What absorbs at 668 cm⁻¹ in an FTIR spectrum?
A band near 668 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 320,000+ reference spectra and knowledge graph.
Backed by 5 cited sources
Quick answer
A band near 668 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 |
|---|---|---|---|
| Alkyl C-H | 4 | 4 | 1.0 |
| C-O single bond | 4 | 4 | 1.0 |
| Metal oxygen | 4 | 4 | 1.0 |
| Silicon (Si) | 4 | 4 | 1.0 |
| Aromatic ring | 3 | 3 | 1.0 |
| Hydroxyl (O-H) | 3 | 3 | 1.0 |
| Carbon dioxide | 3 | 3 | 1.0 |
| Silicon-oxygen (Si-O) | 2 | 2 | 1.0 |
| Methoxy (OCH3) | 2 | 2 | 1.0 |
| Methacrylate | 2 | 2 | 1.0 |
| Acetate | 2 | 2 | 1.0 |
| Amide | 2 | 1 | 1.0 |
| C#c | 1 | 1 | 1.0 |
| Thiol | 1 | 1 | 1.0 |
| C-S single bond | 1 | 1 | 1.0 |
| N-O bond | 1 | 1 | 1.0 |
| Metalloid oxygen | 1 | 1 | 1.0 |
| Siloxane (Si-O-Si) | 1 | 1 | 1.0 |
| Water (H2O) | 1 | 1 | 1.0 |
| Metal sulfur | 1 | 1 | 1.0 |
| Sulfate (SO4) | 1 | 0 | 1.0 |
| Ammonium | 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 |
|---|---|---|---|
| Co3O4 | 668, 1047, 4000 | Metal oxygen | 1 |
| polyhydroxybutyrate | 668, 3440, 1728 | Alkyl C-H | 1 |
| GO | 668, 1720, 1725 | C-O single bond, 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 668 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
-
Silicon (Si)
- Reported band
- 668 cm-1
- Assignment
- Silicon (Si)
- Material / context
- cubic AlN films deposited; cubic AlN films
-
Metal oxygen
- Reported band
- 668 cm-1
- Assignment
- Metal oxygen
- Material / context
- BaTiO3Li0; BaTiO3; BaTiO3-Li0.5Fe2.5O4 composites
-
Metal sulfur
- Reported band
- 668 cm-1
- Assignment
- Metal sulfur
- Material / context
- CdCl2; CdTe
-
Alkyl C-H
- Reported band
- 668 cm-1
- Assignment
- Alkyl C-H
- Material / context
- polyhydroxyalkanoate; polyhydroxybutyrate; polyhydroxybutyrate (PHB)
-
Hydroxyl (O-H)
- Reported band
- 668 cm-1
- Assignment
- Hydroxyl (O-H)
- Material / context
- Co3O4; GO
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