What absorbs at 669 cm⁻¹ in an FTIR spectrum?
A band near 669 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 669 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 |
|---|---|---|---|
| Carbon dioxide | 5 | 5 | 1.0 |
| Adsorbed carbon monoxide | 4 | 4 | 1.0 |
| Carbonyl (C=O) | 3 | 3 | 1.0 |
| Alkyl C-H | 3 | 3 | 1.0 |
| Hydroxyl (O-H) | 2 | 2 | 1.0 |
| Silicon-oxygen (Si-O) | 2 | 2 | 1.0 |
| N h | 2 | 2 | 1.0 |
| Methacrylate | 2 | 2 | 1.0 |
| Acetate | 2 | 2 | 1.0 |
| Alkene (C=C) | 2 | 2 | 1.0 |
| Silicon (Si) | 2 | 2 | 1.0 |
| Amine primary | 1 | 1 | 1.0 |
| Ketone | 1 | 1 | 1.0 |
| Ester | 1 | 1 | 1.0 |
| Carboxyl (COOH) | 1 | 1 | 1.0 |
| Amide | 1 | 1 | 1.0 |
| Urea | 1 | 1 | 1.0 |
| Silicon nitrogen | 1 | 1 | 1.0 |
| Silicon carbon | 1 | 1 | 1.0 |
| Methoxy (OCH3) | 1 | 1 | 1.0 |
| C-O single bond | 1 | 1 | 1.0 |
| Metal oxygen | 1 | 1 | 1.0 |
| Aromatic ring | 1 | 1 | 1.0 |
| Siloxane (Si-O-Si) | 1 | 1 | 0.9 |
| Carbonate | 1 | 1 | 0.7 |
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 |
|---|---|---|---|
| PET | 669, 1408, 800 | Alkyl C-H, Carbonyl (C=O) | 1 |
| silver nanoparticles | 669, 1636, 1631 | Alkyl C-H | 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 669 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
-
Alkyl C-H confidence 1.0
“A very prominent band at 669 could Downloaded C-H be related to aromatic out-of-plane deformations, rather than to mineral matter, because its intensity decreases in the FTIR spectra of the insoluble fractions (e.g., samples”
Georgakopoulos 等 - 2003 - Study of low rank Greek coals using FTIR spectrosc DOI: 10.1080/00908310390232442 -
confidence 1.0
“The disappearance of O-H band and the appearance of a cm-1 new band at 668.56 is associated with Ag-O bond which confirms the incorporation of”
Haq 等 - 2018 - Fabrication of pure and moxifloxacin functionalize DOI: 10.1016/j.jphotobiol.2018.07.011 -
Acetate confidence 1.0
“The Mn(CO) 5X correlation indi5 458s 457s M-COstretching 669cm-1 band is an E mode involving Mn cates that the C O”
Mahmood 等 - 2006 - Comparative vapour phase FTIR spectra and vibratio DOI: 10.1016/j.saa.2005.11.024 -
Silicon (Si) confidence 1.0
“The peaks corresponding to Si-C are 611, 653, cm-1 669, 740, 834 and the peaks corresponding to Si-N are in the”
Optical and photoelectrochemical properties of nitrogen-doped a-SiC thin films deposited by reactive DOI: 10.1080/2374068X.2021.1890420 -
Alkene (C=C) confidence 1.0
“cm-1 cm-1 cm-1 aromatic), 1635 (C=C stretch, aromatic), 1514 (NH), 1016 (C-O stretch, cm-1 hydroxyl), 669 (=C-H bending, alkene) (Masum et al., 2019).”
Pathak 等 - 2020 - Attenuation of hepatic and breast cancer cells by DOI: 10.1016/j.bcab.2020.101863 -
Silicon (Si) confidence 1.0
“According to the literatures [34, 36], the cm-1 presence of weak absorption peaks at about 669 and cm-1 684 is related to the vibrations modes of Si-O band.”
Magnetron sputtering technique for analyzing the influence of RF sputtering power on microstructural surface morphology of aluminum thin films deposited on SiO2/Si substrates DOI: 10.1007/s00339-021-04892-0 -
Urea confidence 1.0
“The peaks due to the urea domain cm-1 cm-1, were characterized by bands at 1626-1645 and 1651-1669 attributed to the ordered and were characterized by bands at 1,626-1,645 cm-1 and 1,651-1,669 cm-1, attributed to the ordered and disordered”
Che 等 - 2019 - Investigations on Aging Behavior and Mechanism of DOI: 10.3390/ma12213636 -
Hydroxyl (O-H) confidence 1.0
“2 3/2 1/2 distinguish27,28, (952.2eV) are too close to we intuitively dried samples showed a characteristic O-H stretching vibration 669cm-1, Cu+ /Cu0 which was ascribed to the copper silicate ratio by Cu LMM X-ray induced Auger determined”
Converting waste PET plastics into automobile fuels and antifreeze components DOI: 10.1038/s41467-022-31078-w
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