Carbonyl (C=O) in FTIR: reading the band between 1820 and 1600 cm-1

Direct answer

The carbonyl stretch is one of the strongest bands in an FTIR spectrum, and it sits between 1820 and 1600 cm-1. Its position sets the candidate list: anhydrides and lactones sit highest, esters 1730-1750 cm-1, saturated ketones and aldehydes 1715-1725 cm-1, carboxylic acids 1700-1725 cm-1, conjugated carbonyls 1660-1700 cm-1, and amide I 1620-1690 cm-1. Those ranges overlap, and hydrogen bonding moves a band by tens of wavenumbers: the same resin can show a free carbonyl at 1728 cm-1 and a hydrogen-bonded one at 1704 cm-1. So the position narrows the search and a companion band makes the call: ester C-O at 1050-1300 cm-1, amide N-H at 3300 cm-1 with amide II at 1540 cm-1, the very broad acid O-H from 2500 to 3300 cm-1, aldehyde C-H near 2720 cm-1, or the anhydride pair near 1840 and 1765 cm-1.

What decides the assignment

1. Pin the position down before choosing a group

   The carbonyl window is not one band but a set of overlapping ranges, so read the measured position to the nearest few wavenumbers and start from the matching row of the table below. The published values sit close together: esters at 1735-1746 cm-1, a ketone at 1720 cm-1, an unconjugated aldehyde or ketone at 1710 cm-1, a carboxylic acid at 1710-1713 cm-1, and amide I from 1623 to 1695 cm-1 depending on the secondary structure.

2. Find the band that belongs with it

   Each carbonyl family travels with a partner band, and that partner is what turns a position into an assignment. An ester brings C-O stretching at 1050-1300 cm-1, an amide brings N-H near 3300 cm-1 and amide II near 1540 cm-1, a carboxylic acid brings a very broad O-H from 2500 to 3300 cm-1, an aldehyde brings a weak C-H band near 2720 cm-1 with its weaker partner near 2820 cm-1, and an anhydride brings two C=O bands of its own.

3. Check whether the carbonyl is one band or two

   Hydrogen bonding often splits the band into a free and a bonded component. A phenolic resin showed 1728 cm-1 for the free carbonyl and 1704 cm-1 for the hydrogen-bonded one; a polyurethane showed 1699 cm-1 for the bonded carbonyl against about 1715 cm-1 for the free one; a polyvinyl alcohol film showed 1737 cm-1 for the free carbonyl and about 1640 cm-1 for the bonded one. Two bands inside the window usually mean free plus hydrogen-bonded, or the anhydride pair, rather than two unrelated groups.

4. Rule out the neighbours before you commit

   Amide I lies inside the C=O window, so a protein-containing sample puts a carbonyl exactly where a conjugated C=O would be expected. Water bending overlaps 1630-1650 cm-1, and aromatic C=C sits near 1600 cm-1. In low-rank coal work the band near 1700 cm-1 covered carboxyl, aldehyde and ketone structures together, while the band near 1650 cm-1 was assigned to highly conjugated carbonyls; a second, independent band is what separates these cases.

Where the band falls

The ranges overlap on purpose: that is why a single position rarely settles the assignment.

Where this group sits in the reference spectra

Showing the 30 best-supported peaks of 1083 total for Carbonyl (C=O).

Possible materials

Check the bands around it

• 1050-1300 cm-1

  Ester C-O stretching. A polyester decomposition study lists the C-O bands from 1050 up to 1210 cm-1 next to the carbonyl bands, and a weathered composite shows the ester group at 1737 cm-1 with the ester C-O bands in the same fingerprint region.

• 1537-1550 cm-1

  Amide II, the N-H bend coupled with C-N stretching. It is reported at 1550-1555 cm-1 and 1539-1546 cm-1 for proteins, and at 1537-1550 cm-1 for the peptide groups of proteins, which is what tells an amide I band apart from a conjugated C=O.

• 3300-3400 cm-1

  N-H stretching. An amide brings a band here together with amide II, so a carbonyl in the 1620-1690 cm-1 range with an N-H band and an amide II band is a peptide, not a conjugated carbonyl.

• 2720 and 2820 cm-1

  Aldehyde C-H stretching. A decomposition study lists the band at 2720 cm-1 as the stretching vibration of C-H in aldehyde groups, with the weaker partner of the pair near 2820 cm-1, and a fingerprint table of a plant extract gives 2740.07 cm-1 as an aldehyde C-H band.

• 1600 cm-1

  Aromatic C=C stretching, which appears next to a conjugated carbonyl at 1600 and 1590 cm-1 in the lignin band tables and supports the assignment of a lowered carbonyl to a conjugated structure.

Published spectra

• Published spectrum

  Three absorption peaks are observed in the carbonyl regions. The 1720 cm-1 peak is assigned to the carbonyl in the ketone group. The carbonyl peak at 1735 cm-1 belongs to the ester. C=O γ-lactone stretch appears at 1769 cm-1.

  Read from the caption of the carbonyl region figure in aged cable insulation, where the same page reports that carbonyl groups increase over the range 1850-1650 cm-1. The extraction interleaved the two columns of the page and moved the unit to its own line; both are restored here.

  Influence of Oxygen Diffusion on Thermal Ageing of Cross-Linked Polyethylene Cable Insulation DOI: 10.3390/ma13092056 · p. 6
• Published spectrum

  In the carbonyl region (1750 to 1710 cm-1), a sharp peak was observed at 1713 and 1737 corresponding to the hydrogen-bonded carboxylic acid and the ester groups, respectively [46].

  One spectrum with two carbonyl assignments: the acid group sits lower than the ester group. The extraction dropped the unit after 1710 onto the next line; it is restored here.

  Ultraviolet Weathering Performance of High-Density Polyethylene/Wood-Flour Composites with a Basalt-Fiber-Included Shell DOI: 10.3390/polym10080831 · p. 8
• Published spectrum

  1710 Carbonyl (C=O) stretching in unconjugated aldehyde and ketone [34]

  One row of the band-assignment table of that paper, given here with the wavenumber and the assignment as printed. The surrounding rows place the hydroxyl band at 3359 cm-1, methylene C-H at 2918 cm-1 and methoxy C-H at 2854 cm-1.

  Physical and Chemical Properties of Acacia mangium Lignin Isolated from Pulp Mill Byproduct for Potential Application in Wood Composites DOI: 10.3390/polym14030491 · p. 8
• Published spectrum

  3 2740.07 C-H Stretching w Aldehyde; 5 1711.37 C=O Stretching s Carboxylic acid

  Two rows of the fingerprint table of that paper. The aldehyde C-H row at 2740.07 cm-1 is the companion band that a carbonyl assignment has to be checked against; the C=O row at 1711.37 cm-1 belongs to a carboxylic acid. The row between them, at 2673.24 cm-1, is an O-H band of chelated beta-diketones and is not an aldehyde band.

  FTIR fingerprint analysis of Sida cordifolia L. and Withania somnifera (L.) Dunal root used in Balarista formulation DOI: 10.54085/ap.2022.11.1.57 · p. 2
• Published spectrum

  the bands at 1700-1790 cm-1 (the stretching vibrations of carbonyl groups), the bands from 1050 up to 1210 cm-1 (the stretching vibrations of C-O), the band at 2720 cm-1 (the stretching vibrations of C-H in aldehyde groups) and the bands above 3500 cm-1 (the stretching vibrations of OH)

  One sentence that lists the carbonyl window together with the bands that confirm or contradict it. The extraction interleaved the two columns of the page; the sentence is given in reading order.

  TG/DSC/FTIR/QMS studies on the oxidative decomposition of terpene acrylate homopolymers DOI: 10.1007/s10973-016-5753-7 · p. 7
• Published spectrum

  When pristine MWNT was oxidized, an IR band at 1713 cm-1 appeared indicating the formation of carboxylic acid groups.

  The same page adds that an acid carbonyl band at 1710 cm-1 disappears when the acid is amidated and that an ester carbonyl band appeared at 1787 cm-1, which is how the group was tracked through the chemistry.

  Analytical strategies for characterizing the surface of carbon nanotubes and the immobilization of biomolecules DOI: 10.1007/s00216-009-2996-1. · p. 4
• Published spectrum

  The carbonyl stretching mode of IB showed a consistent red shift of 17 cm-1 from 1710 cm-1 to 1727 cm-1

  A measured shift of the acid carbonyl when hydrogen bonding changes, and the reason a carbonyl position on its own is not an identity. The extraction interleaved the two columns of the page.

  Mutual Effects of Hydrogen Bonding and Polymer Hydrophobicity on Ibuprofen Crystal Inhibition in Solid Dispersions with Poly(N-vinyl pyrrolidone) and Poly(2-oxazolines) DOI: 10.3390/pharmaceutics13050659 · p. 8
• Published spectrum

  hydrogen bonded carbonyl groups appear at 1699 cm-1, and the value is slightly lower than that of the free PU carbonyl groups (~1715 cm-1) [36].

  The polyurethane case: two carbonyl bands in one window, one from the free group and one from the hydrogen-bonded group. The paper uses the ratio of the two band areas to describe how much of the carbonyl is engaged in hydrogen bonding.

  Preparation and Characterization of Tris(trimethylsiloxy)silyl Modified Polyurethane Acrylates and Their Application in Textile Treatment DOI: 10.3390/polym12081629 · p. 8
• Published spectrum

  All spectra exhibit two bands at the carbonyl stretching region. One band at around 1728 cm-1 comes from the free carbonyl groups, another centered at around 1704 cm-1 corresponds to the hydrogen bonded carbonyl [32].

  Heating the composite moves the intensity from the hydrogen-bonded band to the free one, which is what makes the pair readable as a hydrogen-bonding measurement rather than as two groups.

  Effect of Phenolic Resin Oligomer Motion Ability on Energy Dissipation of Poly(Butyl Methacrylate)/Phenolic Resins Composites DOI: 10.3390/polym12020490 · p. 8
• Published spectrum

  In all spectra of samples C0-C3, the peak that appear at ~1640 cm-1 can be assigned to C=C and corresponded to the bonded carbonyl groups, and the peak at 1737 cm-1 related to the free carbonyl groups [40-42]. The modification of carbonyl band, after electrodialysis process (1737 cm-1), confirmed the presence of hydrogen bonding between carbonyl and hydroxyl groups.

  A polymer film where the free carbonyl stays at 1737 cm-1 while a second component appears near 1640 cm-1, which the authors link to hydrogen bonding between the carbonyl and the hydroxyl groups.

  Study of Polyvinyl Alcohol-SiO2 Nanoparticles Polymeric Membrane in Wastewater Treatment Containing Zinc Ions DOI: 10.3390/polym13111875 · p. 11
• Published spectrum

  In particular, carbonyl groups were resolved into different types: the carbonyl absorbance peak at ~1700 cm-1 showed the existence of carboxyl, aldehyde and ketone structures, while the peak at ~1650 cm-1 represented the highly conjugated carbonyl groups in the coal structure.

  One band, three possible groups: a good example of why a single position in the C=O window does not identify a compound.

  Surface Chemical Heterogeneity of Low Rank Coal Characterized by Micro-FTIR and Its Correlation with Hydrophobicity DOI: 10.3390/min11030239 · p. 4
• Published spectrum

  The decrease in absorbance at 1726 cm-1 (region of unconjugated carboxyl and ester groups) and the increase at 1639 cm-1 (region of conjugated carbonyl) in mode Seifert's cellulose.

  Conjugation and the carbonyl position move together: the unconjugated band falls while the conjugated band rises.

  Impact of Steaming Mode on Chemical Characteristics and Colour of Birch Wood DOI: 10.3390/f11040478 · p. 7
• Published spectrum

  In the next step, citraconic anhydride was observable by the band at 1840 cm-1 and 1765 cm-1 (C=O stretching of anhydride compounds) and 920 cm-1 (C-O stretching of the C-O-C) [43,44].

  The anhydride pair, with the C-O band that belongs to it. Two carbonyl bands in the window with a third band at 920 cm-1 is the pattern to look for when an anhydride is proposed.

  Bio-Based Phosphate-Containing Polyester for Improvement of Fire Reaction in Wooden Particleboard DOI: 10.3390/polym15051093 · p. 11
• Published spectrum

  C=O 1735-1736 Carbonyl stretch Triglycerides, phospholipids, cholesterol esters; C=O 1680-1710 Carbonyl stretch Free fatty acids, nucleic acids (DNA); Proteins (anti-parallel b-pleated sheet structures) C=O 1670-1695 stretch + N-H Carbonyl bend (amide I) b-turns and structures; (a-helix 1648-1670 Amide I Proteins structure); (b-pleated 1623-1637 Amide I Proteins sheet structures)

  Rows of the general assignment table of that paper, given here as printed. It is the clearest published reminder that the amide I band sits inside the carbonyl window and that free fatty acids, lipids and proteins share the 1623-1710 cm-1 region. The extraction renders the Greek letter beta as a plain b and the double bond as a code, and it runs the parenthesised protein descriptions together with the following row; the words are split where the table did that, and the wording is otherwise unchanged.

  Optimization and validation of cryostat temperature conditions for trans-reflectance mode FTIR microspectroscopic imaging of biological tissues DOI: 10.1016/j.mex.2017.01.006 · p. 8
• Published spectrum

  Saturated ester C=O stretch of lipids, phospholipids, triglycerides, and cholesterol esters | 1745, 1743, 1740, 1738 | Amide I, which arises mainly from C=O stretching vibrations of the protein peptide backbone, coupled weakly with C-N stretch, N-H bend, and C-N-C deformation | 1657, 1650, 1646

  Rows of the band-assignment table of that review. The same table gives amide II at 1550, 1546, 1540 and 1537 cm-1, which is the band to check against a carbonyl in the amide I range.

  The Convergence of FTIR and EVs: Emergence Strategy for Non-Invasive Cancer Markers Discovery DOI: 10.3390/diagnostics13010022 · p. 10
• Published spectrum

  1765-1700 C=O Carbonyl stretch; 1720 C=O Carboxylic acid stretch; 1710 C=O Free organic acid carboxylic; 1700 C=O Carboxylic acid, ketone, aldehyde stretch; 1660-1630 C=O Amide stretch (amide I); 1650 C=O Quinone ketone stretch

  Rows of the assignment table for soil organic matter, given as printed. The same table lists 2600-2500 cm-1 for the hydrogen-bonded O-H of carboxylic acids, which is the partner band for the acid carbonyls in that window. The extraction renders the double bond sign as a code.

  Soil Chemical Insights Provided through Vibrational Spectroscopy: Part I DOI: 10.1016/B978-0-12-800132-5.00001-8 · p. 38

Expert reading

The C=O band is easy to find and hard to name, because one window carries seven families. The habits that keep the reading honest are simple. First, report the position with its range, not as a single number: a carbonyl quoted at 1710 cm-1 may be a ketone, an aldehyde or a carboxylic acid, and the published cases above place all three inside ten wavenumbers. Second, name the partner band in the same sentence, because the partner is what carries the identity: C-O for an ester, N-H and amide II for an amide, the very broad O-H for an acid, aldehyde C-H for an aldehyde, two C=O bands for an anhydride. Third, treat a second band in the same window as information rather than as noise, because a free and a hydrogen-bonded carbonyl are routinely tens of wavenumbers apart, and their ratio is a usable measurement of hydrogen bonding. Fourth, remember that amide I occupies the low end of the window, so a protein-bearing sample will always put a carbonyl where a conjugated C=O would be expected, and check amide II before calling it conjugated. And finally, when the band sits near 1650 cm-1 and the sample is a solid or a powder, consider adsorbed water bending at 1630-1650 cm-1 before assigning anything. The peak positions linked below open the corresponding peak pages, where the other groups reported at the same wavenumber are listed.