What absorbs at 1639 cm⁻¹ in an FTIR spectrum?
A band near 1639 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 1639 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 | 38 | 35 | 1.0 |
| Carbonyl (C=O) | 20 | 19 | 1.0 |
| Methacrylate | 17 | 17 | 1.0 |
| Acetate | 17 | 17 | 1.0 |
| Carboxyl (COOH) | 16 | 16 | 1.0 |
| Water (H2O) | 15 | 14 | 1.0 |
| Hydroxyl (O-H) | 14 | 14 | 1.0 |
| Ketone | 14 | 14 | 1.0 |
| Ester | 14 | 14 | 1.0 |
| Alkene (C=C) | 13 | 10 | 1.0 |
| Protein beta sheet | 7 | 7 | 1.0 |
| N h | 6 | 6 | 1.0 |
| Secondary amine | 6 | 6 | 1.0 |
| Alkyl C-H | 5 | 5 | 1.0 |
| Methoxy (OCH3) | 4 | 4 | 1.0 |
| C-O single bond | 4 | 4 | 1.0 |
| Protein | 4 | 4 | 1.0 |
| Aromatic ring | 4 | 4 | 1.0 |
| C n single bond | 4 | 4 | 1.0 |
| Protein random coil | 3 | 3 | 1.0 |
| Protein alpha helix | 2 | 2 | 1.0 |
| C c single bond | 2 | 2 | 1.0 |
| Urea | 2 | 2 | 1.0 |
| Hydrogen bond | 1 | 1 | 1.0 |
| Methyl | 1 | 1 | 1.0 |
| Urethane | 1 | 1 | 1.0 |
| Amine primary | 1 | 1 | 1.0 |
| Thiol | 1 | 1 | 1.0 |
| Nitro (NO2) | 1 | 1 | 1.0 |
| Isocyanate | 1 | 1 | 1.0 |
| Ring structure | 1 | 0 | 1.0 |
| Nitrogen heterocycle | 1 | 0 | 1.0 |
| Lipid | 1 | 0 | 1.0 |
| N n bond | 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 |
|---|---|---|---|
| TiO | 1639, 1730, 1699 | Methacrylate, Acetate | 1 |
| hydroxyapatite | 1639, 1620, 1630 | Methacrylate, Acetate, Amide | 1 |
| protein | 1639, 1650, 1640 | Amide, Carbonyl (C=O), Methacrylate | 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 1639 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
-
Water (H2O) confidence 1.0
“It relaxometry were for the first time combined to NMR T 2 cm-1 may also is important to point out that the band at 1639 facilitate a simultaneous characterization of protein structures be due to water remaining in the tissue sections after”
Bertram 等 - 2006 - Heat-induced changes in myofibrillar protein struc DOI: 10.1021/jf0514726 -
Acetate confidence 1.0
“Aliphatic CH stretching was observed as two peaks at 2943 and 2885 and and the peak at 1639 cm-1 indicates the stretching vibration of C=O.”
Bio_Fabricated Levan Polymer from Bacillus subtilis MZ292983.1 with Antibacterial, Antibiofilm, and Burn Healing Properties DOI: 10.3390/app12136413 -
Acetate confidence 1.0
“2 cm-1 cm-1, 1639 C=O stretching band vibration, 1494 1011 cm-1 cm-1 cm-1 bands, 733 bands, and 438 bands were”
Reactive Oxygen Species (ROS) Are Generated as Organic Contaminants Are Broken Down: SnO DOI: 10.1007/s12668-023-01111-3 -
Hydroxyl (O-H) confidence 1.0
“Meanwhile, for the SLN spectrum, cm-1 the absorption peak centered at 3420 corresponds -C-OH stretching vibration, and weak cm-1 represent C=C stretching vibration, -OH absorption peaks at 1639, 1370 and 1090 deformation vibration and -C-O”
Kim 和 Kang - 2012 - Fabrication of large area photonic crystal with su DOI: 10.1117/12.930432 -
Carbonyl (C=O) confidence 1.0
“Small band at 1639 belongs to carbonyl (-C=O) with C=O-NH arrangements of traces of urea which may be present due to the reaction of excess isocyanate with moisture in air.”
Preparation and characterization of aliphatic polyurethane and modified hydroxyapatite composites for bone tissue engineering DOI: 10.1007/s00289-019-03067-5 -
C c single bond confidence 1.0
“(1687/1643→1663/1639cm-1, C-C/C-N vibration see middle of Change of the solvent from CH 2Cl to CCl slightly affected the”
Morishita 和 Tamiaki - 2009 - Specific coupling between the 13-keto carbonyl and DOI: 10.1016/j.saa.2008.09.016 -
Amide confidence 1.0
“We demonstrated that the binding of the antibody to the antigen cm-1), immobilized on a GaAs surface selectively gave rise to (cid:226)-sheet amide I vibrations (1639 and 1690 while no structurally related signals were induced by nonspecifi”
Onodera 等 - 2007 - Label-free detection of protein-protein interactio DOI: 10.1021/la7022192 -
Alkene (C=C) confidence 1.0
“In contrast 0.0 cm-1, to untreated TiO, a strong peak at 1300 -80 -60 -40 -20 0 20 assignable to the Ti-O stretching vibrations, peak at Zeta Potential (mV) cm-1, assignable to the C=O stretching vibrations, 474 3: Zeta potential of TiO nan”
Singh 等 - 2016 - FT-IR and Zeta Potential Measurements on TiO Nanop DOI: 10.1063/1.4948233
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