What absorbs at 685 cm⁻¹ in an FTIR spectrum?
A band near 685 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 685 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 |
|---|---|---|---|
| Silicon-oxygen (Si-O) | 4 | 4 | 1.0 |
| Metal oxygen | 3 | 3 | 1.0 |
| Phosphate (PO4) | 2 | 2 | 1.0 |
| Alkyl C-H | 1 | 1 | 1.0 |
| Hydroxyl (O-H) | 1 | 1 | 1.0 |
| Ring structure | 1 | 1 | 1.0 |
| Aromatic ring | 1 | 1 | 1.0 |
| Borate | 1 | 1 | 1.0 |
| Carbonyl (C=O) | 1 | 1 | 1.0 |
| Urea | 1 | 1 | 1.0 |
| Phosphorus | 1 | 1 | 1.0 |
Ranking reflects accumulated literature evidence, not a single authoritative rule. Always confirm against your sample context.
Possible materials
| Material | Supporting peaks | Overlapping groups | Cited sources |
|---|---|---|---|
| silver nanoparticles | 685, 1636, 1631 | 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 685 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
-
confidence 1.0
“Similarly by GGA cm(cid:2)1 (i) The frequencies due to Si-O in-plane bending vibrations, method, the peak position is 685.2 in [P] and shifts to”
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 -
confidence 1.0
“[3] made a MgO 10.46% 15.88% comparison between the FTIR spectra of Mg-rich and Mg-poor K2O 0.71% 0.25% 3585cm-1 palygorskites, and concluded that the vibration was Na2O 0.02% 0.01% 685cm-1 Fe)(cid:2)OH Mg)(cid:2)OH, P2O5 0.04% 0.34% caused”
Cai 等 - 2007 - A Fourier transform infrared spectroscopic study o DOI: 10.1016/j.saa.2006.02.053 -
confidence 1.0
“(2006), similar bands observed at 685 and are attributed to the Fe-O”
do Nascimento-Dias 等 - 2020 - Detection of organic or inorganic material in Mart DOI: 10.1017/S147355042000021X -
Phosphate (PO4) confidence 1.0
“The two peaks at 611 and 685 cm-1 cm-1 692 caused by P-O bending in phosphate.”
Ashemary 等 - 2023 - Preparation and characterization of mesoporous Lan DOI: 10.15251/DJNB.2023.182.681 -
Urea confidence 1.0
“The wavenumber around 1,673-1,685 cm-1 corresponded to free hydrogen-bonded urea carbonyl.”
Che 等 - 2019 - Investigations on Aging Behavior and Mechanism of DOI: 10.3390/ma12213636 -
Phosphate (PO4) confidence 1.0
“The spectral bands associated with polyphosphates, namely P=O stretching (1165 cm-1) and P-O-P stretching (685 cm-1), were weak and Int.”
Assessment of Biotechnologically Important Filamentous Fungal Biomass by Fourier Transform Raman Spectroscopy DOI: 10.3390/ijms -
Borate confidence 1.0
“(cm-1) Frequencies Assignment Reference ~685 B-O-B bending vibration [29]”
Facile Synthesis of Calcium Borate Nanoparticles and the Annealing Effect on Their Structure and Size DOI: 10.3390/ijms131114434 -
Aromatic ring confidence 1.0
“The peak at cm-1 685 may be assigned due to the stretching vibration of aromatic ring that might be attached to the free O-H Gracilaria group.”
Kumar 等 - 2013 - Seaweed-mediated biosynthesis of silver nanopartic DOI: 10.1007/s13204-012-0151-3
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