What absorbs at 3650 cm⁻¹ in an FTIR spectrum?
A band near 3650 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 3650 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) | 23 | 20 | 1.0 |
| Silanol (Si-OH) | 5 | 5 | 1.0 |
| Adsorbed carbon monoxide | 2 | 2 | 1.0 |
| Alkyl C-H | 2 | 2 | 1.0 |
| Hydrogen bond | 2 | 2 | 1.0 |
| Water (H2O) | 2 | 2 | 1.0 |
| Methacrylate | 1 | 1 | 1.0 |
| Ketone | 1 | 1 | 1.0 |
| Ester | 1 | 1 | 1.0 |
| Carboxyl (COOH) | 1 | 1 | 1.0 |
| Carbonyl (C=O) | 1 | 1 | 1.0 |
| Amide | 1 | 1 | 1.0 |
| Acetate | 1 | 1 | 1.0 |
| N-O bond | 1 | 1 | 1.0 |
| Silicon (Si) | 1 | 1 | 1.0 |
| Secondary amine | 1 | 1 | 1.0 |
| N h | 1 | 1 | 1.0 |
Ranking reflects accumulated literature evidence, not a single fixed rule. Always confirm against your sample context.
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 3650 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
“3658.8 3200 3450 3394.1 - 3300 Due to O-H stretching in water 3650 3643.6 3648.7 - Due to formation of Ca(OH)2 300a”
Theoretical and experimental study of structures and properties of cement paste The nanostructural aspects of C–S–H DOI: 10.1016/j.jpcs.2011.05.001 -
Hydroxyl (O-H) confidence 1.0
“namely, the small d-OH humplet 2 on water-free MgO substrates revealed that any N 2O deposited (cid:24)3700 (cid:24)3650 cm-1 cm-1.”
Kumi 等 - 2006 - Amorphous solid water films Transport and guest-h DOI: 10.1021/jp058234y -
Hydroxyl (O-H) confidence 1.0
“(cid:2) mode is sufficient to lower the symmetric stretching O-H (cid:1)the cm-1 frequency at 3650 symmetric stretching frequency cm-1.31 vapor(cid:2) of water by 60 obThis lowered trend was works.34-36 served in many previous In this work,”
Lu 等 - 2008 - Molecular events in deliquescence and efflorescenc DOI: 10.1063/1.2973623(cid:5) -
Hydroxyl (O-H) confidence 1.0
“LLM confirmed rule peak-group candidate”
Temperature dependent degradation of phenolic stabilizers and ageing behaviour of PP-R micro-specimen DOI: 10.1016/j.polymdegradstab.2023.110311 -
N h confidence 1.0
“On the other hand, PEI functionalized NDPs produced successful cm-1, loading with amoxicillin as indicated by the presence of the β-lactam peak at 1770 amide cm-1, peak at 1680 and bond between PEI NH stretching and amoxicillin -COOH group”
Rouhani 等 - 2016 - Purification and functionalization of nanodiamond DOI: 10.1016/j.msec.2016.02.075 -
Water (H2O) confidence 1.0
“Repeated measurements of FTIR spectra were performed to obtain a series of spectra of the Repeated measurements of FTIR spectra were performed to obtain a series of spectra of the dehydrated dehydrated samples, starting with the hydrated sa”
The Application of ATR-FTIR Spectroscopy and the Reversible DNA Conformation as a Sensor to Test the Effectiveness of Platinum(II) Anticancer Drugs DOI: 10.3390/s18124297 -
Silanol (Si-OH) confidence 1.0
“cm-1 at 2180 If the pressure of CO in the cell increases up to 1-5 Torr an intense band at 2158 cm-1 grows simultaneously with the band at about 3650 and the decrease of silanol band at ca cm-1.”
Belykh 等 - 2019 - Deep insight into CO adsorption on Ni-USY zeolite DOI: 10.1016/j.apcata.2019.117140 -
confidence 1.0
“As shown in the As shown in Figure 4, most (i.e., 90 %) of the IOT IOTs, the band at 3650 cm-1 corresponds to and HCA samples had particles with diameters less kaolinite (DICK et al., 2008), the band at 3150 than 36.63 µm and 25.03 µm, resp”
Bezerra 等 - 2019 - DOI: 10.1590/s1678-86212019000300327
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