What absorbs at 1651 cm⁻¹ in an FTIR spectrum?

Quick answer

A band near 1651 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 1651 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

• Amide confidence 1.0

  “In general, chitosan powder show characteristic peaks at 3425.58 cm-1 cm-1 cm-1 (-OH and -NH stretching), 2877.79 (-CH stretching), 1651.07 (amide I/C=O stretching), 2 cm-1 cm-1 cm-1 1597.06 (amide II/-NH bending), 1072.42 (C-O-C stretching”

  Cahyono 等 - 2018 - Characteristics of eugenol loaded chitosan-tripoly DOI: 10.1088/1757-899X/349/1/012010
• Water (H2O) confidence 1.0

  “The thermal-dependent FT-IR spectra for raffinose cmK1 8C, was reduced gradually with temperature up to 50 decreased significantly above pentahydrate revealed that the peak intensity at 1651 cmK1 cmK1 8C 8C, 8C, was shifted to 1639 from 48”

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

  “Table 2: Experimental results of the absorbance measurements of the 1651 and 1593 cm-1 peaks after normalization to the cellulose peak.”

  FTIR as an easy and fast analytical approach to follow up microbial growth during fungal pretreatment of poplar wood with Phanerochaete chrysosporium DOI: 10.1016/J.MIMET.2018.01.004
• Amide confidence 1.0

  “pling (amide I and amide II bands at 1651 and 1590 [17] J.E.Guillet,PureAppl.Chem.63(7)(1991)917.”

  Ferreira 等 - 2013 - Surface modification of poly(dimethylsiloxane) by DOI: 10.1016/j.surfcoat.2013.02.035
• Sulfonyl confidence 1.0

  “Meanwhile, the presence of the peak at the 1651 cm-1, were assigned asymmetric O=S=O sulfonic group in SPEEK was confirmed by the sharp peak at 1024.13 cm-1, which were stretching vibration.”

  Hasbullah 等 - 2016 - Synthesis and Characterizations of Electrospun Sul DOI: 10.1063/1.4948867
• Acetate confidence 1.0

  “NH3+ 1520 1520 1520 1520 [54] Overall, it can be observed that the response of the AS peak height 1644 1629 C--O Amide (stretch) 1651 1692 [53] 1685 1640”

  Probing the interaction of glutathione with different shape of silver-nanoparticles by optical spectroscopy DOI: 10.1016/j.mtcomm.2021.102137
• Amide confidence 1.0

  “Fourier transform-infrared spectroscopy (FTIR) analysis was used to determine the change in important, organic functional groups and highlighted cm-1 the disappearance of a peak at 1530 corresponding to the primary protein (C-N) and the app”

  Khoironi 等 - 2023 - Impact of disposable mask microplastics pollution DOI: 10.1007/s11356-023-27651-5
• Acetate confidence 1.0

  “Generally, the absorption peak at 1220 peakat1651cm-1 can be attributed to stretching of the C-O bond in phenol.”

  Enzymatic Browning in Wheat Kernels Produces Symptom of Black Point Caused by Bipolaris sorokiniana DOI: 10.3389/fmicb.2020.526266