What absorbs at 2931 cm⁻¹ in an FTIR spectrum?
A band near 2931 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 2931 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 | 32 | 30 | 1.0 |
| Carboxyl (COOH) | 4 | 4 | 1.0 |
| Hydroxyl (O-H) | 4 | 3 | 1.0 |
| Methyl | 3 | 2 | 1.0 |
| Alkene (C=C) | 2 | 2 | 1.0 |
| Aromatic ring | 1 | 1 | 1.0 |
| N h | 1 | 1 | 1.0 |
| Methacrylate | 1 | 1 | 1.0 |
| Ketone | 1 | 1 | 1.0 |
| Ester | 1 | 1 | 1.0 |
| Carbonyl (C=O) | 1 | 1 | 1.0 |
| Amide | 1 | 1 | 1.0 |
| Acetate | 1 | 1 | 1.0 |
| Chlorine | 1 | 1 | 1.0 |
| Hydrogen bond | 1 | 1 | 1.0 |
| C c single bond | 1 | 1 | 1.0 |
| Amino acid | 1 | 0 | 1.0 |
| Fatty acid | 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 |
|---|---|---|---|
| starch | 2931, 1650, 1540 | Hydroxyl (O-H), Alkyl C-H | 1 |
| AgNPs | 2931, 1232, 1048 | Alkyl C-H, Hydroxyl (O-H) | 1 |
| rGO | 2931, 2358, 2930 | Hydroxyl (O-H), Alkyl C-H | 1 |
| hydrogels | 2931, 3281, 1700 | Alkyl C-H, Carboxyl (COOH) | 1 |
| composite | 2931, 1703, 1295 | Alkyl C-H, Hydroxyl (O-H) | 1 |
| glycerol | 2931, 1585, 1630 | Alkyl C-H, Hydroxyl (O-H), Carboxyl (COOH) | 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 2931 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 bands at around 2931 were attributed to the stretching vibrations of methylene groups, the methyl group of cm(cid:0) 1 [61-63].”
Optimization of citron peel pectin and glycerol concentration in the production of edible film using response surface methodology DOI: 10.1016/j.heliyon.2023.e13724 -
C c single bond confidence 1.0
“The C C bonds appear in the weak bands 2931 and 3079cm-1 ∼1635cm-1 at (C C H) and (C C).”
Font 等 - 2007 - Fourier transform infrared spectroscopy as a suita DOI: 10.1016/j.aca.2007.07.021 -
Alkyl C-H confidence 1.0
“(C-H stretch) only bind with tional peak at 2931 2”
Screening and stability testing of commercially applicable Heliotropium crispum silver nanoparticle formulation with control over aging and biostability DOI: 10.1007/s13204-020-01333-x -
Alkyl C-H confidence 1.0
“The band comBandcenter(cm¡1) ponent analysis of DMPS in the CH stretching region reveals 2931”
Modification of Chrysotile Surface by Organosilanes: An IR–Photoacoustic Spectroscopy Study DOI: 10.1006/jcis.2001.7524 -
Alkyl C-H confidence 1.0
“cm-1, cm-1, ing of amide at 3182 C-H stretching at 2931”
Owonubi 等 - 2018 - Characterization and in vitro release kinetics of DOI: 10.1007/s40090-018-0139-2 -
Alkyl C-H confidence 1.0
“LLM confirmed rule peak-group candidate”
Physicochemical properties of starches with variable amylose contents extracted from bambara groundnut genotypes DOI: 10.1016/j.carbpol.2015.06.100 -
Alkyl C-H confidence 1.0
“The set-up 2 cm-1 the band at 2931 was due to C-H stretching vibrations.”
Structural, optical and electrical properties of Ni doped ZnO nanostructures synthesized by solution combustion method DOI: 10.1007/s10854-017-8038-4 -
Alkyl C-H confidence 1.0
“LLM confirmed rule peak-group candidate”
Armynah 等 - 2016 - Study of Chloride Ion Transport of Composite by Us DOI: 10.1063/1.4943752
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