What absorbs at 650 cm⁻¹ in an FTIR spectrum?
A band near 650 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 650 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 | 15 | 15 | 1.0 |
| Alkyl C-H | 7 | 7 | 1.0 |
| Silicon (Si) | 6 | 5 | 1.0 |
| Hydroxyl (O-H) | 6 | 5 | 1.0 |
| Methoxy (OCH3) | 5 | 5 | 1.0 |
| Methacrylate | 5 | 5 | 1.0 |
| Acetate | 5 | 5 | 1.0 |
| C-O single bond | 5 | 5 | 1.0 |
| Amide | 4 | 4 | 1.0 |
| Silicon-oxygen (Si-O) | 4 | 4 | 1.0 |
| Alkene (C=C) | 3 | 3 | 1.0 |
| Aromatic ring | 3 | 3 | 1.0 |
| N h | 2 | 2 | 1.0 |
| Chlorine | 2 | 1 | 1.0 |
| C c single bond | 1 | 1 | 1.0 |
| C-S single bond | 1 | 1 | 1.0 |
| Azide | 1 | 1 | 1.0 |
| Protein | 1 | 1 | 1.0 |
| Protein beta turn | 1 | 1 | 1.0 |
| Chitosan | 1 | 1 | 1.0 |
| Carbohydrate | 1 | 1 | 1.0 |
| Siloxane (Si-O-Si) | 1 | 1 | 1.0 |
| Carbonyl (C=O) | 1 | 1 | 1.0 |
| Acetyl | 1 | 1 | 1.0 |
| Hydrogen bond | 1 | 1 | 1.0 |
| Phosphorus | 1 | 1 | 1.0 |
| Phosphate (PO4) | 1 | 1 | 1.0 |
| Lipid | 1 | 1 | 1.0 |
| Phospholipid | 1 | 1 | 1.0 |
| Carboxyl (COOH) | 1 | 1 | 0.9 |
| Silicon silicon | 1 | 0 | 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 |
|---|---|---|---|
| PVA | 650, 1660, 2930 | Alkyl C-H, Hydroxyl (O-H) | 1 |
| CuO | 650, 453, 1000 | Metal oxygen, 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 650 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
-
Alkyl C-H confidence 1.0
“stretching), while the peaks within the range of 1000The pre-processing also significantly showed that the cm-1 could be seen through the absorption of alkenes 650 quantified FTIR spectra were distributed normally with functional group C-H”
Aziz 等 - 2020 - FTIR and HPLC-Based Metabolomics of Yacon Leaves E DOI: 10.22146/ijc.43453 -
Phospholipid confidence 1.0
“LLM confirmed rule peak-group candidate”
Depciuch 和 Parlinska-Wojtan - 2018 - Comparing dried and liquid blood serum samples of DOI: 10.1016/j.jpba.2017.11.074 -
Metal oxygen confidence 1.0
“the metal-oxygen peak at 650 The catalytic effect of 16.”
Gandhi 等 - 2010 - Ultrasound assisted one pot synthesis of nano-size DOI: 10.1007/s10853-009-4158-4 -
Metal oxygen confidence 1.0
“Noh, "Frequency shifts of the internal phonon modes in La Ca MnO ," ), characteristics of Mn-O-Mn phonon stretching and 650 cm”
Gorman 等 - 2007 - Study of magneto-transmission spectra of La0.7Pb0. DOI: 10.1109/TMAG.2007.892181 -
Phosphate (PO4) confidence 1.0
“Water-amide on the phosphate index (WAMPI) was calculated by dividing the intensity of the band at 1,630-1,650cm-1 by the intensity of the phosphate band at 604-605 cm-1 (Snoeck and Pellegrini, 2015;”
Li 等 - 2023 - Fourier-transformed infrared spectroscopy study of DOI: 10.3389/feart.2022.1008139 -
Metal oxygen confidence 1.0
“that copper oxide nanoparticles formed were cm-1 The prominent peak was observed to be 650 spherical in shape and were 10-30nm in size, while FTIR analysis of copper oxide nanoparticles.”
Majeed 等 - 2022 - Green Approach for the Synthesis of Copper Oxide N DOI: 10.22207/JPAM.16.1.74 -
Hydroxyl (O-H) confidence 1.0
“The sharp peaks at cm-1 649.6 to the bending mode of O-H.”
Mu 等 - 2009 - Synthesis, characterization and photoluminescence DOI: 10.1088/0957-4484/20/34/345602 -
Alkyl C-H confidence 1.0
“The CCC assigned to C-H in-plane bending and the bands are identified at in-planebendingvibrationsareobservedat650,620and540cm-1 Fig.9.”
Vibrational spectroscopy [FTIR and FTRaman] investigation, computed vibrational frequency analysis and IR intensity and Raman activity peak resemblance analysis on 4-chloro 2-methylaniline using HF and DFT [LSDA, B3LYP and B3PW91] calculations DOI: 10.1016/j.saa.2010.12.068
Have a spectrum with this band?
Upload your FTIR spectrum and get a full interpretation report — peak assignments with literature citations, library matches, and a literature-backed analysis — in seconds.