What absorbs at 1681 cm⁻¹ in an FTIR spectrum?

Quick answer

A band near 1681 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

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 1681 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

• Carboxyl (COOH) confidence 1.0

  “LLM confirmed rule peak-group candidate”

  Mahapatro 等 - 2006 - Surface modification of functional self-assembled DOI: 10.1021/la052817h
• Carbonyl (C=O) confidence 1.0

  “The peak at 1681 was correlated with the stretching vibration of carbonyl (C=O) and/or (C-N, amide 1) [59,60].”

  Synergistic Effects of n-Hexane Fraction of Parkia biglobosa (Jacq.) Bark Extract and Selected Antibiotics on Bacterial Isolates DOI: 10.3390/su9020228
• Alkyl C-H confidence 1.0

  “FT-IR 3051 (C-H, s, stretching), 1681, (C=N, m, bending), 1601, 1473 (C=C, m, bending), 1173 (C-N, m, 1H-NMR”

  A Comprehensive Approach to Derivatization: Elemental Composition, Biochemical, and In Silico Studies of Metformin Derivatives Containing Copper and Zinc Complexes DOI: 10.3390/molecules28031406
• Water (H2O) confidence 1.0

  “Bands at 1681.00 and 1618 in MRSG sample could be attributed to the presence of two types of water molecules in gypsum.”

  The Physico-Chemical and Mineralogical Characterization of Mg-Rich Synthetic Gypsum Produced in a Rare Earth Refining Plant DOI: 10.3390/su13094840
• Acetate confidence 1.0

  “The peak at 1681 due to the Molecules2019,24,4245 12of22 cm-1 = -C O mode of amide moiety of acetamide shifts to 1667 in the cocrystal demonstrating its = involvement in the hydrogen bonding in the cocrystal structure.”

  Budziak 等 - 2019 - Formation of Prenylated Chalcone Xanthohumol Cocry DOI: 10.3390/molecules24234245
• Acetate confidence 1.0

  “cm-1, t Aromatic ring C=C-C stretching vibrations located at 1594 aliphatic C-H cm-1, stretching vibrations arises at 2945 and C-H bending vibration appears at 1444 and cm-1.5,39,40 cm-1 1375 The FTIR spectra of ADP showed C=O stretching vi”

  Polymer-Assisted Aripiprazole-Adipic Acid Cocrystals Produced by Hot Melt Extrusion Techniques DOI: 10.1021/acs.cgd.0c00020.
• Acetate confidence 1.0

  “4 that the peaks at 3364.1 belonged to O-H stretching vibration absorption, the absorption cm-1 peak at 1681.4 contributed to C=O stretching vibration cm-1 of COOH, the absorption peak at 1627.7 contributed to C=N stretching vibration due t”

  Cai - 2014 - Synthesis and Structure Optimization of Schiff Bas DOI: 10.14233/ajchem.2014.17137
• Acetate confidence 1.0

  “spectrum of the isolated 11,15-dimethyl-hexadeca-8,10,14sp3 C-H stretch 2956.34, 2927.31, 2857.46 = C O stretch 1680.83 trien-7-one isomers.”

  Chong 等 - 2016 - An improved method for aldol condensation catalyse DOI: 10.1515/gps-2015-0126