What absorbs at 3660 cm⁻¹ in an FTIR spectrum?
A band near 3660 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 3660 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) | 7 | 6 | 1.0 |
| Water (H2O) | 3 | 3 | 1.0 |
| Bronsted acid site | 1 | 1 | 1.0 |
| Sulfate (SO4) | 1 | 1 | 1.0 |
| N h | 1 | 1 | 1.0 |
| Silicon hydride | 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 |
|---|---|---|---|
| hematite | 3660, 1030, 469 | Hydroxyl (O-H), Water (H2O) | 1 |
| goethite | 3660, 1470, 696 | 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 3660 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 hydride confidence 1.0
“cm-1) cm-1) 3660 and Si-H peak (observed at 2160 417 (1997) relative to Si-O-Si peak as a function of film thickness.”
Bougharouat 等 - 2011 - Plasma polymerization of TEOS for QCM-based VOC va DOI: 10.1051/epjap/2011110144 -
Hydroxyl (O-H) confidence 1.0
“O-H) 3440-cm-1 (bonded might be responsible for the O-H) 3660-cm-1 (unbonded bands and a 1:2 and the adductofwater-etheroxygensisresponsibleforthe3500H-O-H cm-1 3660-cm-1 (symmetric stretching) and the H-O-H (asymmetric stretching) bands.”
Maeda - 2001 - IR spectroscopic study on the hydration and the ph DOI: 10.1021/la001346q -
Hydroxyl (O-H) confidence 1.0
“bands also move closer together, with the separation decreasing and two far less intense bands at 3660 and 3484cm(cid:3)1 97cm(cid:3)1 94cm(cid:3)1 from that can be attributed to the surface hydroxyl groups for well crystallized goethite to”
Spectroscopic characterization of iron ores formed in different geological environments using FTIR, XPS, Mössbauer spectroscopy and thermoanalyses DOI: 10.1016/j.saa.2014.10.090 -
confidence 1.0
“In the difference spectra of aqueous (NH 4) 2SO solutions, a peak at two shoulders N-H cm-1, cm-1 and a peak at were ascribed to stretching and bending vibrations, at and (cid:24)3660 cm-1 respectively.”
Wei 等 - 2005 - Observation of the first hydration layer of isolat DOI: 10.1021/jp0453865 -
Water (H2O) confidence 1.0
“25 26 cm-1 Since the increase of the band at 3660 cannot be attributed to water itself or to the increase of 27 28”
Acidic Properties of Alkaline-Earth Phosphates Determined by an Experimental-Theoretical Approach DOI: 10.1021/acs.jpcc.9b10472 -
Hydroxyl (O-H) confidence 1.0
“cm-1 In Figure 5, the infrared absorption at 3700 is associated with the inner-surface cm-1 O-H stretching, and that at position 3660 is attributed to the inner-cage O-H cm-1 stretching of kaolinite [33].”
Kinetics of Dehydroxylation and Decarburization of Coal Series Kaolinite during Calcination: A Novel Kinetic Method Based on Gaseous Products DOI: 10.3390/ma14061493 -
Hydroxyl (O-H) confidence 1.0
“The broad band between 3660 and 2990 is associated with complex vibrational stresses of free, inter-, and intramolecular bonds of cm-1 hydroxyl groups [49,50].”
Morphology and Selected Properties of Modified Potato Thermoplastic Starch DOI: 10.3390/polym15071762 -
Bronsted acid site confidence 1.0
“However, some decrease was observed for of Brønsted acid sites, agrees well with the values for the exchange degree calculated on cm-1 the band at 3660 due to aluminol groups.”
Mihaylov 等 - 2022 - Purification of Hydrogen from CO with CuZSM-5 Ads DOI: 10.3390/molecules27010096
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