What absorbs at 1690 cm⁻¹ in an FTIR spectrum?
A band near 1690 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 1690 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) | 31 | 26 | 1.0 |
| Amide | 29 | 25 | 1.0 |
| Carboxyl (COOH) | 16 | 14 | 1.0 |
| Ester | 13 | 11 | 1.0 |
| Methacrylate | 12 | 10 | 1.0 |
| Acetate | 12 | 10 | 1.0 |
| Ketone | 11 | 9 | 1.0 |
| Protein | 6 | 5 | 1.0 |
| N h | 5 | 5 | 1.0 |
| Protein beta sheet | 5 | 5 | 1.0 |
| Alkene (C=C) | 5 | 4 | 1.0 |
| Hydroxyl (O-H) | 5 | 4 | 1.0 |
| Alkyl C-H | 4 | 4 | 1.0 |
| Protein beta turn | 3 | 3 | 1.0 |
| Secondary amine | 3 | 2 | 1.0 |
| N-O bond | 3 | 2 | 1.0 |
| Aromatic ring | 3 | 1 | 1.0 |
| Methoxy (OCH3) | 2 | 2 | 1.0 |
| C-O single bond | 2 | 2 | 1.0 |
| C n single bond | 2 | 1 | 1.0 |
| Water (H2O) | 2 | 1 | 1.0 |
| Ring structure | 2 | 1 | 1.0 |
| Phosphate (PO4) | 1 | 1 | 1.0 |
| Silicon-oxygen (Si-O) | 1 | 1 | 1.0 |
| Silicon (Si) | 1 | 1 | 1.0 |
| Hydrogen bond | 1 | 1 | 1.0 |
| C-S single bond | 1 | 1 | 1.0 |
| Protein alpha helix | 1 | 1 | 1.0 |
| C=n | 1 | 1 | 1.0 |
| Urea | 1 | 1 | 1.0 |
| Adsorbed carbon monoxide | 1 | 1 | 1.0 |
| Imide | 1 | 1 | 0.9 |
| Carbonate | 1 | 1 | 0.8 |
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 |
|---|---|---|---|
| chitosan | 1690, 1650, 1655 | Amide | 1 |
| polyurethane | 1690, 2270, 1700 | Carbonyl (C=O), Amide | 1 |
| polycaprolactone | 1690, 1580, 1751 | Carbonyl (C=O), Amide, Ester | 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 1690 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
“The peaks at 1690 and 3691 correspond to C=C and N-H vibrations cm-1 respectively.”
Highly Biological Active Antibiofilm, Anticancer and Osteoblast Adhesion Efficacy from MWCNT/PPy/Pd nanocomposite DOI: 10.1016/j.apsusc.2017.10.142 -
Protein beta turn confidence 1.0
“Peaks between 1690 and 1665 are generally assigned to β-turn structures.”
Fevzioglu 等 - 2020 - Quantitative approach to study secondary structure DOI: 10.1016/j.ijbiomac.2020.07.299 -
Adsorbed carbon monoxide confidence 1.0
“This is based on our assignment of the 1690 band in the commercial Au/TiO catalyst DRIFT spectrum actually being 2 bridging CO between two Au atoms.”
Gaur 等 - 2013 - CO oxidation studies over cluster-derived AuTiO2 DOI: 10.1016/j.cattod.2012.10.029 -
Urea confidence 1.0
“1690cm-1 absence of peaks at The PU was cast onto the screen-printed electrode (SPE) representing urea (C=O) in and analyzed using a single voltammetric cycle between this study indicated there is no urea formation as a bypro- -1200 and +15”
Munir 等 - 2022 - Design and Synthesis of Conducting Polymer Bio-Bas DOI: 10.1155/2022/6815187 -
Carbonyl (C=O) confidence 1.0
“In the literature, FTIR analysis of natural resins featured broad hycm(cid:2)1, droxyl stretch at around 3400 strong carbonyl stretch at 1690cm(cid:2)1, cm(cid:2)1, C¼C 1740 weak signal of vibrations at about 1610 methyl cm(cid:2)1,”
Geographical variations and correlation among some chemical and thermal properties of Almaciga (Agathis philippinensis Warb.) resins from selected commercial sites in the Philippines DOI: 10.1016/j.heliyon.2021.e06413 -
confidence 1.0
“FTIR absorption peak cm-1 at around 1690 was attributed to the bending vibration of N-H from PEI.”
Plasma-Induced Graft Polymerization of Polyethylenimine onto Chitosan/Polycaprolactone Composite Membrane for Heavy Metal Pollutants Treatment in Industrial Wastewater DOI: 10.3390/coatings12121966 -
Protein confidence 1.0
“(1985) PolyprolineIIdriedfilm 1640 The methods for processing mineralized tissues for FT-IR have PolyprolineIIafterheating 1650 1660cm-1/1690cm-1 used a peak of area ratio to detect collagen (Gly-Pro-Ala)n 1630,1656 inD2Osolution maturation”
Vidal 和 Mello - 2011 - Collagen type I amide I band infrared spectroscopy DOI: 10.1016/j.micron.2010.09.010 -
Carbonyl (C=O) confidence 1.0
“To clarify in 2O, should also be visible but was overlapped by the H 2 cm(cid:2)1.[20] which steps O strong carbonyl absorption at 1690 is incorporated into the product acids, we Other spectra in 2 further examined the possible intermediate”
An Unexplored O2Involved Pathway for the Decarboxylation of Saturated Carboxylic Acids by TiO2 Photocatalysis: An Isotopic Probe Study DOI: 10.1002/chem.201001704
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