What absorbs at 3690 cm⁻¹ in an FTIR spectrum?

Quick answer

A band near 3690 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 3690 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

• Hydroxyl (O-H) confidence 1.0

  “Band centred around 3690 assigned to O-H stretching vibration cm-1 [hydroxyl or water] present in the glass.”

  Red light Emission from Europium doped Zinc Sodium Bismuth Borate Glasses DOI: 10.1016/j.physb.2017.09.113
• Hydroxyl (O-H) confidence 1.0

  “These results are consistent with those of HREELS.23 On TMIn on the GaAs(100) surface measured by (cid:24)3690 cm-1 the HO-covered surface, the OH stretch peak was found to be removed effectively by TMIn due to facile reaction, as will be d”

  Wang 和 Lin - 2005 - Reactions of trimethylindium on TiO2 nanoparticles DOI: 10.1021/jp050892v
• Metal hydroxyl confidence 1.0

  “absorption at 830 could be due to a librational Al-OH mode m cm(cid:3)1 The absorption about 3690 in muscovite [39].”

  Phyllosilicate-content influence on the spectroscopic properties and antioxidant capacity of Iberian Cretaceous clays DOI: 10.1016/j.saa.2021.119472
• Hydroxyl (O-H) confidence 1.0

  “LLM confirmed rule peak-group candidate”

  Morphology and Selected Properties of Modified Potato Thermoplastic Starch DOI: 10.3390/polym15071762
• Hydrogen bond confidence 1.0

  “In the hydrogen-bond region band at 3690 spectra of soil fractions, a similar pattern is observed for all the (3775-3100cm-1),RSDvaluesforthebandintegralsarehigher”

  Krivoshein 等 - 2022 - FTIR Photoacoustic and ATR Spectroscopies of Soils DOI: 10.1021/acsomega.1c05702
• confidence 1.0

  “In the sample, a narrow absorption band at 3690 cm-1 was due to the stretching mode of the oxygen-hydrogen present.”

  Prewetting Induced Hydrophilicity to Augment Photocatalytic Activity of Nanocalcite @ Polyester Fabric DOI: 10.3390/polym14020295
• Hydroxyl (O-H) confidence 1.0

  “The characteristic gether with the carbonyl group at 1737 formed on sample WP1iii_2A showed the distribution of calcium cm-1 cm-1, andtheband at1032 sugbands of OH stretching at3690”

  Sotiropoulou 等 - 2018 - Advanced analytical investigation on degradation m DOI: 10.1016/j.microc.2018.03.007
• Hydroxyl (O-H) confidence 1.0

  “The stretching peak at 3690 thus corresponds to the "free" OH bond of water molecules that straddle the interface between the water and hydrophobic surface of H-DNDs.”

  Water interaction with hydrogenated and oxidized detonation nanodiamonds — Microscopic and spectroscopic analyses DOI: 10.1016/j.diamond.2014.09.013.