What absorbs at 661 cm⁻¹ in an FTIR spectrum?

Quick answer

A band near 661 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 authoritative 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 661 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

• confidence 1.0

  “Major peaks appeared at 661 showing stretching-vibration cm-1 cm-1, cm-1 stretching), peaks corresponding to Ca-Halogen, 767 (Ca-O-bond 809 897”

  Antibacterial and Antibiofilm Activity of Ficus carica-Mediated Calcium Oxide (CaONPs) Phyto-Nanoparticles DOI: 10.3390/molecules28145553
• confidence 1.0

  “The by itself doesn't show photoluminescent owing to locally g cm-1 peak at 661 is due to the disorder effect or the forbidden d-d transition, resonant energy transfer between a-Fe presence of 2O nanoparticles.”

  Screening of In Vitro Antibacterial Property of Hematite (α-Fe2O3) Nanoparticles: A Green Approach DOI: 10.1007/s40995-020-00995-0
• confidence 1.0

  “The peaks at 246 cm-1, cm-1, cm-1 296 411 and 611 correspond to the Eg vibrational mode, and the peaks cm-1 at 661 are related to the disorder effect or the presence of Fe 2O nanocrystals.”

  Low-Temperature CO Oxidation over CuO-CeO2/Fe2O3 Catalyst: Effect of KMnO4 Modification DOI: 10.3390/met13020186
• Alkyl C-H confidence 1.0

  “The peak at 661 corresponds to C-H (alkane) and C=H bonding (alkene).”

  Jalal 等 - 2018 - Biosynthesis of Silver Nanoparticles from Orophary DOI: 10.3390/nano8080586
• confidence 1.0

  “Stretching O-C-O 661”

  Noriega 等 - 2019 - Production and Characterization of Non-Isocyanate DOI: 10.3390/polym11101596
• Sulfate (SO4) confidence 1.0

  “Finally, the bands at 661 and 618 can acidsinthezwitterionform(protonsoncarboninαpositiontobothcaralso be attributed to sulfate (Lal and Garg, 2017;”

  Comprehensive characterization of industrial wastewaters using EEM fluorescence, FT-IR and 1H NMR techniques DOI: 10.1016/j.scitotenv.2021.150417
• confidence 0.9

  “Explicit assignment in text: 'stretching vibration of VO tetrahedrons ... split into three bands at 661, 746 and ...'”

  Phuruangrat 等 - 2007 - Characterization of nano-crystalline LiNiVO4 synth DOI: 10.1016/j.matlet.2006.12.037
• confidence 0.7

  “Weak band assigned to bending and wagging modes.”

  Hyperoxaluria is related to whewellite and hypercalciuria to weddellite: What happens when crystalline conversion occurs? DOI: 10.1016/j.crci.2015.12.011