What absorbs at 1665 cm⁻¹ in an FTIR spectrum?
A band near 1665 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 1665 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 | 13 | 10 | 1.0 |
| Carbonyl (C=O) | 11 | 8 | 1.0 |
| Methacrylate | 10 | 8 | 1.0 |
| Acetate | 10 | 8 | 1.0 |
| Ketone | 7 | 5 | 1.0 |
| Ester | 7 | 5 | 1.0 |
| Carboxyl (COOH) | 7 | 5 | 1.0 |
| Alkene (C=C) | 6 | 5 | 1.0 |
| C-O single bond | 4 | 4 | 1.0 |
| Methoxy (OCH3) | 3 | 3 | 1.0 |
| N h | 2 | 2 | 1.0 |
| Alkyl C-H | 2 | 2 | 1.0 |
| C=n | 2 | 2 | 1.0 |
| Protein alpha helix | 2 | 1 | 1.0 |
| Protein | 2 | 1 | 1.0 |
| Oxygen heterocycle | 2 | 1 | 1.0 |
| N-O bond | 1 | 1 | 1.0 |
| Protein beta sheet | 1 | 1 | 1.0 |
| Aromatic ring | 1 | 1 | 1.0 |
| Protein beta turn | 1 | 1 | 1.0 |
| Hydroxyl (O-H) | 1 | 1 | 1.0 |
| Bromine | 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 |
|---|---|---|---|
| PVA | 1665, 1660, 2930 | Acetate, Methacrylate, Carbonyl (C=O) | 1 |
| hydroxyapatite | 1665, 1620, 1639 | Methacrylate, Acetate, Amide | 1 |
| glycerol | 1665, 1585, 1630 | Methacrylate, Acetate, Carbonyl (C=O) | 1 |
| water | 1665, 1018, 2945 | Carbonyl (C=O), 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 1665 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
-
Alkyl C-H confidence 1.0
“assigned to CH bending deformation, belonging to the The band at 1665 has”
Enhanced dielectric and ferroelectric properties of PVDF-BiFeO3 composites in 0–3 connectivity DOI: 10.1016/j.jallcom.2017.04.310 -
Alkene (C=C) confidence 1.0
“The broadband 1665cm(cid:3)1 [21-25], from 1645 to is attributed to C=C and C=N stretching mode 28.”
Majumdar 等 - 2012 - Chemical synthesis and surface morphology of amorp DOI: 10.1080/15685543.2012.699751 -
Acetate confidence 1.0
“In pristine lignin-reinforced films, urea crosslinking withstarch was confirmed through cm-1, cm-1 appearance of FTIR peaks at 1625 belonged to N-H bond vibration and at 1665 cm-1 related to stretching of C=O bondinurea.”
Majeed 等 - 2017 - Lignin macromolecule's implication in slowing the DOI: 10.1002/star.201600362 -
Amide confidence 1.0
“It also 2-P (thederivative of LA) stimulate collagen hydroxylationbymodifycm-1) to the ascertained the ratio of the amide I band area (~1665 ingprolineandlysineresidues[32].LAisfundamentalforcollagensyncm-1).”
The role of topically applied l-ascorbic acid in ex-vivo examination of burn-injured human skin DOI: 10.1016/j.saa.2017.05.055 -
C=n confidence 1.0
“Moreover, the band at 1665 can be assigned to -C=N which indicates”
Heteroployacid on the composite of boehmite and polyionic liquid as a catalyst for alcohol oxidation and tandem alcohol oxidation Knoevenagel condensation reactions DOI: 10.1038/s41598-022-20699-2 -
Alkyl C-H confidence 1.0
“• COO- • The COOstretch, C-H rock, and C-O-C stretch in the unplasticized sample (SC0) may The stretch, C-H rock, and C-O-C stretch in the unplasticized sample (SC0) may cm-1, cm-1, cm-1, be found around 1665 cm-1, 1438 cm-1, and 1014 cm-1,”
Adam 等 - 2023 - Insight into the Effect of Glycerol on Dielectric DOI: 10.3390/molecules28083461 -
Amide confidence 1.0
“c cm-1), cm-1) peaks for amide I (1665 amide III (1255-1272 of collagen and the four r ip t 3vibrational modes of phosphate group (symmetric stretching vibration of of HA) at”
A comparative study on silicon nitride, titanium and polyether ether ketone on mouse pre-osteoblast cells DOI: 10.1002/mds3.10139. -
Carbonyl (C=O) confidence 1.0
“LLM confirmed rule peak-group candidate”
Benabou 等 - 2018 - Study of light-induced formation of photodimers in DOI: 10.1039/x0xx00000x
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