What absorbs at 640 cm⁻¹ in an FTIR spectrum?
A band near 640 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 640 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 |
|---|---|---|---|
| Metal oxygen | 7 | 6 | 1.0 |
| Silicon hydride | 5 | 3 | 1.0 |
| Silicon (Si) | 4 | 4 | 1.0 |
| Alkyl C-H | 3 | 3 | 1.0 |
| N-O bond | 2 | 2 | 1.0 |
| Siloxane (Si-O-Si) | 2 | 2 | 1.0 |
| Silicon-oxygen (Si-O) | 2 | 2 | 1.0 |
| Chlorine | 2 | 2 | 1.0 |
| Phosphorus | 2 | 1 | 1.0 |
| Phosphate (PO4) | 2 | 1 | 1.0 |
| Ring structure | 2 | 1 | 1.0 |
| Nitrogen heterocycle | 1 | 1 | 1.0 |
| Amide | 1 | 1 | 1.0 |
| Protein random coil | 1 | 1 | 1.0 |
| Aromatic ring | 1 | 1 | 1.0 |
| Hydroxyl (O-H) | 1 | 1 | 1.0 |
| N h | 1 | 1 | 1.0 |
| Amino acid | 1 | 1 | 1.0 |
| Methoxy (OCH3) | 1 | 1 | 1.0 |
| Methacrylate | 1 | 1 | 1.0 |
| C-O single bond | 1 | 1 | 1.0 |
| Acetate | 1 | 1 | 1.0 |
| Silicon nitrogen | 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 |
|---|---|---|---|
| olivine | 640, 3525, 3564 | 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 640 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 (Si) confidence 1.0
“118 Materials Engineering and Mechanical Automation cm-1, C Further, H content ( ) can be calculated according to the absorption intensity at about 640 H * C R which is due to the Si-H wagging mode [9].”
Chen 等 - 2014 - Effect of Deposition Power on the Structure Proper DOI: 10.4028/www.scientific.net/AMM.442.116 -
Silicon nitrogen confidence 1.0
“cm-1 wagging peak at 640 The shift in the Si-N-Si stretching mode from 942 to 881 was”
Deposition of Silicon Nitride Films using Chemical Vapor deposition for Photovoltaic Applications DOI: 10.1016/j.rinp.2016.11.029 -
N-O bond confidence 1.0
“The strongly shocked olivine with 80Fo% A.Kereszturietal./SpectrochimicaActaPartA:MolecularandBiomolecularSpectroscopy173(2017)637-646 640 cm-1 cm-1) cm-1 shocked olivines have no peaks above 980 and below 830 shifted with values between +1”
Analyzing Raman - Infrared spectral correlation in the recently found meteorite Csátalja DOI: 10.1016/j.saa.2016.10.012 -
Alkyl C-H confidence 1.0
“2000 cm-1 The absorption peak of the 640 mode was used to determine the total content of bound hydrogen, C H.”
Study of boron doped amorphous silicon lightly hydrogenated prepared by DC magnetron sputtering for infrared detectors applications DOI: 10.1016/j.infrared.2020.103556 -
confidence 1.0
“In Fe(CO) 5, IR studies, a very intense band is found at 640cm-1, it is assigned as a degenerate Fe C O bending”
Mahmood 等 - 2006 - Comparative vapour phase FTIR spectra and vibratio DOI: 10.1016/j.saa.2005.11.024 -
confidence 1.0
“In excellent agreement with theoretically peak at 640 K corresponds to reduction of the nickel ions that Ni+(12CO)(13CO) calculated values, the complexes were found do not adsorb CO, whereas the high-temperature tail corresponds”
Penkova 等 - 2007 - Effect of the preparation method on the state of n DOI: 10.1021/jp071927p -
Silicon (Si) confidence 1.0
“cm-1 880 2000 and 2090 The bands at 640 are attributed to the wagging modes of cm-1 all Si-H vibrations, the peak at 880 is assigned to bending mode of the Si-H bonds and the”
Seba 等 - 2009 - Correlation between physicochemical and electrical DOI: 10.4028/www.scientific.net/MSF.609.129 -
confidence 1.0
“Asymmetric stretching of Si-Ois indicated by a peak at 921 [24], symmetric bending of Si-O bond in the SiO4 1 3 cm(cid:0) cm(cid:0) 1 1 units is shown by the peak at 640 [25], asymmetric bending of Si-O-Si bond is shown by the peak at 491 [”
Reddish-orange emission from sol-gel derived Sm3+-doped Sr2La8(SiO4)6O2 phosphors DOI: 10.1016/j.ijleo.2020.165935
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