What absorbs at 1731 cm⁻¹ in an FTIR spectrum?

Quick answer

A band near 1731 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 1731 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

• Carbonyl (C=O) confidence 1.0

  “Initiator Dicumyl Peroxide which are characterized by the appearance of wave 100 phr (33 g) cyclic natural rubber (CNR) is cm-1 number absorption peaks in the area of 1731 weighed, then CNR is slowly inserted into the internal (absorption o”

  Aritonang 等 - 2019 - Grafting of Oleic Acid On Cyclic Natural Rubber Re DOI: 10.13005/ojc/350119
• confidence 1.0

  “cm-1 FTIR spectroscopy confirms that a band at 1731 represents α-amino characteristic of acid hydrochlorides.”

  Crystal Growth, Spectral, Optical, Laser damage, Photoconductivity and Dielectric Properties of Semiorganic L-cystine hydrochloride Single Crystal DOI: 10.1016/j.saa.2015.06.113
• Acetate confidence 1.0

  “cm-1 1721cm-1 cm-1 shoulder to the main C=O peaks, 1731 in 1 and in 2, at ~1790 cm-1 (1) and at ~1784 (2), also detectable in Nujol, is probably due to a ''free" cm-1”

  2,2′,2″-Thiotris(acetic acid) betaine, S(CH2COOH)2(CH2COO), and 2,2′,2″-selenotris(acetic acid) betaine, Se(CH2COOH)2(CH2COO) DOI: 10.1016/j.molstruc.2016.12.067
• Acetate confidence 1.0

  “addition, the the imide group C=O peaks at 1731 and 1787 cm-' became very broad (see Table I IR for the band assignment).”

  Ha 等 - 2001 - Spectroscopic stuides on the liquid crystal alignm DOI: 10.1080/10587250108028259
• confidence 1.0

  “Since this peak was not overlapped 1731C intensity at due to the cocrystal formation of TP-CA was with the known TP and CA, and the peak temperature was 1C.”

  Solid-state thermal behavior and stability studies of theophylline–citric acid cocrystals prepared by neat cogrinding or thermal treatment DOI: 10.1016/j.jssc.2012.04.010
• Carbonyl (C=O) confidence 1.0

  “LLM confirmed rule peak-group candidate”

  Kapon 等 - 2017 - Vibrational Strong Light-Matter Coupling Using a W DOI: 10.1021/acs.jpcc.7b06999
• Acetate confidence 1.0

  “Fourier transform infrared spectroscopy (FTIR) analysis of the polymer cm-1, Co-CultureofHalotolerantBacteria produced by the consortium showed a significant peak at 1731 representing the C=O group.”

  Co-Culture of Halotolerant Bacteria to Produce Poly(3- hydroxybutyrate-co-3-hydroxyvalerate) Using Sewage Wastewater Substrate DOI: 10.3390/polym14224963
• Carbonyl (C=O) confidence 1.0

  “Soluplus1was cm(cid:1)1, to dispersion with 1:1 (w/w) weight ratios of ester carbonyl stretching of shifted to 1731 cm(cid:1)1attributed However, as a slight excess of amorphous INDO formed (109.3%) was shown in Fig.”

  Thermoanalytical and Fourier transform infrared spectral curve-fitting techniques used to investigate the amorphous indomethacin formation and its physical stability in Indomethacin-Soluplus® solid dispersions DOI: 10.1016/j.ijpharm.2015.10.042