What absorbs at 2840 cm⁻¹ in an FTIR spectrum?
A band near 2840 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 2840 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 | 22 | 22 | 1.0 |
| Hydroxyl (O-H) | 3 | 3 | 1.0 |
| Methoxy (OCH3) | 2 | 2 | 1.0 |
| Hydrogen bond | 1 | 1 | 1.0 |
| Aromatic ring | 1 | 1 | 1.0 |
| C c single bond | 1 | 1 | 1.0 |
| Methacrylate | 1 | 1 | 1.0 |
| C-O single bond | 1 | 1 | 1.0 |
| Acetate | 1 | 1 | 1.0 |
| N h | 1 | 1 | 1.0 |
| Water (H2O) | 1 | 1 | 1.0 |
| Silicon-oxygen (Si-O) | 1 | 1 | 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 |
|---|---|---|---|
| silica | 2840, 1100, 1080 | Hydroxyl (O-H), Alkyl C-H | 1 |
| polypropylene | 2840, 1030, 1600 | Alkyl C-H, Methoxy (OCH3), Hydroxyl (O-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 2840 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
-
Methoxy (OCH3) confidence 1.0
“Specifically, the methoxy bands (2840 and precise calculation of the LO modes and is beyond the cm(cid:2)1) 820 scope were no longer present after 16 days (Fig.”
Structural studies of alkoxysilyl-functionalised ionic liquid and its application in a hybrid electrochromic device DOI: 10.1016/j.materresbull.2012.08.036 -
Alkyl C-H confidence 1.0
“On the shortwave shoulder of the broad OH peak, two peaks of C-H We believe that this region division better suits the 2840cm-1 stretching vibrations at 2930 (asymmetric) and studied soil samples.”
FTIR photoacoustic spectroscopy for identification and assessment of soil components_ Chernozems and their size fractions DOI: 10.1016/j.pacs.2020.100162 -
Alkyl C-H confidence 1.0
“The stretching C-H alkane group was represented by the medium peak at 2840 cm-1.”
Bio-Fabrication of ZnONPs from Alkalescent Nucleoside Antibiotic to Control Rice Blast: Impact on Pathogen (Magnaporthe grisea) and Host (Rice) DOI: 10.3390/ijms24032778 -
Alkyl C-H confidence 1.0
“The peaks observed at 2840 and 2920 are respectively related to the symmetric and antisymmetric stretching vibrational of C-H from alkyl groups.”
Awada 和 Daneault - 2015 - Chemical Modification of Poly(Vinyl Alcohol) in Wa DOI: 10.3390/app5040840 -
Alkyl C-H confidence 1.0
“LLM confirmed rule peak-group candidate”
Synthesis of New Homopiperazine-1.4-Diium Tetrachloridromercurate (II) Monohydrate (C5H14N2)[HgCl4]H2O, Crystal Structure, Hirshfeld Surface, Spectroscopy, Thermal Analysis, Antioxidant Activity, Electric and Dielectric Behavior DOI: 10.3390/cryst12040486 -
Alkyl C-H confidence 1.0
“- 626.8 626.8 2950cm(cid:1)1 C-H corresponding to stretches of tween 2840 and - 657.7 657.7”
New approaches to identification and characterization of tioconazole in raw material and in pharmaceutical dosage forms DOI: 10.1016/j.jpha.2018.11.006 -
Alkyl C-H confidence 1.0
“Results and Discussion with the spectrum of DTAT, two new peaks located at 2931 cm-1 and 2840 cm-1 (C-H of -CH2-) appeared and one peak at 848 cm-1 (C-Cl) disappeared in the PETAT spectrum, showing that a reaction had occurred between DTAT”
The Effects of a Macromolecular Charring Agent with Gas Phase and Condense Phase Synergistic Flame Retardant Capability on the Properties of PP/IFR Composites DOI: 10.3390/ma11010111 -
confidence 1.0
“cm-1 Disappearance of the band of Si-O-CH groups at 2840 were also evidenced together with the”
El Hadad 等 - 2011 - Preparation of sol-gel hybrid materials from gamma DOI: 10.1007/s00396-011-2504-y
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