What absorbs at 1640 cm⁻¹ in an FTIR spectrum?
A band near 1640 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 1640 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
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 | 1640, 1722, 1383 | Carbonyl (C=O) | 1 |
| PET | 1640, 1408, 800 | Amide, Carbonyl (C=O) | 1 |
| Cellulose | 1640, 1735, 1650 | Hydroxyl (O-H), Amide | 1 |
| Methacrylate | 1640, 873, 1725 | Carbonyl (C=O), Hydroxyl (O-H) | 1 |
| nanotube | 1640, 1540, 1080 | Amide, Hydroxyl (O-H) | 1 |
| SiO2 | 1640, 1739, 699 | 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 1640 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
-
Alkene (C=C) confidence 1.0
“cm-1 In all spectra of samples C0-C3, the peak that appear at ~1640 can be assigned to cm-1 C=C and corresponded to the bonded carbonyl groups, and the peak at 1737 related to the free carbonyl groups [40-42].”
Study of Polyvinyl Alcohol-SiO2 Nanoparticles Polymeric Membrane in Wastewater Treatment Containing Zinc Ions DOI: 10.3390/polym13111875 -
Hydroxyl (O-H) confidence 1.0
“Nevertheless, considering the absorbed moisture amount observation, the peak at approximately 1640 cm-1 is more appropriate to analyze, as it is assigned to H-O-H angle vibrations Materials2020,13,4573 7of22 in water molecules [35].”
IR Study on Cellulose with the Varied Moisture Contents: Insight into the Supramolecular Structure DOI: 10.3390/ma13204573 -
Urea confidence 1.0
“Furthermore, the sharp peak at 1640 cm-1, amine-isocyanate generated by the ing at 1640 is attributed to the urea bond created by the reaction reaction, could not be found in the spectrum of the between the amine group and HDI.”
Cohn 和 Stern - 2000 - Sequential surface derivatization of PET films DOI: 10.1021/ma9900784 -
Acetate confidence 1.0
“cm-1 to the C=O stretching mode of the PMMA changed into a doublet of 1725 and 1640 cm-1 Li+ confirming the formation of ion-dipolar coordination between the and C=O in”
Effects of different inorganic nanoparticles on the structural, dielectric and ion transportation properties of polymers blend based nanocomposite solid polymer electrolytes DOI: 10.1016/j.electacta.2017.07.051 -
Acetate confidence 1.0
“They showed intense quantitative FTIR analyses included the applications of Fourier self1730cm-1) C=O peaks (centered at approximately deconvolution, area integration methods, and the calculation of area indicating 1640cm-1 ratios (e.g., D'”
Fossil cutin of Johnstonia coriacea (Corystospermaceae, Upper Triassic, Cacheuta, Argentina) DOI: 10.1016/j.coal.2018.02.016 -
Water (H2O) confidence 1.0
“The absorption at ~1640 here that area covered under crystalline and amorphous may be related to bending mode of absorbed water and phase may not be the best parameter to use since it is cr/ carbonyl groups of hemicelluloses [49-51].”
Das 等 - 2017 - Extraction of xylem fibers from Musa sapientum and DOI: 10.1007/s12221-017-1187-5 -
Metal oxygen confidence 1.0
“However, as O-H bending vibrations absorb in the same region, cm-1 and spectral contributions of carbonyl groups may affect the 1640 peak, alternative peak cm-1 selections are recommended [15].”
Methacrylate peak determination and selection recommendations using ATR-FTIR to investigate polymerisation of dental methacrylate mixtures DOI: 10.1371/journal.pone.0252999 -
Amide confidence 1.0
“FTIR spectrum of these conjugated nanotubes showed an additional peak at 1640 cm-1 corresponding to amide II vibration which is due to reaction of carboxylic groups of FA”
Covalent diphenylalanine peptide nanotube conjugated to folic acid/magnetic nanoparticles for anti-cancer drug delivery DOI: 10.1016/j.jddst.2017.06.005
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