What absorbs at 1688 cm⁻¹ in an FTIR spectrum?
A band near 1688 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 1688 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) | 8 | 8 | 1.0 |
| Amide | 4 | 4 | 1.0 |
| Protein beta turn | 3 | 3 | 1.0 |
| Hydroxyl (O-H) | 3 | 2 | 1.0 |
| Methacrylate | 2 | 2 | 1.0 |
| Ketone | 2 | 2 | 1.0 |
| Ester | 2 | 2 | 1.0 |
| Carboxyl (COOH) | 2 | 2 | 1.0 |
| Acetate | 2 | 2 | 1.0 |
| Alkyl C-H | 2 | 2 | 1.0 |
| Aromatic ring | 1 | 1 | 1.0 |
| Protein alpha helix | 1 | 1 | 1.0 |
| N h | 1 | 1 | 1.0 |
| C=n | 1 | 0 | 1.0 |
| Thiol | 1 | 0 | 1.0 |
| Carbonate | 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 1688 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
-
Amide confidence 1.0
“15 cm-1 V ) of the 1 states of the two acetyl end peaks at 1664 and The peak at 1634 originates from the amide-I stretching carbonyl,13Cd cm-1 1688 mode of the N-methylamino (isotopically labeled are 800 fs and 1 ps, respectively.”
Bagchi 等 - 2007 - Two-dimensional infrared investigation of N-acetyl DOI: 10.1021/jp067348m -
Alkyl C-H confidence 1.0
“While increasing the 47 48 CH 3COOK salt in the polymer salt complex the intensity of Raman modes get widen at 1054.3 49 cm-1 and 1688.3 because of C-H bending vibrations.”
Kumar 和 Rao - 2018 - Structural and electrochemical properties of PVP-C DOI: 10.1088/2053-1591/aace40 -
Amide confidence 1.0
“(cm-1) Wavenumber Band assignment β - 1688 (shoulder) Amide I sheets protein A α - A27 1654 (shoulder) Amide I helix protein β - IgG425,37 1634 (main peak) Amide I sheets - IgG427 1545 (main peak) Amide II unordered”
Beattie 等 - 2021 - Insight into purification of monoclonal antibodies DOI: 10.1039/d1an00985k -
confidence 1.0
“Peak cm-1 intensities at 3461 and 3361 (-N-H) were the strongest for samples close to the source cm-1 of dimer 2 (0 cm), while the peaks at 1688 (C=O) were strongest for samples close to N the entry port of dimer 1 (12.7 cm) (Fig.”
Elkasabi 等 - 2011 - Cellular transduction gradients via vapor-deposite DOI: 10.1016/j.biomaterials.2010.10.046. -
Carbonyl (C=O) confidence 1.0
“LLM confirmed rule peak-group candidate”
Fabry 等 - 2019 - Adaptive and automated system-optimization for het DOI: 10.1039/c9re00032a -
Carbonyl (C=O) confidence 1.0
“(C=O) The amide I band originates from symmetric stretching vibrations of the carbonyl functional cm-1), β-turn α-helical group and indicates changes in anti-parallel sheets and (1688, 1681, 1669 cm-1), cm-1),”
Grunert 等 - 2020 - Vibrational Spectroscopy of Peritoneal Dialysis Ef DOI: 10.3390/biom10060965 -
Protein beta turn confidence 1.0
“The amide I spectral region is commonly used for the analysis of secondary protein cm-1 β-sheet structures [41-43], wherein the amide I bands at 1624 and 1620 usually correspond to cm-1 β-turn structures [26,35,44,45], the bands between 168”
Ultrafine and High-Strength Silk Fibers Secreted by Bimolter Silkworms DOI: 10.3390/polym12112537 -
Protein beta turn confidence 1.0
“gel ranging from 4000 to 400 The distinctive central cm-1 band for CWS at 1665.34 determined for β-turn structures, which indicating a more packed structure and cm-1 central bands for UM at 1688.07 indicates more β-turn”
Janarthanan 等 - 2023 - Exploration and Evaluation of Pony Fish (Leiognath DOI: 10.18805/IJAR.B-4999.
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