What absorbs at 1687 cm⁻¹ in an FTIR spectrum?
A band near 1687 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 1687 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) | 12 | 11 | 1.0 |
| Carboxyl (COOH) | 9 | 8 | 1.0 |
| Amide | 8 | 6 | 1.0 |
| Methacrylate | 5 | 4 | 1.0 |
| Ketone | 5 | 4 | 1.0 |
| Ester | 5 | 4 | 1.0 |
| Acetate | 5 | 4 | 1.0 |
| N h | 2 | 2 | 1.0 |
| Aromatic ring | 2 | 2 | 1.0 |
| Protein beta sheet | 2 | 2 | 1.0 |
| Protein | 2 | 2 | 1.0 |
| Alkyl C-H | 2 | 2 | 1.0 |
| Hydroxyl (O-H) | 1 | 1 | 1.0 |
| Lipid | 1 | 1 | 1.0 |
| Carbohydrate | 1 | 1 | 1.0 |
| Hydrogen bond | 1 | 1 | 1.0 |
| Methoxy (OCH3) | 1 | 1 | 1.0 |
| Secondary amine | 1 | 1 | 1.0 |
| C n single bond | 1 | 1 | 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 1687 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
-
Protein beta sheet confidence 1.0
“cm21) cm21) b-sheet [ a [ Table III summarizes the differences of the auto-peak inten- (1687 helix(1650 side chain cm21) sities in the 2D correlation spectra of different elastin samples (1609 and cm21.”
Structural analysis of some soluble elastins by means of FTIR and 2D IR correlation spectroscopy DOI: 10.1002/bip.21524 -
Carbonyl (C=O) confidence 1.0
“The absorption cm-1 bands at 1687 and 1655 are attributed to the conjugated C=O band from the aromatic carboxyl groups [41,42].”
HPTLC and ATR/FTIR Characterization of Antioxidants in Different Rosemary Extracts DOI: 10.3390/molecules26196064 -
Carboxyl (COOH) confidence 1.0
“indicated that the interaction between DES and vibration of carboxylic acid of BA at BA affected the formation of internal crystal cm-1 1686.82 and shifting of two peaks at structures.”
Ainurofiq 等 - 2018 - Synthesis, characterization, and stability study o DOI: 10.1080/10837450 -
Carbonyl (C=O) confidence 1.0
“carboxylic acid salts at 1594.23 and 1552.76 In BA, the neutral carboxylic group (-COOH) a group of carboxylic acids of BA at 1686.82 cm-1 that were not found, replaced by the band cm-1 had a strong carbonyl (C=O) with stretching at a peak”
A Novel Desloratadine-Benzoic Acid Co-Amorphous Solid: Preparation, Characterization, and Stability Evaluation DOI: 10.3390/pharmaceutics10030085 -
Acetate confidence 1.0
“The band assigned 3468 and whereas OH bending of the phenol function was at 1383 The band assigned cm-1, to the stretching C=O vibration of the carboxylic group is located at 1726 cm-1, whereas the band at to the stretching C=O vibration of”
Synthesis of Bioactive Chlorogenic Acid-Silica Hybrid Materials via the Sol–Gel Route and Evaluation of Their Biocompatibility DOI: 10.3390/ma10070840 -
confidence 1.0
“The regression coefficients coefficients (Figure S3b) showed wavenumber values from the 1743 C=O lipid ester cm-1 (Figure S3b) showed wavenumber values from the 1743 cm-1 C=O lipid ester carbonyl, carbonyl, 1687, 1665, 1630 and 1555 from C=”
Detection of Human Cholangiocarcinoma Markers in Serum Using Infrared Spectroscopy DOI: 10.3390/cancers13205109 -
Methoxy (OCH3) confidence 1.0
“This result can be related to the presence of the methoxy groups in its weakening of the bands at 1688 cm-1 and 1687 cm-1, for SA and GA, respectively, of the structure.”
Vibrational Study (Raman, SERS, and IR) of Plant Gallnut Polyphenols Related to the Fabrication of Iron Gall Inks DOI: 10.3390/molecules27010279 -
Alkyl C-H confidence 1.0
“The KPI packing index is 69.1 and this is reflected The FT-IR spectrum of monomer 3c has been shown in in the multitude of weak but cumulatively important C-H cm-1 Figure 1 which showed the distinct band at 1687 … O, C-H … π and π … π stack”
Liquid phase synthesis of aromatic poly(azomethine)s, their physicochemical properties, and measurement of ex situ electrical conductivity of pelletized powdered samples DOI: 10.1080/15685551.2016.1231042
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