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 130,000+ reference spectra and knowledge graph.
Backed by 8 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 |
| Silicon (Si) | 4 | 4 | 1.0 |
| Metal oxygen | 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 |
|---|---|---|---|
| GO | 668, 1720, 1182 | C-O single bond, Alkyl C-H | 1 |
| Co3O4 | 668, 1050, 450 | Metal oxygen | 1 |
| CdS | 668, 1375, 1092 | C-O single bond | 1 |
| Copper | 668, 1636, 1202 | C-O single bond | 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) confidence 1.0
“The epitaxcm-1 cm-1, absorption peaks at 668 and 951 corresponding 2O (0001) [6], ial growths of cubic AlN on Si (100) [5], Al 3 to the transverse and longitudinal optical vibration modes.”
Fu 等 - 2012 - Structural, optical and electrical properties of c DOI: 10.1007/s00339-011-6713-5 -
Metal oxygen confidence 1.0
“The absorption bands at 668 1502 cm-1 cm-1 and 1440 indicate the metallic stretching vibrations in Fe - O.”
Effect of Gd and Nb on Dielectric and Magnetic Transition temperature of BaTiO3- Li0.5Fe2.5O4 Composites DOI: 10.1016/j.physb.2019.02.035 -
Metal sulfur confidence 1.0
“It may be noted here that cm(cid:3)1 cm(cid:3)1 absorption peaks at 668 and (cid:2)703 for Cd-S (cid:2) stretching modes have been observed by Kawar et al.”
Utilization of residual CdCl2 in CBD-CdS to realize grain growth in CdTe: A novel route DOI: 10.1016/j.materresbull.2013.08.006 -
Alkyl C-H confidence 1.0
“On the other hand, the peaks at cm-1 668.45 and 671.73 are related to the C-H groups of aromatic compounds [57].”
Characterization of Polyhydroxybutyrate, PHB, Synthesized by Newly Isolated Haloarchaea Halolamina spp. DOI: 10.3390/molecules27217366 -
Hydroxyl (O-H) confidence 1.0
“5 UV-Vis absorption spectra of GO cm(cid:2)1, whereas for rGO-Co 3O 4, the peaks of Co-O and Co548 cm(cid:2)1 along with the peaks of O-H, O appeared at 668 and 578”
Synthesis of Self-Assembled rGO-Co3O4 Nanoparticles in Nanorods Structure for Supercapacitor Application DOI: 10.1007/s11665-018-3361-3 -
confidence 1.0
“cm-1 LO optical mode, whereas the absorption peak at 668 corresponds to the TO mode of Ti-O stretching vibration in the titanium”
Structural and optical properties investigation of DC magnetron sputtered β-TiO2 thin film DOI: 10.1016/j.apsusc.2014.08.131 -
confidence 1.0
“The absorption band seen at 668 Based on the surface investigation, AFM pictures of cm-1 corresponds to the typical stretching collision of OMnO 2, Cu, and Cu doped MnO nanoparticles were analysed Mn-O, indicating that MnO is present in the”
Shoba 等 - 2022 - Copper nanoparticles incorporated manganese dioxid DOI: 10.14447/jnmes.v25i3.a05 -
confidence 1.0
“O bond is longer than the Si-O bond [25, 26], the partial cm-1, In particular, new peaks at 668 and 419 which Si4+ Mg2+ replacement of in Si-O-Si linkages by would are attributed to the formation of Si-O-Mg bonds [27-”
Zhang 等 - 2019 - Influence of Magnesia on Demoulding Strength of Co DOI: 10.1515/rams-2019-0008
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