What absorbs at 1646 cm⁻¹ in an FTIR spectrum?
A band near 1646 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 1646 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 | 20 | 19 | 1.0 |
| Hydroxyl (O-H) | 17 | 16 | 1.0 |
| Carbonyl (C=O) | 15 | 15 | 1.0 |
| Alkene (C=C) | 11 | 10 | 1.0 |
| Alkyl C-H | 9 | 9 | 1.0 |
| Methacrylate | 9 | 9 | 1.0 |
| Acetate | 9 | 9 | 1.0 |
| Water (H2O) | 9 | 9 | 1.0 |
| Carboxyl (COOH) | 8 | 8 | 1.0 |
| N h | 8 | 8 | 1.0 |
| Ketone | 6 | 6 | 1.0 |
| Ester | 6 | 6 | 1.0 |
| C-O single bond | 4 | 4 | 1.0 |
| Aromatic ring | 3 | 3 | 1.0 |
| Secondary amine | 3 | 3 | 1.0 |
| Methoxy (OCH3) | 3 | 3 | 1.0 |
| Carbohydrate | 3 | 3 | 1.0 |
| Hydrogen bond | 2 | 2 | 1.0 |
| Amine primary | 2 | 2 | 1.0 |
| Protein | 2 | 2 | 1.0 |
| Lignin | 2 | 2 | 1.0 |
| N-O bond | 2 | 2 | 1.0 |
| Silicate | 1 | 1 | 1.0 |
| C n single bond | 1 | 1 | 1.0 |
| Nucleic acid | 1 | 1 | 1.0 |
| Aldehyde (CHO) | 1 | 1 | 1.0 |
| Ring structure | 1 | 1 | 1.0 |
| Protein random coil | 1 | 1 | 1.0 |
| Silanol (Si-OH) | 1 | 1 | 1.0 |
| Amino acid | 1 | 1 | 1.0 |
| Chitosan | 1 | 1 | 1.0 |
| C c single bond | 1 | 1 | 1.0 |
| Protein beta turn | 1 | 1 | 1.0 |
| Protein alpha helix | 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 |
|---|---|---|---|
| starch | 1646, 1650, 1540 | Hydroxyl (O-H), Amide | 1 |
| epoxy | 1646, 1660, 1735 | Amide, Carbonyl (C=O) | 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 1646 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
-
Acetate confidence 1.0
“The C-O stretching from the asymmetric oxygen bridge at 1646 is bands in pure MC.”
Aziz 等 - 2022 - The Study of Ion Transport Parameters in MC-Based DOI: 10.3390/membranes12020139 -
Carbonyl (C=O) confidence 1.0
“A peak at 1646 3500 3000 2500 2000 1500 1000 500 ─C appeared due to O amidic carbonyl group.”
Influence of amine‐terminated additives on thermal and mechanical properties of diglycidyl ether of bisphenol A (DGEBA) cured epoxy DOI: 10.1002/app.48404 -
N-O bond confidence 1.0
“This indicated polymeric enzyme cleavage posure there were also no peaks between 1646 and 1132 space-defined”
Bawa 等 - 2013 - A novel stimuli-synchronized alloy-treated matrix DOI: 10.1016/j.jconrel.2012.12.011 -
Water (H2O) confidence 1.0
“The absorption bands at 1645.58 and were GidleyandBulpin[41]foundthatachainlengthofatleast10 attributed to the tightly bound water present in the starch, 1384.55cm(cid:1)1 glucose units is required for crystallization and formation and the”
Characterization of nanoscale retrograded starch prepared by a sonochemical method DOI: 10.1002/star.201500313 -
Alkene (C=C) confidence 1.0
“peaks at 1608 and 1646 Both peaks can be assigned to carbon-carbon double bond stretching, the cm(cid:2)1 peak at 1608 arising from a carbon-carbon double bond of an aromatic ring and the peak at 1646 cm(cid:2)1 arises from the alkene group”
Fox 等 - 2004 - Investigation of failure mechanisms in aged aerosp DOI: 10.1016/j.engfailanal.2003.05.010 -
Hydroxyl (O-H) confidence 1.0
“LLM confirmed rule peak-group candidate”
Synthesis, characterization and functional properties of low substituted acetylated corn starch DOI: 10.1016/j.ijbiomac.2012.02.030 -
Protein beta turn confidence 1.0
“(+) (-) to normalize the population of D447 photoconverted to S390 of turn structure, and the bands at 1660 and 1646 cm-1 to those at the higher temperatures as described above, and belong to those of a helical structure.”
Iwata 等 - 2003 - Light-induced structural changes in the LOV2 domai DOI: 10.1021/bi0345135 -
Water (H2O) confidence 1.0
“and bending mode at 1646 A broad hump of water stretching mode stretching mode was observed at about 3400 reveals weak hydrogen bonding.”
Synthesis and characterization of cobalt ferrite nanoparticles for hyperthermia treatment application DOI: 10.1063/1.5052096
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