What absorbs at 1620 cm⁻¹ in an FTIR spectrum?
A band near 1620 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 1620 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 | 34 | 30 | 1.0 |
| Hydroxyl (O-H) | 29 | 24 | 1.0 |
| Carbonyl (C=O) | 26 | 24 | 1.0 |
| Carboxyl (COOH) | 24 | 24 | 1.0 |
| Water (H2O) | 23 | 20 | 1.0 |
| Aromatic ring | 22 | 15 | 1.0 |
| Alkyl C-H | 19 | 16 | 1.0 |
| Alkene (C=C) | 16 | 14 | 1.0 |
| N h | 16 | 13 | 1.0 |
| Methacrylate | 14 | 14 | 1.0 |
| Acetate | 14 | 14 | 1.0 |
| Ester | 13 | 12 | 1.0 |
| Ketone | 12 | 12 | 1.0 |
| Protein beta sheet | 9 | 8 | 1.0 |
| C c single bond | 9 | 7 | 1.0 |
| Secondary amine | 7 | 6 | 1.0 |
| C=n | 7 | 6 | 1.0 |
| Hydrogen bond | 4 | 3 | 1.0 |
| C-O single bond | 3 | 3 | 1.0 |
| Methoxy (OCH3) | 3 | 3 | 1.0 |
| N-O bond | 3 | 3 | 1.0 |
| C n single bond | 3 | 3 | 1.0 |
| Carbohydrate | 3 | 2 | 1.0 |
| Methyl | 2 | 2 | 1.0 |
| Amine primary | 2 | 2 | 1.0 |
| Oxalate | 2 | 1 | 1.0 |
| N n bond | 2 | 1 | 1.0 |
| Amino acid | 1 | 1 | 1.0 |
| Protein beta turn | 1 | 1 | 1.0 |
| Carbonate | 1 | 1 | 1.0 |
| Protein alpha helix | 1 | 1 | 1.0 |
| Nitrile | 1 | 1 | 1.0 |
| Lignin | 1 | 1 | 1.0 |
| Silicon-oxygen (Si-O) | 1 | 1 | 1.0 |
| Fluorine (C-F) | 1 | 1 | 1.0 |
| Metal hydroxyl | 1 | 1 | 1.0 |
| Ring structure | 1 | 1 | 1.0 |
| Phenolic | 1 | 1 | 0.9 |
| Phosphate (PO4) | 1 | 0 | 1.0 |
| Lewis acid site | 1 | 0 | 0.9 |
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 | 1620, 1722, 1383 | Carbonyl (C=O), Carboxyl (COOH) | 1 |
| Polyaniline | 1620, 1565, 1570 | Amide | 1 |
| nanocellulose | 1620, 1540, 1646 | Amide, Hydroxyl (O-H) | 1 |
| soil | 1620, 575, 1240 | Hydroxyl (O-H), Carboxyl (COOH) | 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 1620 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
-
C c single bond confidence 1.0
“LLM confirmed rule peak-group candidate”
Electrically conductive nanocomposites of PMMA and carbon nanotubes prepared by in situ polymerization under probe sonication DOI: 10.1007/s11696-018-0443-5 -
Carboxyl (COOH) confidence 1.0
“COO-/C=C C-H vibrations 3rd peak showing the three main vibrational modes of CO (in order 2 (1620cm-1), 1525cm-1 (C=C).”
Demyan 等 - 2013 - Combining a coupled FTIR-EGA system and in situ DR DOI: 10.5194/bg-10-2897-2013 -
Aromatic ring confidence 1.0
“5 show also an overlapC@C cm-1, feature is connected with the stretching of quinoid rings that are ping band at 1620 undetected in Fig.”
Revisiting the Redox Transitions of Polyaniline. Semiquantitative Interpretation of Electrochemically Induced IR Bands DOI: 10.1016/j.jelechem.2021.115593 -
N-O bond confidence 1.0
“No peak was seen at 1620 due to ular hydrogen bonds get disturbed by LiCl/DMAc solvent system and (C\\O amide I stretching vibrations), verifying the complete removal Clformed a hydrogen bond between the anions and the hydroxyl”
Kale 和 Gorade - 2019 - Potential application of medical cotton waste for DOI: 10.1016/j.ijbiomac.2018.11.196 -
Protein beta sheet confidence 1.0
“The the band at 1668 During the binding of Cu(II) ions, the intensity of the band at cm-1 cm-1, β-sheet R-helix, are assigned to the which is assigned to an decreased while the bands at 1620 and 1680 1650 conformation.Thevalueof1620cm-1fora”
Kim 等 - 2010 - Molecular Rearrangement of Metal-Chelating Lipid M DOI: 10.1021/la902052f -
Hydroxyl (O-H) confidence 1.0
“The fundamental absorption peaks at 1620 & 3400 are assigned to the bending and stretching vibrations of O-H group.”
Kundu 等 - 2018 - Synthesis and characterization of iron and copper DOI: 10.1063/1.5051300 -
Carbohydrate confidence 1.0
“LLM confirmed rule peak-group candidate”
Lapuz 等 - 2022 - Production of Nanocellulose Film from Abaca Fibers DOI: 10.3390/cryst12050601 -
Carbonyl (C=O) confidence 1.0
“N A The infrared spectra of COM and COD have strong absorption peaks at 1620 and 1640 M cm-1, respectively, which are due to antisymmetric carbonyl stretching motions [23].”
Quantitative analysis of calcium oxalate hydrate urinary stones using FTIR and 950/912 cm-1 peak ratio DOI: 10.1016/j.vibspec.2019.03.006
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