What absorbs at 1238 cm⁻¹ in an FTIR spectrum?

Quick answer

A band near 1238 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 1238 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

• Protein confidence 1.0

  “The intensity of the cm-1 bands in the 1238-1244 region also decreased in the purified samples, which may be due to the elimination of a small fraction of protein.”

  Ashurov 等 - 2021 - Characterization of polysaccharides from Eremurus DOI: 10.21285/2227-2925-2021-11-2-281-289
• Alkyl C-H confidence 1.0

  “Furthermore, peaks originating from cm-1) cm-1) Si-CH 2-CH vibrations (at 1238 and Si-CH (at 1277 and 880 are much more 3 3”

  Mechanical properties, chemical analysis and evaluation of antimicrobial response of Si-DLC coatings fabricated on AISI 316 LVM substrate by a multi-target DC-RF magnetron sputtering method for potential biomedical applications DOI: 10.1016/j.apsusc.2017.03.223
• C c single bond confidence 1.0

  “The peak at 1238cm-' may -0 C -C asymmetric stretching [18].”

  Ha 等 - 1998 - Polarized Infrared Spectroscopic Studies of Polari DOI: 10.1080/10587259808025380
• Ester confidence 1.0

  “Absorption at cm-1 cm-1 1238 was due to the ester sulfate group, and absorption at 1048 was due”

  Preparation and Characterization of Super-Absorbing Gel Formulated from -Carrageenan–Potato Peel Starch Blended Polymers DOI: 10.3390/polym13244308
• confidence 1.0

  “assigned to stretching CO”

  Rukman 等 - 2019 - GO-Fe3O4 Nanocomposite from coconut shell Synthes DOI: 10.1088/1755-1315/217/1/012008
• Amide confidence 1.0

  “of amide III P=O asymmetric stretching of - - - - 1238 1235 phosphodiester in phospholipids”

  Sukprasert 等 - 2020 - Synchrotron FTIR Light Reveals Signal Changes of B DOI: 10.1155/2020/6149713
• Amide confidence 1.0

  “Notably, The bands at 1326 cm-1 and 1238 cm-1 are attributed to the C-N stretching of the aromatic cm-1 cm-1 1633 is due to the C=C stretching of the quinoid, and 1428 is the characteristic benzenoid rings.”

  Preparation of Anionic Surfactant-Based One-Dimensional Nanostructured Polyaniline Fibers for HydrogenStorage Applications DOI: 10.3390/polym15071658
• Amide confidence 1.0

  “while the ASC and PSC-amide III bands were located at wavenumbers of 1237.94 and stretching vibration [50], and amide III peak reflects intermolecular interactions in collagen, including cm-1, 1239.84 respectively.”

  Ben Slimane 和 Sadok - 2018 - Collagen from Cartilaginous Fish By-Products for a DOI: 10.3390/md16060211