What absorbs at 1721 cm⁻¹ in an FTIR spectrum?

Quick answer

A band near 1721 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 1721 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

• 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

  “cm-1 observed at 1721 is assigned to C O stretching vibration.”

  Laccase-assisted grafting of poly(3-hydroxybutyrate) onto the bacterial cellulose as backbone polymer: Development and characterisation DOI: 10.1016/j.carbpol.2014.07.003
• Acetate confidence 1.0

  “cm-1 vibrational band at 2943 refers to the C-H stretching of alkyl groups, including 178 Accepted cm-1, the C=O stretching band appearing at 1721 the bending band of CH at 1448 179 2”

  Synthesis of lanthanum tungstate interconnecting nanoparticles by high voltage electrospinning DOI: 10.1016/j.apsusc.2015.05.194
• Acetate confidence 1.0

  “Sharp and strong peak at 1721 corresponds -O-C=O stretching vibration and broad and strong peak between approximately cm-1 represents Si-O-Si stretching vibration.”

  Kim 和 Kang - 2012 - Fabrication of large area photonic crystal with su DOI: 10.1117/12.930432
• Carbonyl (C=O) confidence 1.0

  “Also, we observe the characteristic bands corresponding to the stretching modes of the ethylene glycol and ester groups at ~1126 and ~1256 cm-1 the aromatic and ethylene glycol CH2 stretching mode (1658 cm-1) and the carbonyl (C=O) stretchi”

  Laskarakis 等 - 2004 - A Spectroscopic Ellipsometry study of PET membrane DOI: 10.1002/masy.200450109
• Carboxyl (COOH) confidence 1.0

  “LLM confirmed rule peak-group candidate”

  Makhliyo 等 - 2023 - Preparation of oxidized nanocellulose by using pot DOI: 10.21203/rs.3.rs-2033622/v1
• Carbonyl (C=O) confidence 1.0

  “LLM confirmed rule peak-group candidate”

  Noh 等 - 2005 - Keto defect sites in fluorene-based organic field- DOI: 10.1063/1.1897068
• Alkyl C-H confidence 1.0

  “ging mode at The peaks at 2951, 1721, 1449 and 1159cm-1 are assigned to CH stretching, C O stretching, CH 3 stretching and O CH stretching vibrations, respectively, in 3 Fig.5.”

  Ramesh 等 - 2007 - FTIR studies of PVCPMMA blend based polymer elect DOI: 10.1016/j.saa.2006.06.012