What absorbs at 1691 cm⁻¹ in an FTIR spectrum?
A band near 1691 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 1691 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 |
|---|---|---|---|
| Amide | 10 | 8 | 1.0 |
| Carbonyl (C=O) | 7 | 6 | 1.0 |
| Methacrylate | 6 | 6 | 1.0 |
| Acetate | 6 | 6 | 1.0 |
| Ketone | 5 | 5 | 1.0 |
| Ester | 5 | 5 | 1.0 |
| Carboxyl (COOH) | 5 | 5 | 1.0 |
| Protein beta sheet | 2 | 2 | 1.0 |
| Protein | 2 | 2 | 1.0 |
| Alkene (C=C) | 1 | 1 | 1.0 |
| Methoxy (OCH3) | 1 | 1 | 1.0 |
| C-O single bond | 1 | 1 | 1.0 |
| Ring structure | 1 | 1 | 1.0 |
| C c single bond | 1 | 1 | 1.0 |
| C=n | 1 | 1 | 1.0 |
| Water (H2O) | 1 | 1 | 1.0 |
Ranking reflects accumulated literature evidence, not a single authoritative 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 1691 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
-
Water (H2O) confidence 1.0
“This side chain is increased, allowing water molecules to fully solvate the D-ringcarbonylandshiftingthevibrationalfrequencyto1691cm-1.”
Chenchiliyan 等 - 2023 - Ground-state heterogeneity and vibrational energy DOI: 10.1063/5.0135268 -
Protein confidence 1.0
“(1656cm(cid:2)1), a-helix b-sheet and Krimm [20] observed that The thyroid spectra, in addition to evident strong protein peaks 1691cm(cid:2)1) (1647cm(cid:2)1) (1626, 1679, and unordered secondary cm(cid:2)1 1659cm(cid:2)1, at 1547 and als”
Das 等 - 2008 - FTIR of touch imprint cytology A novel tissue dia DOI: 10.1016/j.jphotobiol.2008.05.012 -
Protein confidence 1.0
“Another peak at 1691 was likely (cid:2)-sheet indicative of a structure formed by unfolding and aggreof the protein via lyophilization [44].”
Additives affecting thermal stability of salmon calcitonin in aqueous solution and structural similarity in lyophilized solid form DOI: 10.1016/j.procbio.2011.08.017 -
C=n confidence 1.0
“The C=N stretching frequency of free aminoguanidine cm-1, bicarbonate is known to occur at 1691 and a peak was observed in the complexes at cm-1, 1650 indicating that the imine nitrogen atom is involved in the complexation.”
Synthesis and Crystal Structure of [Ni(Amgu)2]X2 – Novel Solid-State Precursors for NiO Nanoparticles DOI: 10.1016/j.ica.2018.07.021 -
C c single bond confidence 1.0
“weak C-H stretching at 3.86.05 3810.57 3859.98 We further observed sharp asymmetric C = O cm-1, cm-1 carboxylate at 1691.28 C-C ring stretching at 1419.30 and C-N stretching of a tertiary amine at cm-1.”
Characterizing and demonstrating the role of Klebsiella SSN1 exopolysaccharide in osmotic stress tolerance using neutron radiography DOI: 10.1038/s41598-023-37133-w -
Acetate confidence 1.0
“cm-1 The narrow peak at 1691 fits chiefly to C=O stretching.”
Influence of gamma irradiation on rosin properties and its antimicrobial activity DOI: 10.1038/s41598-023-31372-7 -
Acetate confidence 1.0
“Based on the study of the FTIR spectrum of cm-1, the FTIR spectrum of DMPA, the C=O absorption peak of DMPA is located at 1691 which DMPA, the C=O absorption peak of DMPA is located at 1691 cm-1, which proves that the prepolymers”
Synthesis of Linseed Oil-Based Waterborne Urethane Oil Wood Coatings DOI: 10.3390/polym10111235 -
Amide confidence 1.0
“1940.31 - 1999.62,1951.55 1905.70 Allene Results concerning the banana peels derived activated carbon were stretching) similar to those reported by Zheng and Wang [34], whereas some peaks - 1690.75 - - Primary amide (C--O stretching) were i”
Preparation and characterization of activated carbon from agricultural wastes and their ability to remove chlorpyrifos from water DOI: 10.1016/j.toxrep.2023.01.011
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 confidence-rated evidence chain — in seconds.