What absorbs at 1645 cm⁻¹ in an FTIR spectrum?

Quick answer

A band near 1645 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 1645 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

  “The peaks OH-stretching contribution in the region between ~3700 and ~3000 The cm-1 cm-1 at ~2979 cm-1 and ~2938 cm-1 are assigned to C-H stretching, while the peak centered at peaks at ~2979 and ~2938 are assigned to C-H stretching, while”

  Hydrating Capabilities of the Biopolymers Produced by the Marine Thermophilic Bacillus horneckiae SBP3 as Evaluated by ATR-FTIR Spectroscopy DOI: 10.3390/ma15175988
• Amide confidence 1.0

  “LLM confirmed rule peak-group candidate”

  A Preliminary Study of FTIR Spectroscopy as a Potential Non-Invasive Screening Tool for Pediatric Precursor B Lymphoblastic Leukemia DOI: 10.3390/molecules
• Amide confidence 1.0

  “LLM confirmed rule peak-group candidate”

  Glycation in human fingernail clippings using ATR-FTIR spectrometry, a new marker for the diagnosis and monitoring of diabetes mellitus DOI: 10.1016/j.clinbiochem.2016.09.001
• Carbohydrate confidence 1.0

  “Spectral data treatment included normalization to amide I (about 1645 cm-1), multiple baseline correction and assignment of glucose-specific peaks in the 1000-1160 cm-1 region.”

  Eikje - 2011 - In vivo interstitial glucose characterization and DOI: 10.1117/12.874735
• Amide confidence 1.0

  “1) The relative intensity and relative area under peak parameters were reveal a superior absorption of plasma membrane proteins from madetermined by dividing the band area and band intensity of Amide I 1645cm-1 lignant tissues over those fr”

  Plasma membrane proteins_ A new probe for the characterization of breast cancer DOI: 10.1016/j.lfs.2019.116777
• Water (H2O) confidence 1.0

  “The peak at 1645 is usually assigned to the bound water present in”

  Gani 等 - 2017 - Physico-chemical, structural, pasting and thermal DOI: 10.1016/j.ijbiomac.2016.10.100
• Acetate confidence 1.0

  “( C-H) δ δ ( 1457 1375 In the amber samples a wide absorption shoulder in the ( C-O) 887 2) δ 2852 CH ( δ cm-1, region between 1255 and 1156 related to C-O stretch1645”

  Differentiation between copal and amber by their structure and thermal behaviour DOI: 10.1007/s10973-023-12333-8
• Carbonyl (C=O) confidence 1.0

  “The band at 1645 may indicate -C=O carbonyl groups and -C=Ccm-1 stretching [27].”

  Hieu 等 - 2022 - Microorganism-mediated green synthesis of silver n DOI: 10.15251/DJNB.2022.171.359