What absorbs at 1642 cm⁻¹ in an FTIR spectrum?

Quick answer

A band near 1642 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 1642 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

  “~1642cm-1 in all the treated pulp samples may be attributed to release of free carbonyl groups (C=O) due to the action of enzyme on 5.4.”

  Exploring the lignolytic potential of a new laccase producing strain Kocuria sp. PBS-1 and its application in bamboo pulp bleaching DOI: 10.1016/j.ibiod.2019.104726
• Water (H2O) confidence 1.0

  “Isothermal rehydration process of preheated raffinose cmK1 8C for 81, 125 or 150 IR peak intensity at 1642 (1641) samples preheated sample was gradually increased with time, due to the 8C, absorption of water vapor from 30 70% RH condition.”

  Cheng 和 Lin - 2006 - Processes of dehydration and rehydration of raffin DOI: 10.1016/j.carbpol.2005.11.024
• Acetate confidence 1.0

  “Spectral peaks of FTIR at 1641.79 confirmed the appearance of C--O CO32(cid:0) binding, with strong stretching of in Amide I.”

  Biologically-induced synthetic manganese carbonate precipitate (BISMCP) for potential applications in heavy metal removal DOI: 10.1016/j.heliyon.2023.e15919
• Amide confidence 1.0

  “There was a very strong infrared (IR) band at cm-1 1642 generated primarily by the C = O stretching vibration from the amide I of protein in the spectrum (Tatulian, 2013), which overlapped with asymmetric stretching bands from carboxylate.”

  Changes in chemical composition and structure of root cell wall of citrus rootstock seedlings in response to boron deficiency by FTIR spectroscopy DOI: 10.1080/14620316.2017.1362327
• Acetate confidence 1.0

  “cm-1 However, peaks are seen at 3479 for SnO 2-doped GO cm-1 cm-1 1639 (C=O stretching vibration), and 1642 (OH”

  Reactive Oxygen Species (ROS) Are Generated as Organic Contaminants Are Broken Down: SnO DOI: 10.1007/s12668-023-01111-3
• Hydroxyl (O-H) confidence 1.0

  “5 of 23 https://doi.org/10.33263/BRIAC133.274 cm-1 In FTIR spectra, absorption peaks of 1.5% Tylose MH 50 at 3441 and 2835-2906 cm-1 indicated stretching vibrations of OH and symmetric and asymmetric C-H vibrations, cm-1 whereas peaks at 16”

  Mustary 等 - 2023 - Synthesis and Characterization of Biocomposite Fil DOI: 10.33263/BRIAC133.274
• Ester confidence 1.0

  “The formed band at region broad peak at 1642 cm(cid:1)1 is attributed to formation of ester groups, which of 1642 is the result of reaction between the hydroxyl group of starch and anhydride groups of coupling agent.”

  Physical and thermal mechanical properties of corn starch/LDPE composites DOI: 10.1002/app.37877
• C c single bond confidence 1.0

  “A new peak was detected at 1641.98 in the (C-C-- treated effluent indicating the presence of alkenes C symmetric (C-- stretch), ketones and aldehydes O stretch) (Fig.”

  Greenhouse gases emissions from duckweed pond system treating polyester resin wastewater containing 1,4-dioxane and heavy metals DOI: 10.1016/j.ecoenv.2020.111253