What absorbs at 840 cm⁻¹ in an FTIR spectrum?
A band near 840 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 840 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 | 16 | 15 | 1.0 |
| Silicon (Si) | 8 | 8 | 1.0 |
| C c single bond | 8 | 7 | 1.0 |
| Methoxy (OCH3) | 7 | 7 | 1.0 |
| C-O single bond | 7 | 7 | 1.0 |
| Methacrylate | 6 | 6 | 1.0 |
| Acetate | 6 | 6 | 1.0 |
| Fluorine (C-F) | 5 | 5 | 1.0 |
| Alkene (C=C) | 4 | 4 | 1.0 |
| Siloxane (Si-O-Si) | 4 | 3 | 1.0 |
| Silicon-oxygen (Si-O) | 4 | 3 | 1.0 |
| Aromatic ring | 3 | 3 | 1.0 |
| Methyl | 3 | 3 | 1.0 |
| Metal oxygen | 3 | 3 | 1.0 |
| Hydroxyl (O-H) | 3 | 3 | 1.0 |
| Carbohydrate | 3 | 2 | 1.0 |
| Silicon carbon | 3 | 2 | 1.0 |
| Secondary amine | 2 | 2 | 1.0 |
| Phosphate (PO4) | 2 | 2 | 1.0 |
| Lignin | 2 | 2 | 1.0 |
| Sulfate (SO4) | 2 | 2 | 1.0 |
| Nitrate | 2 | 2 | 0.9 |
| Water (H2O) | 2 | 1 | 1.0 |
| Carbonate | 2 | 1 | 1.0 |
| Silicon hydride | 2 | 1 | 1.0 |
| Hydrogen bond | 1 | 1 | 1.0 |
| Carbonyl (C=O) | 1 | 1 | 1.0 |
| N h | 1 | 1 | 1.0 |
| Silicon nitrogen | 1 | 1 | 1.0 |
| C n single bond | 1 | 1 | 1.0 |
| Amide | 1 | 1 | 1.0 |
| Silanol (Si-OH) | 1 | 1 | 1.0 |
| Borate | 1 | 1 | 0.6 |
| Metal oxo | 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 |
|---|---|---|---|
| PVDF | 840, 1160, 1784 | Alkyl C-H | 1 |
| MgO | 840, 699, 874 | Alkyl C-H, Methoxy (OCH3) | 1 |
| CeO2 | 840, 1554, 1630 | Methoxy (OCH3) | 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 840 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
“LLM confirmed rule peak-group candidate”
Arshad 等 - 2019 - Dielectric and Structural Properties of Poly(vinyl DOI: 10.1155/2019/5961563 -
confidence 1.0
“The presence of peak at enceofresidualorganicsinthesample.Asignificantspectroscopic 460cm-1 band at 840, 654, 600 and corresponds to Al-O vibrationwhereasbandat400cm-1 correspondstoCe-Ovibration.The 400cm-1 band at 460 [16] and [17] which a”
Chandradass 等 - 2009 - Effect of different fuels on the alumina-ceria com DOI: 10.1016/j.jallcom.2008.12.119 -
Metal oxygen confidence 1.0
“So it can be concluded that the growth of 840 cm-1 band is associated with the disappearance of V2, the coefficient 840 0.59 wandering V2 trapped by Oi: V + O → V O”
Chen 等 - 2009 - Neutron irradiation defects in Czochralski silicon DOI: 10.1002/pssc.200880706 -
Alkyl C-H confidence 1.0
“Additionally, a small peak at 840 for C-H drally and graphitically bonded carbons with incorporated bending mode could be observed.”
Gupta 等 - 2004 - Synthesis of DLC films by electrodeposition techni DOI: 10.1016/j.diamond.2004.02.006 -
Silicon (Si) confidence 1.0
“cm-1-890 cm-1 2) the peak centered at 840 due to the Si-H bending modes;”
Hou 等 - 2011 - Evolution of infrared spectra and optical emission DOI: 10.1088/1674-1056/20/7/077802 -
Silicon-oxygen (Si-O) confidence 1.0
“905 and 995 are assigned to Si-O-Ca stretching vibrations of C 2S, and this phase gives rise to cm-1 O-Si-O stretching modes at 840 [31,37].”
The Application of 29Si NMR Spectroscopy to the Analysis of Calcium Silicate-Based Cement using Biodentine™ as an Example DOI: 10.3390/jfb10020025 -
C-O single bond confidence 1.0
“The The band band at at about about 890 890 cm cm' was was attributed attributed to to n-linkage, n-linkage, and and the the 25, 27 Table 25, 27 Table band band at at about about 840 840 cm' cm' was was attributed attributed to to a-glycosi”
Liu 等 - 2006 - Characterization of the basidiomycetes Thelephora DOI: 10.1117/12.710905 -
C-O single bond confidence 1.0
“774 886 802 CeOH brations that are overlapped by stretching vibrations of the 840 (Ka(cid:3)cur(cid:1) akov(cid:1) CeOeC groups and glycosidic bond vibrations a et al., 0.3 2000).”
Structural studies of the arabinogalactans in Mangifera indica L. fruit exudate DOI: 10.1016/j.foodhyd.2016.05.038
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