What absorbs at 1740 cm⁻¹ in an FTIR spectrum?
A band near 1740 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 1740 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 |
|---|---|---|---|
| Carbonyl (C=O) | 151 | 129 | 1.0 |
| Ester | 108 | 97 | 1.0 |
| Carboxyl (COOH) | 90 | 82 | 1.0 |
| Amide | 90 | 81 | 1.0 |
| Methacrylate | 81 | 74 | 1.0 |
| Acetate | 81 | 74 | 1.0 |
| Ketone | 72 | 65 | 1.0 |
| Lipid | 19 | 17 | 1.0 |
| Alkyl C-H | 16 | 14 | 1.0 |
| Methoxy (OCH3) | 14 | 13 | 1.0 |
| C-O single bond | 14 | 13 | 1.0 |
| Carbohydrate | 12 | 12 | 1.0 |
| Hydroxyl (O-H) | 9 | 8 | 1.0 |
| N h | 9 | 7 | 1.0 |
| Acetyl | 6 | 6 | 1.0 |
| Alkene (C=C) | 5 | 5 | 1.0 |
| Lignin | 4 | 4 | 1.0 |
| Urethane | 4 | 4 | 1.0 |
| C c single bond | 4 | 3 | 1.0 |
| Fatty acid | 4 | 3 | 1.0 |
| Aldehyde (CHO) | 4 | 3 | 1.0 |
| Aromatic ring | 3 | 3 | 1.0 |
| Protein | 3 | 3 | 1.0 |
| Water (H2O) | 3 | 3 | 1.0 |
| Triglyceride | 3 | 3 | 1.0 |
| Secondary amine | 3 | 2 | 1.0 |
| Hydrogen bond | 2 | 2 | 1.0 |
| C n single bond | 2 | 2 | 1.0 |
| N-O bond | 2 | 2 | 1.0 |
| Phospholipid | 2 | 2 | 1.0 |
| Methyl | 2 | 2 | 1.0 |
| Silanol (Si-OH) | 2 | 1 | 1.0 |
| Silicon (Si) | 2 | 1 | 1.0 |
| Ring structure | 1 | 1 | 1.0 |
| Protein beta sheet | 1 | 1 | 1.0 |
| Oxygen heterocycle | 1 | 1 | 1.0 |
| Protein alpha helix | 1 | 1 | 1.0 |
| Amine primary | 1 | 1 | 1.0 |
| Metal oxygen | 1 | 1 | 1.0 |
| Chitosan | 1 | 1 | 1.0 |
| C=n | 1 | 1 | 0.9 |
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 |
|---|---|---|---|
| PET | 1740, 1408, 800 | Amide, Carbonyl (C=O) | 1 |
| PHB | 1740, 3440, 1724 | Carbonyl (C=O), Ester | 1 |
| PLLA | 1740, 1182, 921 | Amide | 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 1740 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
-
C n single bond confidence 1.0
“Nevertheless, the stretching vibrations of C=O cm-1, bond [34], C-O bond [34, 35] and C-N bond [35] are found at peak positions of 1740 1375 cm-1”
Aziz 等 - 2023 - Water remediation capability of cubic-phase CdS na DOI: 10.15251/DJNB.2023.181.203 -
Amide confidence 1.0
“(a) after 4h of exposure to 50Hz EMF at 50𝜇T;(b)after4hofexposureto50HzEMFat0.8mT;(c)after4hofexposureto50HzEMFat1.4mT.Theincreasein𝛽-sheetcontentwith 1740cm-1 𝛼-helix vibration band in the amide I region appeared clearly after the respect”
Calabro 等 - 2013 - 50 Hz Electromagnetic Field Produced Changes in FT DOI: 10.1155/2013/414393 -
Alkyl C-H confidence 1.0
“the region 3000 cm-1 and 1740 cm-1 were related to CH2 symmetric stretching Also, the wavenumbers 2851 cm-1”
The Effect of High-Intensity Ultrasound and Natural Oils on the Extraction and Antioxidant Activity of Lycopene from Tomato (Solanum lycopersicum) Waste DOI: 10.3390/antiox11071404 -
Acetate confidence 1.0
“vera, with the FTIR revealing peaks at 1739.94 cm (C O stretching vibration), 1370 = -1 1H (deformation of the H-C-OH bonds), and 1370 cm (C-O-C asymmetric stretching vibration);”
Chou 等 - 2023 - Potential of methacrylated acemannan for exerting DOI: 10.1186/s12906-023-04022-8 -
Ester confidence 1.0
“PET exhibited its characteristic cm-1, due to the stretching of the ester bands at 1740”
Cohn 和 Stern - 2000 - Sequential surface derivatization of PET films DOI: 10.1021/ma9900784 -
Ester confidence 1.0
“Hydrocolloids, 12, doi:10.1016/S0268-005X(98) Mahmoud, Reg, Peter, and Keith (1992) also indi00014-9 cm-1 cated that the carboxylic ester peak at 1740 in”
Changes in chemical composition and structure of root cell wall of citrus rootstock seedlings in response to boron deficiency by FTIR spectroscopy DOI: 10.1080/14620316.2017.1362327 -
Acetate confidence 1.0
“cm-1 C-O-C are related to C = O bond in intensity at 1740 The stretching band of groups in the crystalline cm-1).”
El-Hadi - 2019 - Miscibility of CrystallineAmorphousCrystalline B DOI: 10.1080/03602559.2018.1455863 -
Acetyl confidence 1.0
“In the beginning of the treatment, hemicelluloses are the first to degrade, as proved here by the cm-1 initial decrease of the 1740 peak attributed to the cleavage of acetyl or acetoxy groups in xylan.”
Esteves 等 - 2013 - CHEMICAL CHANGES OF HEAT TREATED PINE AND EUCALYPT DOI: 10.4067/S0718-221X2013005000020
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