What absorbs at 1638 cm⁻¹ in an FTIR spectrum?
A band near 1638 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 1638 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 | 41 | 37 | 1.0 |
| Carbonyl (C=O) | 33 | 27 | 1.0 |
| Alkene (C=C) | 31 | 29 | 1.0 |
| Methacrylate | 29 | 27 | 1.0 |
| Acetate | 28 | 26 | 1.0 |
| Carboxyl (COOH) | 26 | 24 | 1.0 |
| Ketone | 22 | 20 | 1.0 |
| Ester | 21 | 19 | 1.0 |
| Water (H2O) | 18 | 17 | 1.0 |
| Hydroxyl (O-H) | 13 | 13 | 1.0 |
| Alkyl C-H | 11 | 11 | 1.0 |
| Aromatic ring | 10 | 10 | 1.0 |
| N h | 10 | 10 | 1.0 |
| Methoxy (OCH3) | 7 | 7 | 1.0 |
| C-O single bond | 7 | 7 | 1.0 |
| Protein beta sheet | 5 | 3 | 1.0 |
| Secondary amine | 4 | 4 | 1.0 |
| C c single bond | 4 | 4 | 1.0 |
| Protein | 4 | 4 | 1.0 |
| Carbohydrate | 4 | 4 | 1.0 |
| Ring structure | 3 | 3 | 1.0 |
| Protein alpha helix | 3 | 3 | 1.0 |
| C=n | 3 | 3 | 1.0 |
| Silicon-oxygen (Si-O) | 2 | 2 | 1.0 |
| Silanol (Si-OH) | 2 | 2 | 1.0 |
| C n single bond | 1 | 1 | 1.0 |
| Chitosan | 1 | 1 | 1.0 |
| Methyl | 1 | 1 | 1.0 |
| Siloxane (Si-O-Si) | 1 | 1 | 1.0 |
| Silicon (Si) | 1 | 1 | 1.0 |
| Amine primary | 1 | 1 | 1.0 |
| N-O bond | 1 | 1 | 1.0 |
| C-S single bond | 1 | 1 | 1.0 |
| Metal oxygen | 1 | 1 | 1.0 |
| Quinone | 1 | 1 | 1.0 |
| Thiol | 1 | 1 | 1.0 |
| Metal hydroxyl | 1 | 1 | 1.0 |
| Ammonium | 1 | 1 | 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 |
|---|---|---|---|
| PVA | 1638, 1660, 2930 | Methacrylate, Carbonyl (C=O), Amide | 1 |
| glycerol | 1638, 1585, 1630 | Methacrylate, Carbonyl (C=O) | 1 |
| gold nanoparticles | 1638, 3252, 1749 | Amide, Methacrylate, Carbonyl (C=O) | 1 |
| AuNPs | 1638, 1646, 1721 | Amide, Methacrylate, Alkene (C=C) | 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 1638 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 shift in the peak and the new peak in cm-1 C=O stretching of the acetate group is related to the absorption band at 1638 the plasticized PVA shows the interaction between the NH 4SCN and the PVA [9].”
Characteristics of a Plasticized PVA-Based Polymer Electrolyte Membrane and H+ Conductor for an Electrical Double-Layer Capacitor: Structural, Morphological, and Ion Transport Properties DOI: 10.3390/membranes -
Acetate confidence 1.0
“Free esters and acids C=O Hemicellulose - 1722.43 1728.22 stretch C=O stretching Lignin 1637.56 1653.00 1658.78 [6]”
Firsoni 等 - 2019 - Fiber Content and Relative Feed Value Estimation o DOI: 10.1088/1757-899X/546/4/042008 -
Acetate confidence 1.0
“Antidiabetic bands at around 1638 (C=O stretching) whereas circular dichroism (CD) spectra exhibited α-helix content of secondary structure of standard 80.59% and 84.59% of column purified C-PC.”
Husain 等 - 2021 - Physicochemical characterization of C-phycocyanin DOI: 10.14715/cmb/2021.67.4.8 -
Alkyl C-H confidence 1.0
“þ A A f c cm-1), cm-1), C=O stretching (1638 CH wagging (1437 and 2 cm-1) C-N stretching (1292 of optimized sample CPN-0 a Fig.”
Jothi 等 - 2021 - Proton conducting polymer electrolyte based on cor DOI: 10.1007/s11581-020-03792-2 -
Hydroxyl (O-H) confidence 1.0
“The maximum of 73.1% of antiindicates asymmetric O-H stretching, 1637.7 oxidation activity was indicated by EPsC at 1000 µg/mL but cm-1 represents the Amide I band, and 1408.08 represents”
Lavania 等 - 2023 - Anatomizing extracellular polymer of Calothrix des DOI: 10.14719/pst.2615 -
Amide confidence 1.0
“32(cid:2) (c) CO at 1413 produced by integrating different vibrations, including: (b) the amide I band at 1638”
Li 等 - 2014 - Mineralogical characterization of calcification in DOI: 10.1180/minmag.2014.078.4.01 -
Amide confidence 1.0
“interactions.33 Accumulating lines of eviprotein-ligand cm-1 originates band at 675 corroborating the presence dences reported AuNPs pivotal role in rapid biomoleccm-1 nanoparticles44 of Au and an amide I band at 1638 recognition.34(cid:2)3”
Hetero-Functionalized Gold Nanoparticles to Silence AU-Rich Element Containing mRNAs in RAGE-Expressing Inflammatory Cells: Preparation, Characterization, and <i>In</i>-<i>Vitro</i> Evaluation DOI: 10.1166/jnn.2018.14648 -
Water (H2O) confidence 1.0
“The dominant cm-1 stichtite peak at 1638 is a deformation mode of molecular water.”
Melchiorre 等 - 2020 - Insights on Structure and Threshold Detection Limi DOI: 10.3390/min10030215
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