What absorbs at 1621 cm⁻¹ in an FTIR spectrum?
A band near 1621 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 1621 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) | 8 | 6 | 1.0 |
| Amide | 8 | 6 | 1.0 |
| Hydroxyl (O-H) | 7 | 6 | 1.0 |
| Carboxyl (COOH) | 7 | 5 | 1.0 |
| Alkyl C-H | 6 | 5 | 1.0 |
| Methacrylate | 6 | 4 | 1.0 |
| Acetate | 6 | 4 | 1.0 |
| Ketone | 5 | 3 | 1.0 |
| Ester | 5 | 3 | 1.0 |
| Aromatic ring | 4 | 4 | 1.0 |
| Water (H2O) | 4 | 3 | 1.0 |
| Ring structure | 3 | 2 | 1.0 |
| Carbohydrate | 2 | 2 | 1.0 |
| Protein beta sheet | 2 | 2 | 1.0 |
| Alkene (C=C) | 2 | 2 | 1.0 |
| Metal oxygen | 1 | 1 | 1.0 |
| Nitride | 1 | 1 | 1.0 |
| Aldehyde (CHO) | 1 | 1 | 1.0 |
| Methoxy (OCH3) | 1 | 1 | 1.0 |
| C-O single bond | 1 | 1 | 1.0 |
| Quinone | 1 | 1 | 1.0 |
| N-O bond | 1 | 1 | 1.0 |
| Secondary amine | 1 | 1 | 1.0 |
| N h | 1 | 1 | 1.0 |
| Thiol | 1 | 1 | 1.0 |
| C c single bond | 1 | 1 | 0.9 |
| C=n | 1 | 1 | 0.9 |
| Methyl | 1 | 1 | 0.8 |
| Imide | 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 |
|---|---|---|---|
| humic acid | 1621, 1660, 1220 | 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 1621 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
-
Hydroxyl (O-H) confidence 1.0
“-CH 2107-110 PAN 2937 asymmetrical stretching of groups C^N -C^N 104-106 2244 stretching of groups O-H 1621 bending of the absorbed water”
Electrocatalytic and structural properties and computational calculation of PAN-EC-PC-TPAI-I2 gel polymer electrolytes for dye sensitized solar cell application DOI: 10.1039/d1ra01983j -
confidence 1.0
“assigned at 1633 while in Co-3AP-Ni and Cu-3AP-Ni complexes corresponding bands were observed at 1621 and 1615cm(cid:3)1, respectively.”
Synthesis, spectroscopic, thermal and structural properties of [M(3-aminopyridine)2Ni(μ-CN)2(CN)2]n (M(II)=Co and Cu) heteropolynuclear cyano-bridged complexes DOI: 10.1016/j.saa.2014.12.117 -
Amide confidence 1.0
“The deconvolution in the cm-1, cm-1 cm-1 cm-1 amide region shows peaks at 1621 1637 due to β sheet;”
Spectral profile index changes as biomarker of toxicity in Catla catla (Hamilton, 1822) edible fish studied using FTIR and principle component analysis DOI: 10.1007/s42452-020-3001-z -
Thiol confidence 1.0
“It clearly indicates a peak at (1621) 2573 or buffer solution with an increase cm-' assigned to the S-H stretching vibration of captopril (Fig.”
Vibrational Spectroscopic Studies on the Disulfide Formation and Secondary Conformational Changes of Captopril–HSA Mixture after UV-B Irradiation DOI: 10.1562/2005-04-25-RN-497 -
Alkyl C-H confidence 1.0
“that the branch chain in the raw material was broken by the co-thermal oxidation of HNO which were the deformation of the aliphatic C-H, the extension of the aromatic C = C bond, HA showed strong bands at 1621 cm-1 (C = C stretching vibrati”
Co-Thermal Oxidation of Lignite and Rice Straw for Synthetization of Composite Humic Substances: Parametric Optimization via Response Surface Methodology DOI: 10.3390/ijerph192416875 -
Alkyl C-H confidence 1.0
“1621.22 and C-H stretching at 2929.00 While other NAD NADH Quinol NADPH NADP Control peaks were either distorted or removed in the spectrum of Redox Mediators (1 mM) the metabolites, there was a sharp increase in the intensity”
Influence of redox mediators and media on methyl red decolorization and its biodegradation by Providencia rettgeri DOI: 10.1007/s42452-019-0668-0 -
N-O bond confidence 1.0
“Figure 5 shows the nent.24 The NO stretch frequencies in Mb variants are sum- - cos2 dependence of C on (3 1) for the peaks at 1621 and ł”
Park 等 - 2000 - Vibrational Stark spectroscopy of NO bound to heme DOI: 10.1021/ja0014741 -
Alkene (C=C) confidence 1.0
“Strong and broad band cm-1 A at 1621 due to C=C stretching of phenyl nucleus and C=C stretching of lignin in CoirBA(Py-Acetone)-C-BPO, indicating introduction of mono substituted benzene onto coir arising M”
Rout 等 - 2017 - A green approach to produce silver nano particles DOI: 10.1016/j.surfin.2017.03.004
Have a spectrum with this band?
Upload your FTIR spectrum and get a full interpretation report — peak assignments with literature citations, library matches, and a literature-backed analysis — in seconds.