What absorbs at 721 cm⁻¹ in an FTIR spectrum?
A band near 721 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 721 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 |
|---|---|---|---|
| Alkyl C-H | 15 | 15 | 1.0 |
| Methacrylate | 6 | 6 | 1.0 |
| Acetate | 6 | 6 | 1.0 |
| Carboxyl (COOH) | 5 | 5 | 1.0 |
| Methoxy (OCH3) | 4 | 4 | 1.0 |
| C-O single bond | 4 | 4 | 1.0 |
| Alkene (C=C) | 3 | 3 | 1.0 |
| Ketone | 2 | 2 | 1.0 |
| Ester | 2 | 2 | 1.0 |
| Carbonyl (C=O) | 2 | 2 | 1.0 |
| Amide | 2 | 2 | 1.0 |
| C c single bond | 2 | 2 | 1.0 |
| N h | 2 | 2 | 1.0 |
| Aromatic ring | 1 | 1 | 1.0 |
| Hydroxyl (O-H) | 1 | 1 | 1.0 |
| Phosphate (PO4) | 1 | 1 | 1.0 |
| Fatty acid | 1 | 1 | 1.0 |
| Carbonate | 1 | 1 | 1.0 |
| Silicon (Si) | 1 | 1 | 1.0 |
| Secondary amine | 1 | 1 | 1.0 |
| Nucleic acid | 1 | 1 | 0.8 |
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 |
|---|---|---|---|
| Polyester | 721, 1700, 1250 | Alkyl C-H, Methacrylate, Acetate | 1 |
| calcite | 721, 2850, 2864 | Methacrylate, Acetate, Alkyl C-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 721 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
-
N h confidence 1.0
“Other less intense bands are those located 721cm-1 The effects of the number of accumulated scans and that at 1359, 1240, 984, 839 and due to NH in-plane of the nominal resolution employed for FT-Raman data bending, CH bending, cyclopropyl”
Armenta 等 - 2004 - Determination of cyromazine in pesticide commercia DOI: 10.1016/j.aca.2004.02.063 -
Acetate confidence 1.0
“cm-1 Olefinic carbons -HC=CH-cis 721 (m) 721 (m) 719 (m) C-O-C stretching in esters”
Barnaby 等 - 2018 - Free Radical Scavenging Capacity, Carotenoid Conte DOI: 10.1155/2018/1762342 -
Silicon (Si) confidence 1.0
“381.74 Si-Ob-Si bending vibration 720.8 128.70 725.9 110.96 766.2”
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 -
C c single bond confidence 1.0
“cm(cid:0) cm(cid:0) cm(cid:0) 1, 1 1 and sharp absorption peak at 1743 the absorption peak at 1154 and the absorption peak at 721 were assigned to C-- C-O O stretching vibration peak, stretching vibration peak and the deformation vibration”
Microencapsulation of ginger-based essential oil (Zingiber cassumunar roxb) with chitosan and oil palm trunk waste fiber prepared by spray-drying method DOI: 10.1016/j.csite.2020.100606 -
Carbonate confidence 1.0
“The identities of the carboncm-1 rarely feature carbonate in its pure form, developing a reliates were verified through analysis of the 877 ± 6 and cm-1 able technique for carbonate quantification in matrices will 721 ± 9 carbonate bands in”
Carbonate mineral identification and quantification in sediment matrices using diffuse reflectance infrared Fourier transform spectroscopy DOI: 10.1007/s10311-020-01027-4 -
Alkyl C-H confidence 1.0
“1 (a), the FTIR spectrum showed that no absorption peaks in the same range with sericin wave number, cm-1, but instead show typical peaks of the polyester molecule, such as the CH strain at 2964.59 CH bending at cm-1 cm-1, 721.38 and the CC”
Surface modification of polyester fiber with sericin for cold reactive dyeing DOI: 10.1063/1.5082406 -
Carboxyl (COOH) confidence 1.0
“The peak at 721.10 cm-1 can be attributed to the in-plane bending vibration of -COO-.”
Effects and Mechanism of Fe3+ on Flotation Separation of Feldspar and Epidote with Sodium Oleate at Natural pH DOI: 10.3390/separations9050110 -
Carboxyl (COOH) confidence 1.0
“The FTIR spectra of UiO-66-10BA and UiO-66-60BA samples synthesized with the addition of benzoic acid contained peaks associated with benzoates: 1590, 1174, 1072, 934 and 721 cm-1 [30,47-49] (see also Supplementary Materials, Figure S4).”
Butova 等 - 2023 - In Situ FTIR Spectroscopy for Scanning Accessible DOI: 10.3390/nano13101675
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