What absorbs at 3645 cm⁻¹ in an FTIR spectrum?
A band near 3645 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 3645 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) | 10 | 9 | 1.0 |
| Silicon (Si) | 1 | 1 | 1.0 |
| Metal oxygen | 1 | 1 | 1.0 |
| Water (H2O) | 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 |
|---|---|---|---|
| water | 3645, 1018, 2945 | Hydroxyl (O-H), Water (H2O) | 1 |
| montmorillonite | 3645, 1637, 523 | Hydroxyl (O-H), Silicon (Si), Water (H2O) | 1 |
| ionic liquids | 3645, 938, 1040 | Hydroxyl (O-H), Metal oxygen, Silicon (Si) | 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 3645 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
-
Hydroxyl (O-H) confidence 1.0
“It is known from the literature that 356 aluminium in highly distorted coordination might become NMR silent and the observed cm-1 357 band at 3645 can be associated to OH groups connected to the so called 'invisible' 358 aluminium species [”
New synthesis routes and catalytic applications of ferrierite crystals. Part 1: 1,8-Diaminooctane as a new OSDA DOI: 10.1016/j.micromeso.2019.109987 -
Hydroxyl (O-H) confidence 1.0
“The band at 3645cm intime of 3h gave the lowest RPO conversion of 47% because dicated OH stretching vibration of the absorbed water”
Chokpanyarat 等 - 2018 - Synthesis of Ca-Doped Three-Dimensionally Ordered DOI: 10.1155/2018/8056701 -
Hydroxyl (O-H) confidence 1.0
“t h o cm-1 bacteriorhodopsin78) The apparent depletion band at ~3645 (first observed in was at r M Tyr-5778”
Dioumaev 等 - 2010 - Low-Temperature FTIR Study of Multiple K Intermedi DOI: 10.1021/jp908698f. -
Hydroxyl (O-H) confidence 1.0
“cm-1 In all the paste samples, the peak at 3645 indicates the presence of free OH In all the paste samples, the peak at 3645 cm-1 indicates the presence of free OH groups, which suggests the presence of the portlandite phase.”
Evaluation of Mechanical and Microstructural Properties and Global Warming Potential of Green Concrete with Wheat Straw Ash and Silica Fume DOI: 10.3390/ma15093177 -
Hydroxyl (O-H) confidence 1.0
“This resulted in the loss of all B-type carbonate h o cm-1 cm-1 and interestingly, a new hydroxyl peak at 3645 (FTIR spectra) and at 3615 r M”
Carbonate substitution in the mineral component of bone: Discriminating the structural changes, simultaneously imposed by carbonate in A and B sites of apatite DOI: 10.1016/j.jssc.2017.07.025. -
Hydroxyl (O-H) confidence 1.0
“LLM confirmed rule peak-group candidate”
Morgan 等 - 2007 - Water structural changes in the L and M photocycle DOI: 10.1021/bi0616596. -
Metal oxygen confidence 1.0
“Further, clay had cm-1 β-CD O-H FT-IR spectra related to (a), (b), PVP (c) Cloisite IR peak at 3645 corresponding to the free cm-1 15A and (d) CCP are shown in Fig.”
Fixed bed adsorption column studies and models for removal of ibuprofen from aqueous solution by strong adsorbent Nano-clay composite DOI: 10.1007/s40201-019-00392-9 -
Hydroxyl (O-H) confidence 1.0
“The peak at 3645 was attributed to the hydroxyl groups of the montmorillonite layer [36].”
Adsorption of imidazolium and pyridinium ionic liquids onto montmorillonite: characterization and thermodynamic calculations DOI: 10.1016/j.cej.2012.07.128
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