 2900 [65-69], and the salt content also reduced its intensity.”
Characteristics of Methyl Cellulose Based Solid Polymer Electrolyte Inserted with Potassium Thiocyanate as K+ Cation Provider: Structural and Electrical Studies DOI: 10.3390/ma15165579 -
Silicon carbon confidence 1.0
“LLM confirmed rule peak-group candidate”
Bhagat 等 - 2007 - Rapid synthesis of water-glass based aerogels by i DOI: 10.1016/j.apsusc.2006.07.016 -
Alkyl C-H confidence 1.0
“a peak near 2900 which corresponds to C-H stretching mode and peaks below cm-1, modes.18-20 corresponding to C-C modes and C-H bending Both Ra2000 J.”
DEPOSITION OF WIDE BAND GAP DLC FILMS USING R.F. PECVD AT VERY LOW POWER DOI: 10.1142/S0217979211100102 -
Water (H2O) confidence 1.0
“The peak at 2900 is attributed to the presence of x N AV cell cm(cid:0) 1 moisture in the sample whereas the peak at around 3700 indicate P¼� x� the vibration of hydroxyl group in the sample [26].”
Effect of Crâ Bi substitution on the structural, optical, electrical and magnetic properties of strontium hexaferrites DOI: 10.1016/j.physb.2019.411681 -
Alkyl C-H confidence 1.0
“cm-1 The peaks in and around 2900 shows various stretching modes of C-H (n=1,2,3).”
De 等 - 2011 - Fourier-Transform Infrared Absorption Study of the DOI: 10.1063/1.3605921 -
Alkyl C-H confidence 1.0
“The peak at ~2900 cm-1 is associated with C-H stretching vibrations, and the peak at a challenge for decades and the main barrier for commercial success until the strong impact of”
Filipova 等 - 2018 - Synthesis of Nanofibrillated Cellulose by Combined DOI: 10.3390/nano8090640 -
confidence 1.0
“The spectra of A, S and H4 each show a group cm(cid:1)1 of three small peaks around 2900 which do not correspond to any peaks in the Voloshin et al.”
Groat 等 - 2009 - The crystal chemistry of holtite DOI: 10.1180/minmag.2009.073.6.1033 -
Alkyl C-H confidence 1.0
“LLM confirmed rule peak-group candidate”
Hassan 和 Badri - 2014 - Lignin Recovery from Alkaline Hydrolysis and Glyce DOI: 10.1063/1.4895236
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