What absorbs at 2981 cm⁻¹ in an FTIR spectrum?
A band near 2981 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 2981 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 | 9 | 8 | 1.0 |
| Hydroxyl (O-H) | 3 | 3 | 1.0 |
| Methoxy (OCH3) | 3 | 2 | 1.0 |
| Methacrylate | 3 | 2 | 1.0 |
| Acetate | 3 | 2 | 1.0 |
| Methyl | 3 | 1 | 1.0 |
| C-O single bond | 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 |
|---|---|---|---|
| PMMA | 2981, 1722, 1383 | Methacrylate, Alkyl C-H, Methoxy (OCH3) | 1 |
| PVP | 2981, 2983, 1648 | Alkyl C-H, Hydroxyl (O-H), Methacrylate | 1 |
| chitosan | 2981, 1650, 1655 | Alkyl C-H, Methacrylate, Hydroxyl (O-H) | 1 |
| Al2O3 | 2981, 1722, 1383 | Hydroxyl (O-H), Methacrylate, Alkyl C-H | 1 |
| water | 2981, 1018, 2945 | Hydroxyl (O-H), Methoxy (OCH3), Methacrylate | 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 2981 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
-
Hydroxyl (O-H) confidence 1.0
“And vibrational frequencies are 3767 for surface OH and 2981 cm-1 for in-plane OH at site 1, respectively.”
The investigation of adsorption and dissociation of H2O on Li2O (111) by ab initio theory DOI: 10.1016/j.apsusc.2017.02.133 -
Acetate confidence 1.0
“between host polymer and lithium salt and CH3 stretching mode at 2981.34 XRD analysis had also been performed to study the structural behaviour of the PMMA polymer electrolyte.”
Effect of Al2O3 in poly(methyl methacrylate) composite polymer electrolytes DOI: 10.1063/1.5034559 -
Hydroxyl (O-H) confidence 1.0
“cm-1 cm-1 SM showed many intense peaks at 3324 for the acidic O-H, 2981 for the acidic showed many intense peaks at 3324 cm-1 for the acidic O-H, 2981 cm-1 for the acidic - cm-1 cm-1 -CH=CH- (aryl) str, 2884 for C-H (alkyl) str, 1635 for th”
Boosting the Anticancer Activity of Sunitinib Malate in Breast Cancer through Lipid Polymer Hybrid Nanoparticles Approach DOI: 10.3390/polym14122459 -
Alkyl C-H confidence 1.0
“cm-1 cm-1 spectral regions at 3347 (OH symmetric and asymmetric stretching), 2981 (methyl group cm-1 C-H stretching), 1247 (among others C-H stretching and O-H deformations in carbohydrates), cm-1”
Pleurotus Mushrooms Content in Glucans and Ergosterol Assessed by ATR-FTIR Spectroscopy and Multivariate Analysis DOI: 10.3390/foods9040535 -
Alkyl C-H confidence 1.0
“cm-1 The four sub bands of the C-H absorption are centered at about 2981, 2938, 2911, and 2877 at room temperature.”
The Role of Backbone Hydration of Poly(N-isopropyl acrylamide) Across the Volume Phase Transition Compared to its Monomer DOI: 10.1038/s41598-017-17272-7 -
Alkyl C-H confidence 1.0
“It was noted that CS/PVP composites show a complex region of CH Therefore, the ratio of intensities I 2930/I was chosen to characterise the influence of 2981 and CH2 stretching vibrations with contribution from both type of molecules [36].”
Electrospun Fibres of Chitosan/PVP for the Effective Chemotherapeutic Drug Delivery of 5-Fluorouracil DOI: 10.3390/chemosensors9040070 -
Alkyl C-H confidence 1.0
“We also observed the rising of negative bands at 2981 and cm-1 from CH stretching vibrations (not shown in Figure 3).”
Kumar 和 Barth - 2010 - Following Enzyme Activity with Infrared Spectrosco DOI: 10.3390/s100402626 -
Alkyl C-H confidence 1.0
“The micro-structure and The CH asymmetric stretching band appeared at 2981 AM.”
A KBr-impregnated paper substrate as a sample probe for the enhanced ATR-FTIR signal strength of anionic and non-ionic surfactants in an aqueous medium DOI: 10.1039/d0ra07286a
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