What absorbs at 1680 cm⁻¹ in an FTIR spectrum?
A band near 1680 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 1680 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 |
|---|---|---|---|
| Carbonyl (C=O) | 30 | 22 | 1.0 |
| Amide | 29 | 25 | 1.0 |
| Carboxyl (COOH) | 15 | 12 | 1.0 |
| Methacrylate | 14 | 10 | 1.0 |
| Acetate | 14 | 10 | 1.0 |
| Ketone | 13 | 9 | 1.0 |
| Ester | 12 | 9 | 1.0 |
| Alkyl C-H | 7 | 7 | 1.0 |
| N h | 7 | 7 | 1.0 |
| Alkene (C=C) | 5 | 5 | 1.0 |
| Protein beta sheet | 5 | 5 | 1.0 |
| Secondary amine | 5 | 5 | 1.0 |
| Hydroxyl (O-H) | 4 | 2 | 1.0 |
| Protein beta turn | 3 | 3 | 1.0 |
| C n single bond | 3 | 3 | 1.0 |
| Aromatic ring | 3 | 2 | 1.0 |
| Methoxy (OCH3) | 3 | 2 | 1.0 |
| C-O single bond | 3 | 2 | 1.0 |
| Hydrogen bond | 3 | 2 | 1.0 |
| Ammonium | 2 | 2 | 1.0 |
| Carbohydrate | 2 | 2 | 1.0 |
| N-O bond | 2 | 2 | 1.0 |
| Protein | 2 | 1 | 1.0 |
| Metal oxygen | 1 | 1 | 1.0 |
| Bromine | 1 | 1 | 1.0 |
| Water (H2O) | 1 | 1 | 1.0 |
| Protein alpha helix | 1 | 1 | 1.0 |
| Urethane | 1 | 1 | 1.0 |
| Sulfate (SO4) | 1 | 1 | 1.0 |
| Carbonate | 1 | 1 | 1.0 |
| Aldehyde (CHO) | 1 | 1 | 1.0 |
| Bronsted acid site | 1 | 1 | 1.0 |
| Methyl | 1 | 1 | 1.0 |
| C c single bond | 1 | 0 | 1.0 |
| Nitrate | 1 | 0 | 1.0 |
| Phosphate (PO4) | 1 | 0 | 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 1680 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
-
Alkene (C=C) confidence 1.0
“(cm-1) Component Spectral band Associated vibration C-C/C-O COM 780 single bond stretching C-C/C-O 1318 single bond stretching C-O 1630 double bond stretching C-O COD 1680 double bond stretching”
High precision mapping of kidney stones using mu;IR spectroscopy to determine urinary lithogenesis DOI: 10.1002/jbio.201300201 -
Amide confidence 1.0
“Finally, two major bands, the amide I and II cm-1, 1680-1500 bands observed in are discussed.”
Hirashima 等 - 2005 - ATR-FTIR spectroscopic study on hydrogen bonding o DOI: 10.1021/ma051081s -
Amide confidence 1.0
“aromatic ring and foremost absorption bands at 1692, 1620 and 1600 Both spectrum cm-1 peaks at 1680 and 1550 NNMB and NMB FAXs are related to amide I and II, respectively.”
Biochemical and Structural Characterization of Ferulated Arabinoxylans Extracted from Nixtamalized and Non-Nixtamalized Maize Bran DOI: 10.3390/foods11213374 -
Hydrogen bond confidence 1.0
“amide I band is more sensitive than amide II band concernindicatingthatthecarbonylstretchingvibrationofaminoacid ing the hydrogen bonding-related secondary structure, thus side chains of PGAIPG was exposed markedly in the pH 2.5 1680cm-1 th”
Lin 等 - 2005 - PHand thermal-dependent conformational transitio DOI: 10.1016/j.peptides.2004.11.005 -
Alkyl C-H confidence 1.0
“Drying droplets of mM of intact ketone moieties and the reduced strength of methylglyoxalsolutionsontheATRdiamondcrystalsurface bands in the region we assign to dioxolane structures CdO produced strong absorbance bands in both the (1680 ind”
Loeffler 等 - 2006 - Oligomer formation in evaporating aqueous glyoxal DOI: 10.1021/es060810wCCC -
Amide confidence 1.0
“Note that the subbands of amide I shown as shoulders in the absorption spectrum are seen in the Stark (cid:24)1630 (cid:24)1680 cm-1.”
Park 等 - 2000 - Vibrational Stark spectroscopy of NO bound to heme DOI: 10.1021/ja0014741 -
Alkene (C=C) confidence 1.0
“In contrast cm-1, to the Raman signal, the FT-IR spectrum shows the most intense vibrational mode at 1680 bond.12,13 attributed to the vibrational motion of the alkene group (C=C) and the C=O Although cm-1.”
EXPRESS: Vibrational Spectroscopic Characterization and Coherent Anti-Stokes Raman Spectroscopy (CARS) Imaging of Artepillin C DOI: 10.1177/0003702820904456 -
Bronsted acid site confidence 1.0
“Meanwhile, the peaks at 1467 4+)),20 s(NH while the two peaks at 3161 Brønsted acid sites ((cid:238) cm-1 weaken and remain until 473 K before and 1680 cm-1 and 3246 are attributed to s(NH 3) coordinated to Lewis (cid:238) cm-1 is disappear”
Pena 等 - 2004 - Identification of surface species on titania-suppo DOI: 10.1021/jp0313122
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