What absorbs at 1240 cm⁻¹ in an FTIR spectrum?
A band near 1240 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 1240 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 | 49 | 45 | 1.0 |
| Methacrylate | 42 | 41 | 1.0 |
| Acetate | 42 | 41 | 1.0 |
| C-O single bond | 41 | 38 | 1.0 |
| Methoxy (OCH3) | 36 | 35 | 1.0 |
| Phosphate (PO4) | 16 | 16 | 1.0 |
| Secondary amine | 16 | 14 | 1.0 |
| C n single bond | 16 | 14 | 1.0 |
| Alkyl C-H | 13 | 12 | 1.0 |
| Ester | 12 | 12 | 1.0 |
| Carbonyl (C=O) | 12 | 11 | 1.0 |
| Carboxyl (COOH) | 11 | 11 | 1.0 |
| Phosphorus | 10 | 10 | 1.0 |
| Carbohydrate | 9 | 9 | 1.0 |
| Ketone | 9 | 9 | 1.0 |
| Nucleic acid | 8 | 8 | 1.0 |
| N h | 7 | 6 | 1.0 |
| C c single bond | 7 | 6 | 1.0 |
| Protein | 6 | 6 | 1.0 |
| Hydroxyl (O-H) | 5 | 5 | 1.0 |
| Aromatic ring | 4 | 3 | 1.0 |
| S eq o | 3 | 3 | 1.0 |
| Ring structure | 3 | 3 | 1.0 |
| Acetyl | 3 | 3 | 1.0 |
| Lignin | 3 | 3 | 1.0 |
| Aldehyde (CHO) | 2 | 2 | 1.0 |
| Phospholipid | 2 | 2 | 1.0 |
| Water (H2O) | 2 | 2 | 1.0 |
| Oxygen heterocycle | 1 | 1 | 1.0 |
| Alkene (C=C) | 1 | 1 | 1.0 |
| Methyl | 1 | 1 | 1.0 |
| Nitrate | 1 | 1 | 1.0 |
| Fatty acid | 1 | 1 | 1.0 |
| Borate | 1 | 1 | 1.0 |
| Sulfate (SO4) | 1 | 1 | 1.0 |
| Sulfonyl | 1 | 1 | 1.0 |
| Amino acid | 1 | 1 | 1.0 |
| Metal oxygen | 1 | 1 | 1.0 |
| Protein beta sheet | 1 | 1 | 1.0 |
| Silicon-oxygen (Si-O) | 1 | 1 | 1.0 |
| Lipid | 1 | 1 | 0.6 |
| Silicon carbon | 1 | 0 | 1.0 |
| Silicon (Si) | 1 | 0 | 1.0 |
| N n bond | 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 | 1240, 1650, 1540 | Methacrylate, Acetate, C-O single bond | 1 |
| cells | 1240, 970, 1741 | Amide, 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 1240 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
-
confidence 1.0
“A peak around 1240 P-- RS4 microcapsules enriched extrudates which is characteristic of O P0 P5 P10 P15 P20 bonds (Shalviri et al., 2010) in RS4.”
Encapsulating probiotics in novel resistant starch wall material for production of rice flour extrudates DOI: 10.1016/j.lwt.2020.110839 -
Lignin confidence 1.0
“LLM confirmed rule peak-group candidate”
Physicochemical characterisation of barley straw treated with sodium hydroxide or urea and its digestibility and in vitro fermentability in ruminants DOI: 10.1038/s41598-022-24738-w -
Acetate confidence 1.0
“Second derivative spectra of the bending mode 3 Messman and Storey used the absorbance of the peak at 1240 cm-1 centered at 1454 for pure PLA (100/0-P/L) and two blends of cm-1 C-O-C associated with the ring stretch to follow the PLA and la”
Braun 等 - 2006 - Infrared spectroscopic determination of lactide co DOI: 10.1021/ma061922a -
Alkyl C-H confidence 1.0
“2850cm(cid:2)1 sp3-CH in the region ranging from 2970 to observable which could be attributed to due to sym3 1300cm(cid:2)1 (cid:1)2875cm(cid:2)1 sp3-CH C-H stretching modes and from 1240 to metric vibration at and symmetric (cid:1)2910cm(c”
Das 等 - 2004 - Vibrational spectroscopy characterization of low-d DOI: 10.1016/j.mssp.2004.09.121 -
confidence 1.0
“In the heparin-modified sample, there are peaks around 1168 cm-1 and 1240 cm-1, which correspond to the S-O and S=O bands.”
Surface Heparinization of a Magnesium-Based Alloy: A Comparison Study of Aminopropyltriethoxysilane (APTES) and Polyamidoamine (PAMAM) Dendrimers DOI: 10.3390/jfb13040296 -
Phosphate (PO4) confidence 1.0
“2A-C, there is a significant increase in the intensity of the cm(cid:1)1 absorbance band at 1240 (which represents phosphate, specific vibrational mode) of the infected compared to control 2011.”
Erukhimovitch 等 - 2011 - Infrared spectral changes identified during differ DOI: 10.1039/c1an15319f -
Acetyl confidence 1.0
“If estherification occurs the band at 1240 should also increase due to antisymmetric stretching vibration of the acetyl ester groups which with our samples does not happen.”
Esteves 等 - 2013 - CHEMICAL CHANGES OF HEAT TREATED PINE AND EUCALYPT DOI: 10.4067/S0718-221X2013005000020 -
Hydroxyl (O-H) confidence 1.0
“One notable difference with colophony is the presence of stretching bands of hydroxyl stretching probably due to alco- ∼1240cm-1.”
Font 等 - 2007 - Fourier transform infrared spectroscopy as a suita DOI: 10.1016/j.aca.2007.07.021
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