What absorbs at 4000 cm⁻¹ in an FTIR spectrum?
A band near 4000 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 4000 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 |
|---|---|---|---|
| Hydroxyl (O-H) | 17 | 16 | 1.0 |
| Methacrylate | 11 | 11 | 1.0 |
| Acetate | 11 | 11 | 1.0 |
| C-O single bond | 9 | 9 | 1.0 |
| Methoxy (OCH3) | 8 | 8 | 1.0 |
| Water (H2O) | 8 | 6 | 1.0 |
| Carboxyl (COOH) | 7 | 6 | 1.0 |
| Amide | 6 | 6 | 1.0 |
| Alkyl C-H | 5 | 5 | 1.0 |
| N h | 4 | 4 | 1.0 |
| Ketone | 3 | 3 | 1.0 |
| Ester | 3 | 3 | 1.0 |
| Carbonyl (C=O) | 3 | 3 | 1.0 |
| Hydrogen bond | 2 | 2 | 1.0 |
| N-O bond | 2 | 2 | 1.0 |
| Metal oxygen | 2 | 2 | 1.0 |
| Secondary amine | 1 | 1 | 1.0 |
| C n single bond | 1 | 1 | 1.0 |
| Protein | 1 | 1 | 1.0 |
| Carbohydrate | 1 | 1 | 1.0 |
| Silicon-oxygen (Si-O) | 1 | 1 | 1.0 |
| Silicon (Si) | 1 | 1 | 1.0 |
| Silanol (Si-OH) | 1 | 1 | 1.0 |
| Siloxane (Si-O-Si) | 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 |
|---|---|---|---|
| Chitosan | 4000, 1650, 1655 | Acetate, Methacrylate, Hydroxyl (O-H) | 1 |
| silica | 4000, 1100, 1080 | Hydroxyl (O-H), Methacrylate, Acetate | 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 4000 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
-
C-O single bond confidence 1.0
“Ether band 4000 3500 3000 2500 2000”
Aziz 等 - 2022 - The Study of Ion Transport Parameters in MC-Based DOI: 10.3390/membranes12020139 -
Siloxane (Si-O-Si) confidence 1.0
“The FT-IR data was revealed the existence of peaks at 4000 400 showing the siloxane & silanol groups were present, Fluorescence Spectrophotometer, SEM and EDS.”
Banoth 等 - 2022 - Sustainable Thermochemical Extraction of Amorphous DOI: 10.21203/rs.3.rs-657645/v1 -
Water (H2O) confidence 1.0
“20cm-1 , but only covering the range between 14000 and Interactive baseline rubberband correction was used 4000cm-1 in the midinfrared region, 200 to is used for to minimize the water band contribution to spectra.”
Calabro 等 - 2013 - 50 Hz Electromagnetic Field Produced Changes in FT DOI: 10.1155/2013/414393 -
Hydroxyl (O-H) confidence 1.0
“The peaks near 4000cm-1 2KBr 1651 cmK1 3000 were assigned to the OH stretching vibration of water molecules in the pentahydrate of raffinose;”
Cheng 和 Lin - 2006 - Processes of dehydration and rehydration of raffin DOI: 10.1016/j.carbpol.2005.11.024 -
Hydroxyl (O-H) confidence 1.0
“The wavenumber of O-H stretching decreases after PEG-4000 encapsulated Mg 0.5Ni 0.5Fe 2O because of PEG-4000 reducing water molecule in 4”
Deswardani 等 - 2017 - Study of Structural and Magnetic Properties of Sil DOI: 10.1088/1757-899X/202/1/012047 -
Hydroxyl (O-H) confidence 1.0
“lower than 4500 However, if the low-frequency signals Transmission infrared spectra were recorded using a Jasco IMV-4000 multiare from O-H stretch in H 2O species, their combination with mol”
Le Losq 等 - 2015 - Complex IR spectra of OHgroups in silicate glass DOI: 10.2138/am-2015-5076 -
Carboxyl (COOH) confidence 1.0
“In some reports, the band at resolution in is COO(cid:1) the transmission mode from wave numbers 400 to assigned to asymmetrical stretching of the 4000cm(cid:1)1.”
Majid 和 Arof - 2005 - Proton-conducting polymer electrolyte films based DOI: 10.1016/j.physb.2004.10.025 -
Metal oxygen confidence 1.0
“cm-1, 4000 the bands emerging in relation to stretching collision of Cd-O were observed in the M cm-1 cm-1 range 600-1000 [21].”
Silver modified cadmium oxide - A novel material for enhanced photodegradation of malachite green DOI: 10.1016/j.ijleo.2018.11.066
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