What absorbs at 1643 cm⁻¹ in an FTIR spectrum?
A band near 1643 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 1643 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 | 28 | 26 | 1.0 |
| Carbonyl (C=O) | 20 | 18 | 1.0 |
| Methacrylate | 16 | 15 | 1.0 |
| Acetate | 16 | 15 | 1.0 |
| Ketone | 13 | 12 | 1.0 |
| Carboxyl (COOH) | 13 | 12 | 1.0 |
| Hydroxyl (O-H) | 13 | 12 | 1.0 |
| Ester | 12 | 11 | 1.0 |
| Alkene (C=C) | 10 | 7 | 1.0 |
| N h | 9 | 9 | 1.0 |
| Alkyl C-H | 7 | 7 | 1.0 |
| Methoxy (OCH3) | 5 | 5 | 1.0 |
| C-O single bond | 5 | 5 | 1.0 |
| C c single bond | 4 | 4 | 1.0 |
| Water (H2O) | 4 | 4 | 1.0 |
| Secondary amine | 3 | 3 | 1.0 |
| C n single bond | 3 | 3 | 1.0 |
| Aromatic ring | 2 | 2 | 1.0 |
| Protein | 2 | 2 | 1.0 |
| Carbohydrate | 2 | 2 | 1.0 |
| Ring structure | 1 | 1 | 1.0 |
| Silicon-oxygen (Si-O) | 1 | 1 | 1.0 |
| Silicon (Si) | 1 | 1 | 1.0 |
| Carbon dioxide | 1 | 1 | 1.0 |
| Nucleic acid | 1 | 1 | 1.0 |
| N n bond | 1 | 1 | 1.0 |
| Acetyl | 1 | 1 | 1.0 |
| Amino acid | 1 | 1 | 1.0 |
| Protein random coil | 1 | 1 | 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 |
|---|---|---|---|
| Polyamide | 1643, 1663, 1541 | Amide, Carbonyl (C=O), Methacrylate | 1 |
| AgNPs | 1643, 1232, 1048 | Methacrylate, Acetate, Amide | 1 |
| graphene | 1643, 1720, 2097 | Methacrylate, Acetate, Amide | 1 |
| microplastics | 1643, 1614, 1760 | Carbonyl (C=O), Methacrylate, Acetate | 1 |
| Silver Nanoparticles (AgNPs) | 1643, 1100, 1749 | 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 1643 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 peak at 1705.1 means that there is a bond of cm-1 -C=O, and also absorption at 1643.4 and medium intensity which shows the presence of C=C.”
Aritonang 等 - 2019 - Grafting of Oleic Acid On Cyclic Natural Rubber Re DOI: 10.13005/ojc/350119 -
Alkene (C=C) confidence 1.0
“In the spectra related to the PE fragments and pellets, new weathering bands from After twenty days of photo-exposition of particles in the climate room, the PP showed 1510 to 1770 cm-1 of absorbance, with centred peaks at about 1614 cm-1 (”
Campanale 等 - 2023 - Fourier Transform Infrared Spectroscopy to Assess DOI: 10.3390/polym15040911 -
Carbonyl (C=O) confidence 1.0
“cm-1 were impregnated either by the antibiotic only or Whereas, the band at 1643.2 arises due to by the AgNPs only and also by combinations them carbonyl stretch, unsaturated aldhyde, ketone”
El-Mekkawy 等 - 2021 - Silver Nanoparticles (AgNPs) Biosynthesized by Asp DOI: 10.22207/JPAM.15.1.05 -
confidence 1.0
“Two OH stretching bands were cm(cid:1)1 cm(cid:1)1 observed at 3620-3600 and 3502-3478 along with HOH bending band at 16533 With using DFT-B3LYP and a basis set of 6-31G(d,p), free 1196cm(cid:1)1 cm(cid:1)1 HSO(cid:1) ion adopted here yield”
Futamata 等 - 2005 - ATR-SEIR study of anions and water adsorbed on pla DOI: 10.1016/j.susc.2005.06.020 -
Acetate confidence 1.0
“FTIR cm-1 spectra of copal show a vibrational band at 1643 (C=O stretching) attributed to communic acids, while amber shows cm-1 cm-1, a band at 1735 associated with ester-group vibrations and a shoulder at about 3340 suggesting partial oxi”
Differentiation between copal and amber by their structure and thermal behaviour DOI: 10.1007/s10973-023-12333-8 -
Amide confidence 1.0
“Therein, the 1632~1643 cm-1 wheat, glutinous rice, and mung bean powders with high is assigned to Amide assigned to Amide I, 1532~1535 cm-1 cm-1 carbohydrate contents, the peak intensities for the 1641 is assigned to Amide III, which are II”
JOONSUK - 2019 - ATR-FTIR Analysis of Adhesives Jointing Buddhist S DOI: 10.12654/JCS.2019.35.1.02 -
C c single bond confidence 1.0
“FTIR spectrum showed an cm-1assigned absorption band at 1643.41 to the C C stretching vibration of the hexagonal ring also there were oxygen-containing functional groups such as C O and O H.”
Production of graphene powder by electrochemical exfoliation of graphite electrodes immersed in aqueous solution DOI: 10.1016/j.ijleo.2016.04.101 -
Acetate confidence 1.0
“shifted to a higher wavenumber of 1725 The peared due to the anion exchange reaction taking cm-1 3peak at 1643 corresponds to the C=O in PVP place between the ILs and the NO ions of the TFA4due to the Ag/PVP interaction and the actual peak”
Lakshmanamoorthy 和 Manivannan - 2022 - Ionic liquids assist synthesis of AgAgX (X = Cl, DOI: 10.1007/s10854-021-06774-w
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