What absorbs at 1720 cm⁻¹ in an FTIR spectrum?

Quick answer

A band near 1720 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.

Possible materials

Materials are shown only when the same literature pool supports this band and at least one additional characteristic peak.

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 1720 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”

  Structural modification and functional improvement of starch nanoparticles using vacuum cold plasma DOI: 10.1016/j.ijbiomac.2019.12.167
• Carboxyl (COOH) confidence 1.0

  “cm(cid:1)1 in The appearance of an absorption peak at 1720 purified milled HEC/AA samples was attributed to C¼¼O stretching vibrations of the carboxyl group in AA, which demonstrated the success of grafting.”

  Solid state mechano-chemical grafting copolymerization of hydroxyethyl cellulose with acrylic acid DOI: 10.1002/app.30098
• Carbonyl (C=O) confidence 1.0

  “[32] built a regression model to determine FFA content by identifying the ν(C=O) ester vibration and the ν(C=O) of the carboxylic acid, which corresponded to 1747 and the 1720 cm-1, respectively.”

  Monitoring Enzymatic Hydroesterification of Low-Cost Feedstocks by Fourier Transform InfraRed Spectroscopy DOI: 10.3390/catal9060535
• Acetate confidence 1.0

  “The to 1600 acyl groups, C-O (peak at 1220 and cm-1) = bacteria used LDPE as their source of carbon and made the carbonyl groups, C O (peak near 1720 was mainly necessary changes incorporating the functional groups to modify found.”

  Biodegradation of Unpretreated Low-Density Polyethylene (LDPE) by Stenotrophomonas sp. and Achromobacter sp., Isolated From Waste Dumpsite and Drilling Fluid DOI: 10.3389/fmicb.2020.603210
• Acetate confidence 1.0

  “cm-1, For E1 (Fig.3a), the intensities of the absorptions at 1514 assigned to the aromatic C=C cm-1, deformation, and near 1720 assigned to the C=O stretching band in the lignin, increase from the surface to the depth.”

  FTIR spectroscopy of woods: a new approach to study the weathering of the carving face of a sculpture DOI: 10.1016/j.saa.2014.10.011
• Acetate confidence 1.0

  “(2002) studied the heat degradation of lignin in hardwood and softwood cm-1 and obtained an increase in the peak at 1720 with increasing temperature, which they concluded to be due to the production of C=O bonds in lignin.”

  Esteves 等 - 2013 - CHEMICAL CHANGES OF HEAT TREATED PINE AND EUCALYPT DOI: 10.4067/S0718-221X2013005000020
• Carboxyl (COOH) confidence 1.0

  “This D3 band, which is mainly responsible for symmetric stretching bands occur at 1651 and 1405 in the the severe band overlap in the Raman spectra of both diesel polyitaconic carboxylate and are typical of polycarboxylates acids.12 soot an”

  Gaffney 等 - 2015 - Characterization of Fine Mode Atmospheric Aerosols DOI: 10.1021/jp510361s
• Ester confidence 1.0

  “The spectrum shows reduction in the and appearance of a new cm(cid:3)1 cm(cid:3)1 intensity of the anhydride band at 1780 and 1857 peak at 1720 due to the ester and the carboxylic acid”

  Galgali 等 - 2007 - Sugar-linked biodegradable polymers Regio-specifi DOI: 10.1016/j.carbpol.2006.06.035