What absorbs at 2181 cm⁻¹ in an FTIR spectrum?

Quick answer

A band near 2181 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 2181 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

• Adsorbed carbon monoxide confidence 1.0

  “cm-1-with * stretching vibrations and at 1800-1300 the bending vibrations), carbon dioxide (bands at 0.04 cm-1, 2359-2310 attributed to the asymmetric stretching cm-1, vibrations and at 669 associated with the degenerate 0.02 bending vibrat”

  Studies of thermal properties of di(methacryloyloxymethyl)naphthalene–divinylbenzene (DMN–DVB) copolymer and its alkyl-bonded derivatives DOI: 10.1007/s10973-019-08150-7
• Adsorbed carbon monoxide confidence 1.0

  “Also bands pointing to the for0 cm-1, mation of carbon monoxide (bands at 2181 and 2114 related to the stretching vibrations) appear.”

  Maciejewska 和 Rogulska - 2019 - Insight into functionalized DMN-co-GMA copolymers DOI: 10.1007/s10973-019-08327-0
• Adsorbed carbon monoxide confidence 1.0

  “Moreover, bands typical of carbon dioxide (at 2359-2310 attributed to asymcm-1, metric stretching vibrations and at 669 caused by degenerate bending vibrations), cm-1, carbon monoxide (at 2181 and 2114 related to stretching vibrations) and”

  Regular Polymeric Microspheres with Highly Developed Internal Structure and Remarkable Thermal Stability DOI: 10.3390/ma14092240
• Alkyl C-H confidence 1.0

  “Stretching CO 2181 and 2116 C-H cm-1 Stretching Alkyl groups 3000-2800 C-H cm-1 Carboxylic acids 3000-2800 Stretching cm-1 1780-1640”

  Pyrolysis and Volatile Evolution Behaviors of Cold-Rolling Oily Sludge DOI: 10.3390/pr10030543
• Adsorbed carbon monoxide confidence 1.0

  “cm-1, The peak at 2181 characteristic of carbon monoxide, changes its position in a fashion indicating that FTIR spectra of the sorption products in the gaseous phase the oxidation of the surfactant is a multistage process.”

  Study of effect of phosphate and uranium ions on the thermal properties of surfactant-modified natural red clay using TG–FTIR–MS techniques DOI: 10.1007/s10973-018-7616-x