FTIR Functional Group Frequency Table
This table lists the characteristic infrared absorption frequencies for common functional groups encountered in FTIR spectroscopy. It covers over 80 entries organized by functional group category, with wavenumber ranges in cm⁻¹, typical band intensities, band shapes, and explanatory notes for each entry.
The same data powers SpectralBench's FTIR Peak Identifier — the automated tool that detects peaks in your uploaded spectrum and matches them against these frequency ranges to assign functional groups with confidence scores.
Use the category links below to jump directly to the functional group you need.
How to Use This Table
- Find your peak position (wavenumber in cm⁻¹) from your spectrum.
- Scan the table below for matching wavenumber ranges. Multiple functional groups may absorb at similar positions.
- Use the band shape (broad, sharp, doublet) and intensity (strong, medium, weak) to narrow your assignment — these characteristics are often as diagnostic as position.
- Look for additional corroborating peaks. Most functional groups produce multiple bands — confirming two or three bands from the same group is far more reliable than relying on a single peak.
For a systematic approach to spectrum interpretation, see the FTIR Interpretation Guide.
Functional Group Frequencies
Alcohols, Phenols
| Functional Group | Range (cm⁻¹) | Intensity | Shape | Notes |
|---|---|---|---|---|
| O-H stretch (free) | 3610–3670 | strong | sharp | Sharp absorption of non-hydrogen-bonded O-H. Typically seen in dilute solutions or gas phase. |
| O-H stretch (hydrogen bonded) | 3200–3550 | strong | broad | Broad absorption due to hydrogen-bonded O-H groups. The breadth results from the distribution of H-bond strengths in the sample. |
O-H stretch (free)
3610–3670 cm⁻¹
strongsharp
Sharp absorption of non-hydrogen-bonded O-H. Typically seen in dilute solutions or gas phase.
O-H stretch (hydrogen bonded)
3200–3550 cm⁻¹
strongbroad
Broad absorption due to hydrogen-bonded O-H groups. The breadth results from the distribution of H-bond strengths in the sample.
Carboxylic Acids
| Functional Group | Range (cm⁻¹) | Intensity | Shape | Notes |
|---|---|---|---|---|
| O-H stretch (carboxylic acid) | 2500–3300 | strong | broad | Very broad absorption characteristic of carboxylic acid O-H. Often overlaps with C-H stretches, creating a distinctive broad plateau in the spectrum. |
| C=O stretch (carboxylic acid) | 1700–1725 | strong | sharp | Carbonyl stretch of carboxylic acids. Dimeric hydrogen bonding in the solid and liquid phase lowers the frequency. Confirm with the broad O-H stretch near 2500-3300 cm⁻¹. |
| O-H bend (carboxylic acid) | 1395–1440 | medium | sharp | In-plane bending of the O-H in carboxylic acids, coupled with C-O stretching. This band is less diagnostic than the C=O and broad O-H stretches for identifying carboxylic acids. |
O-H stretch (carboxylic acid)
2500–3300 cm⁻¹
strongbroad
Very broad absorption characteristic of carboxylic acid O-H. Often overlaps with C-H stretches, creating a distinctive broad plateau in the spectrum.
C=O stretch (carboxylic acid)
1700–1725 cm⁻¹
strongsharp
Carbonyl stretch of carboxylic acids. Dimeric hydrogen bonding in the solid and liquid phase lowers the frequency. Confirm with the broad O-H stretch near 2500-3300 cm⁻¹.
O-H bend (carboxylic acid)
1395–1440 cm⁻¹
mediumsharp
In-plane bending of the O-H in carboxylic acids, coupled with C-O stretching. This band is less diagnostic than the C=O and broad O-H stretches for identifying carboxylic acids.
Amines
| Functional Group | Range (cm⁻¹) | Intensity | Shape | Notes |
|---|---|---|---|---|
| N-H stretch (1° amine, asymmetric) | 3350–3500 | medium | doublet | The higher-frequency component of the primary amine doublet, arising from asymmetric N-H stretching. Paired with the symmetric stretch to confirm a primary amine. |
| N-H stretch (1° amine, symmetric) | 3250–3400 | medium | doublet | The lower-frequency component of the primary amine doublet, arising from symmetric N-H stretching. The two-band pattern distinguishes primary amines from secondary amines and alcohols. |
| N-H stretch (2° amine) | 3310–3500 | weak | sharp | A single, weaker N-H stretch band for secondary amines. Because only one N-H bond is present, a single peak appears rather than the doublet seen in primary amines. |
| N-H bend (1° amine, scissors) | 1580–1650 | medium | sharp | Scissoring deformation of the NH₂ group in primary amines. This band helps confirm primary amines when observed with the N-H stretch doublet near 3300-3500 cm⁻¹. |
| C-N stretch (aliphatic amine) | 1020–1250 | medium | sharp | C-N stretching in aliphatic amines. This band is often difficult to identify with certainty because it falls in the fingerprint region where many other single-bond stretches absorb. |
| C-N stretch (aromatic amine) | 1250–1360 | strong | sharp | C-N stretching in aromatic amines (arylamines). The band is stronger and at higher frequency than aliphatic C-N stretches due to partial double-bond character from resonance with the ring. |
| N-H wag (1° amine) | 650–900 | variable | broad | Broad out-of-plane wagging of the NH₂ group in primary amines. This band is often broad and variable in position, making it less reliable than the N-H stretch for identification. |
| N-H wag (2° amine) | 700–750 | weak | broad | Broad, weak out-of-plane wagging of the N-H bond in secondary amines. This band is often obscured by other absorptions in the fingerprint region. |
N-H stretch (1° amine, asymmetric)
3350–3500 cm⁻¹
mediumdoublet
The higher-frequency component of the primary amine doublet, arising from asymmetric N-H stretching. Paired with the symmetric stretch to confirm a primary amine.
N-H stretch (1° amine, symmetric)
3250–3400 cm⁻¹
mediumdoublet
The lower-frequency component of the primary amine doublet, arising from symmetric N-H stretching. The two-band pattern distinguishes primary amines from secondary amines and alcohols.
N-H stretch (2° amine)
3310–3500 cm⁻¹
weaksharp
A single, weaker N-H stretch band for secondary amines. Because only one N-H bond is present, a single peak appears rather than the doublet seen in primary amines.
N-H bend (1° amine, scissors)
1580–1650 cm⁻¹
mediumsharp
Scissoring deformation of the NH₂ group in primary amines. This band helps confirm primary amines when observed with the N-H stretch doublet near 3300-3500 cm⁻¹.
C-N stretch (aliphatic amine)
1020–1250 cm⁻¹
mediumsharp
C-N stretching in aliphatic amines. This band is often difficult to identify with certainty because it falls in the fingerprint region where many other single-bond stretches absorb.
C-N stretch (aromatic amine)
1250–1360 cm⁻¹
strongsharp
C-N stretching in aromatic amines (arylamines). The band is stronger and at higher frequency than aliphatic C-N stretches due to partial double-bond character from resonance with the ring.
N-H wag (1° amine)
650–900 cm⁻¹
variablebroad
Broad out-of-plane wagging of the NH₂ group in primary amines. This band is often broad and variable in position, making it less reliable than the N-H stretch for identification.
N-H wag (2° amine)
700–750 cm⁻¹
weakbroad
Broad, weak out-of-plane wagging of the N-H bond in secondary amines. This band is often obscured by other absorptions in the fingerprint region.
Amides
| Functional Group | Range (cm⁻¹) | Intensity | Shape | Notes |
|---|---|---|---|---|
| N-H stretch (amide) | 3180–3350 | medium | broad | Broad N-H stretch in amides, shifted to lower frequency compared to amines due to resonance with the carbonyl. Primary amides show two bands; secondary amides show one. |
| C=O stretch (amide I) | 1630–1690 | strong | sharp | The amide I band, primarily C=O stretching mixed with some C-N stretching and N-H bending. The low frequency reflects resonance delocalization of the nitrogen lone pair into the carbonyl. |
| N-H bend (amide II) | 1510–1570 | strong | sharp | The amide II band, primarily N-H in-plane bending mixed with C-N stretching. In proteins, this band is used alongside amide I to determine secondary structure. |
N-H stretch (amide)
3180–3350 cm⁻¹
mediumbroad
Broad N-H stretch in amides, shifted to lower frequency compared to amines due to resonance with the carbonyl. Primary amides show two bands; secondary amides show one.
C=O stretch (amide I)
1630–1690 cm⁻¹
strongsharp
The amide I band, primarily C=O stretching mixed with some C-N stretching and N-H bending. The low frequency reflects resonance delocalization of the nitrogen lone pair into the carbonyl.
N-H bend (amide II)
1510–1570 cm⁻¹
strongsharp
The amide II band, primarily N-H in-plane bending mixed with C-N stretching. In proteins, this band is used alongside amide I to determine secondary structure.
Alkanes
| Functional Group | Range (cm⁻¹) | Intensity | Shape | Notes |
|---|---|---|---|---|
| C-H stretch (CH₃ asymmetric) | 2950–2970 | strong | sharp | Asymmetric stretching of the three C-H bonds in a methyl group. This band at ~2960 cm⁻¹ is one of the most common absorptions in organic spectra. |
| C-H stretch (CH₃ symmetric) | 2860–2885 | strong | sharp | Symmetric stretching of a methyl group where all three C-H bonds extend and contract in phase. Its position near 2870 cm⁻¹ helps distinguish CH₃ from CH₂. |
| C-H stretch (CH₂ asymmetric) | 2915–2940 | strong | sharp | Asymmetric stretching of methylene C-H bonds, appearing near 2925 cm⁻¹. Particularly strong in long-chain hydrocarbons due to numerous CH₂ groups. |
| C-H stretch (CH₂ symmetric) | 2845–2870 | medium | sharp | Symmetric stretching of methylene C-H bonds near 2855 cm⁻¹. Slightly lower in frequency than the CH₃ symmetric stretch. |
| C-H bend (CH₃ asymmetric deformation) | 1440–1465 | medium | sharp | Asymmetric bending (deformation) of the methyl group near 1450 cm⁻¹. This band overlaps with the CH₂ scissors mode and is present in virtually all organic molecules containing CH₃. |
| C-H bend (CH₃ symmetric umbrella) | 1370–1390 | medium | sharp | Symmetric bending (umbrella mode) of CH₃ near 1375 cm⁻¹. Gem-dimethyl groups (isopropyl, t-butyl) show a characteristic split of this band into a doublet. |
| C-H bend (CH₂ scissors) | 1440–1475 | medium | sharp | Scissoring deformation of methylene groups near 1465 cm⁻¹. In crystalline long-chain alkanes, this band may split due to crystal field effects. |
C-H stretch (CH₃ asymmetric)
2950–2970 cm⁻¹
strongsharp
Asymmetric stretching of the three C-H bonds in a methyl group. This band at ~2960 cm⁻¹ is one of the most common absorptions in organic spectra.
C-H stretch (CH₃ symmetric)
2860–2885 cm⁻¹
strongsharp
Symmetric stretching of a methyl group where all three C-H bonds extend and contract in phase. Its position near 2870 cm⁻¹ helps distinguish CH₃ from CH₂.
C-H stretch (CH₂ asymmetric)
2915–2940 cm⁻¹
strongsharp
Asymmetric stretching of methylene C-H bonds, appearing near 2925 cm⁻¹. Particularly strong in long-chain hydrocarbons due to numerous CH₂ groups.
C-H stretch (CH₂ symmetric)
2845–2870 cm⁻¹
mediumsharp
Symmetric stretching of methylene C-H bonds near 2855 cm⁻¹. Slightly lower in frequency than the CH₃ symmetric stretch.
C-H bend (CH₃ asymmetric deformation)
1440–1465 cm⁻¹
mediumsharp
Asymmetric bending (deformation) of the methyl group near 1450 cm⁻¹. This band overlaps with the CH₂ scissors mode and is present in virtually all organic molecules containing CH₃.
C-H bend (CH₃ symmetric umbrella)
1370–1390 cm⁻¹
mediumsharp
Symmetric bending (umbrella mode) of CH₃ near 1375 cm⁻¹. Gem-dimethyl groups (isopropyl, t-butyl) show a characteristic split of this band into a doublet.
C-H bend (CH₂ scissors)
1440–1475 cm⁻¹
mediumsharp
Scissoring deformation of methylene groups near 1465 cm⁻¹. In crystalline long-chain alkanes, this band may split due to crystal field effects.
Alkenes
| Functional Group | Range (cm⁻¹) | Intensity | Shape | Notes |
|---|---|---|---|---|
| =C-H stretch (alkene) | 3020–3100 | medium | sharp | Stretching of C-H bonds on sp²-hybridized carbon in alkenes. The higher frequency compared to sp³ C-H reflects the greater s-character in the bond. |
| C=C stretch (alkene) | 1620–1680 | variable | sharp | Stretching of the C=C double bond. The band is often weak in symmetrically substituted alkenes and may be absent if the bond is centrosymmetric. Conjugation lowers the frequency. |
| =C-H oop bend (trans alkene) | 960–975 | strong | sharp | Strong out-of-plane C-H bending for trans-disubstituted alkenes near 970 cm⁻¹. This band is one of the most reliable for assigning trans alkene geometry. |
| =C-H oop bend (cis alkene) | 650–730 | medium | sharp | Out-of-plane C-H bending for cis-disubstituted alkenes. This band is weaker and more variable in position than the trans counterpart, making cis assignment less certain by IR alone. |
| =C-H oop bend (vinyl/terminal) | 905–920 | strong | sharp | Strong out-of-plane C-H bending for monosubstituted (vinyl) alkenes near 910 cm⁻¹. A second oop band near 990 cm⁻¹ further confirms the vinyl group. |
| =C-H oop bend (vinylidene) | 880–900 | strong | sharp | Out-of-plane C-H bending for vinylidene (1,1-disubstituted) alkenes near 890 cm⁻¹. This single strong band is characteristic of the =CH₂ group in geminal-disubstituted alkenes. |
=C-H stretch (alkene)
3020–3100 cm⁻¹
mediumsharp
Stretching of C-H bonds on sp²-hybridized carbon in alkenes. The higher frequency compared to sp³ C-H reflects the greater s-character in the bond.
C=C stretch (alkene)
1620–1680 cm⁻¹
variablesharp
Stretching of the C=C double bond. The band is often weak in symmetrically substituted alkenes and may be absent if the bond is centrosymmetric. Conjugation lowers the frequency.
=C-H oop bend (trans alkene)
960–975 cm⁻¹
strongsharp
Strong out-of-plane C-H bending for trans-disubstituted alkenes near 970 cm⁻¹. This band is one of the most reliable for assigning trans alkene geometry.
=C-H oop bend (cis alkene)
650–730 cm⁻¹
mediumsharp
Out-of-plane C-H bending for cis-disubstituted alkenes. This band is weaker and more variable in position than the trans counterpart, making cis assignment less certain by IR alone.
=C-H oop bend (vinyl/terminal)
905–920 cm⁻¹
strongsharp
Strong out-of-plane C-H bending for monosubstituted (vinyl) alkenes near 910 cm⁻¹. A second oop band near 990 cm⁻¹ further confirms the vinyl group.
=C-H oop bend (vinylidene)
880–900 cm⁻¹
strongsharp
Out-of-plane C-H bending for vinylidene (1,1-disubstituted) alkenes near 890 cm⁻¹. This single strong band is characteristic of the =CH₂ group in geminal-disubstituted alkenes.
Aromatics
| Functional Group | Range (cm⁻¹) | Intensity | Shape | Notes |
|---|---|---|---|---|
| C-H stretch (aromatic) | 3000–3100 | medium | sharp | Stretching of aromatic C-H bonds on sp² carbons. Appears just above 3000 cm⁻¹, helping distinguish aromatic from aliphatic C-H (below 3000 cm⁻¹). |
| C=C stretch (aromatic, ~1600 cm⁻¹) | 1585–1615 | variable | sharp | One of the characteristic aromatic ring C=C stretching vibrations near 1600 cm⁻¹. Intensity depends on the symmetry and substituents of the ring; more polar substituents increase intensity. |
| C=C stretch (aromatic, ~1475 cm⁻¹) | 1450–1510 | variable | sharp | The second major aromatic ring C=C stretching vibration near 1475-1500 cm⁻¹. Together with the ~1600 cm⁻¹ band, these confirm an aromatic ring system. |
| C-H oop bend (monosubstituted aromatic) | 730–770 | strong | sharp | Out-of-plane C-H bending of five adjacent aromatic hydrogens in monosubstituted benzene. This band, combined with the ring oop bend near 690 cm⁻¹, confirms a monosubstituted pattern. |
| Ring oop bend (monosubstituted aromatic) | 690–710 | strong | sharp | Out-of-plane ring bending of monosubstituted benzene near 700 cm⁻¹. This band paired with the C-H oop bend near 750 cm⁻¹ is one of the most reliable aromatic substitution patterns. |
| C-H oop bend (1,2-disubstituted/ortho) | 735–770 | strong | sharp | Out-of-plane C-H bending of four adjacent aromatic hydrogens in ortho-disubstituted benzene. The absence of a strong ring bend near 690 cm⁻¹ distinguishes ortho from mono substitution. |
| C-H oop bend (1,3-disubstituted/meta) | 770–810 | strong | sharp | Out-of-plane C-H bending of three adjacent hydrogens in meta-disubstituted benzene near 780 cm⁻¹. Meta substitution also shows a ring oop bend near 690 cm⁻¹. |
| Ring oop bend (1,3-disubstituted/meta) | 680–720 | strong | sharp | Out-of-plane ring bending for meta-disubstituted benzene. This band near 690 cm⁻¹ overlaps with the monosubstituted ring bend, so the C-H oop pattern must be used to differentiate. |
| C-H oop bend (1,4-disubstituted/para) | 800–860 | strong | sharp | Out-of-plane C-H bending of two adjacent hydrogens in para-disubstituted benzene near 830 cm⁻¹. This single strong band is highly diagnostic for para substitution. |
| Ring breathing (aromatic) | 990–1010 | variable | sharp | Symmetric ring expansion/contraction (breathing mode) of aromatic rings near 1000 cm⁻¹. This vibration involves all ring atoms moving radially in phase. |
C-H stretch (aromatic)
3000–3100 cm⁻¹
mediumsharp
Stretching of aromatic C-H bonds on sp² carbons. Appears just above 3000 cm⁻¹, helping distinguish aromatic from aliphatic C-H (below 3000 cm⁻¹).
C=C stretch (aromatic, ~1600 cm⁻¹)
1585–1615 cm⁻¹
variablesharp
One of the characteristic aromatic ring C=C stretching vibrations near 1600 cm⁻¹. Intensity depends on the symmetry and substituents of the ring; more polar substituents increase intensity.
C=C stretch (aromatic, ~1475 cm⁻¹)
1450–1510 cm⁻¹
variablesharp
The second major aromatic ring C=C stretching vibration near 1475-1500 cm⁻¹. Together with the ~1600 cm⁻¹ band, these confirm an aromatic ring system.
C-H oop bend (monosubstituted aromatic)
730–770 cm⁻¹
strongsharp
Out-of-plane C-H bending of five adjacent aromatic hydrogens in monosubstituted benzene. This band, combined with the ring oop bend near 690 cm⁻¹, confirms a monosubstituted pattern.
Ring oop bend (monosubstituted aromatic)
690–710 cm⁻¹
strongsharp
Out-of-plane ring bending of monosubstituted benzene near 700 cm⁻¹. This band paired with the C-H oop bend near 750 cm⁻¹ is one of the most reliable aromatic substitution patterns.
C-H oop bend (1,2-disubstituted/ortho)
735–770 cm⁻¹
strongsharp
Out-of-plane C-H bending of four adjacent aromatic hydrogens in ortho-disubstituted benzene. The absence of a strong ring bend near 690 cm⁻¹ distinguishes ortho from mono substitution.
C-H oop bend (1,3-disubstituted/meta)
770–810 cm⁻¹
strongsharp
Out-of-plane C-H bending of three adjacent hydrogens in meta-disubstituted benzene near 780 cm⁻¹. Meta substitution also shows a ring oop bend near 690 cm⁻¹.
Ring oop bend (1,3-disubstituted/meta)
680–720 cm⁻¹
strongsharp
Out-of-plane ring bending for meta-disubstituted benzene. This band near 690 cm⁻¹ overlaps with the monosubstituted ring bend, so the C-H oop pattern must be used to differentiate.
C-H oop bend (1,4-disubstituted/para)
800–860 cm⁻¹
strongsharp
Out-of-plane C-H bending of two adjacent hydrogens in para-disubstituted benzene near 830 cm⁻¹. This single strong band is highly diagnostic for para substitution.
Ring breathing (aromatic)
990–1010 cm⁻¹
variablesharp
Symmetric ring expansion/contraction (breathing mode) of aromatic rings near 1000 cm⁻¹. This vibration involves all ring atoms moving radially in phase.
Alkynes
| Functional Group | Range (cm⁻¹) | Intensity | Shape | Notes |
|---|---|---|---|---|
| ≡C-H stretch (terminal alkyne) | 3260–3330 | strong | sharp | Stretching of the C-H bond on sp-hybridized carbon in terminal alkynes. The high frequency reflects the 50% s-character of the bond, and the band is characteristically sharp and strong. |
| C≡C stretch (terminal alkyne) | 2100–2140 | weak | sharp | Stretching of the carbon-carbon triple bond in terminal alkynes. The band is relatively weak because the dipole moment change during vibration is small. |
| C≡C stretch (internal alkyne) | 2190–2260 | weak | sharp | Stretching of the carbon-carbon triple bond in internal alkynes. Symmetrically substituted internal alkynes may show no absorption due to zero dipole moment change. |
≡C-H stretch (terminal alkyne)
3260–3330 cm⁻¹
strongsharp
Stretching of the C-H bond on sp-hybridized carbon in terminal alkynes. The high frequency reflects the 50% s-character of the bond, and the band is characteristically sharp and strong.
C≡C stretch (terminal alkyne)
2100–2140 cm⁻¹
weaksharp
Stretching of the carbon-carbon triple bond in terminal alkynes. The band is relatively weak because the dipole moment change during vibration is small.
C≡C stretch (internal alkyne)
2190–2260 cm⁻¹
weaksharp
Stretching of the carbon-carbon triple bond in internal alkynes. Symmetrically substituted internal alkynes may show no absorption due to zero dipole moment change.
Aldehydes
| Functional Group | Range (cm⁻¹) | Intensity | Shape | Notes |
|---|---|---|---|---|
| C-H stretch (aldehyde, Fermi doublet low) | 2700–2760 | medium | doublet | The lower component of the aldehyde C-H Fermi resonance doublet. This distinctive band near 2720 cm⁻¹ is highly diagnostic for aldehydes, as few other groups absorb in this region. |
| C-H stretch (aldehyde, Fermi doublet high) | 2820–2860 | medium | doublet | The higher component of the aldehyde C-H Fermi resonance doublet, arising from Fermi resonance between the C-H stretch fundamental and the first overtone of the C-H bend. |
| C=O stretch (aldehyde) | 1720–1740 | strong | sharp | Carbonyl stretch of aldehydes, slightly higher than ketones due to less inductive donation. Confirm by looking for the characteristic Fermi doublet near 2720/2850 cm⁻¹. |
C-H stretch (aldehyde, Fermi doublet low)
2700–2760 cm⁻¹
mediumdoublet
The lower component of the aldehyde C-H Fermi resonance doublet. This distinctive band near 2720 cm⁻¹ is highly diagnostic for aldehydes, as few other groups absorb in this region.
C-H stretch (aldehyde, Fermi doublet high)
2820–2860 cm⁻¹
mediumdoublet
The higher component of the aldehyde C-H Fermi resonance doublet, arising from Fermi resonance between the C-H stretch fundamental and the first overtone of the C-H bend.
C=O stretch (aldehyde)
1720–1740 cm⁻¹
strongsharp
Carbonyl stretch of aldehydes, slightly higher than ketones due to less inductive donation. Confirm by looking for the characteristic Fermi doublet near 2720/2850 cm⁻¹.
Nitriles
| Functional Group | Range (cm⁻¹) | Intensity | Shape | Notes |
|---|---|---|---|---|
| C≡N stretch (nitrile) | 2210–2260 | strong | sharp | Strong stretching absorption of the C≡N triple bond. Conjugation lowers the frequency, while electron-withdrawing substituents raise it. The band is much stronger than C≡C due to the large dipole moment change. |
C≡N stretch (nitrile)
2210–2260 cm⁻¹
strongsharp
Strong stretching absorption of the C≡N triple bond. Conjugation lowers the frequency, while electron-withdrawing substituents raise it. The band is much stronger than C≡C due to the large dipole moment change.
Azides
| Functional Group | Range (cm⁻¹) | Intensity | Shape | Notes |
|---|---|---|---|---|
| N₃ asymmetric stretch (azide) | 2090–2160 | strong | sharp | Strong asymmetric stretching of the azide (-N₃) group. This intense band in an otherwise quiet spectral region is highly diagnostic for organic azides. |
N₃ asymmetric stretch (azide)
2090–2160 cm⁻¹
strongsharp
Strong asymmetric stretching of the azide (-N₃) group. This intense band in an otherwise quiet spectral region is highly diagnostic for organic azides.
Isocyanates
| Functional Group | Range (cm⁻¹) | Intensity | Shape | Notes |
|---|---|---|---|---|
| N=C=O stretch (isocyanate) | 2250–2275 | strong | sharp | Intense asymmetric stretching of the cumulated N=C=O system. The very strong, sharp band in a relatively clear spectral region makes isocyanates easy to identify. |
N=C=O stretch (isocyanate)
2250–2275 cm⁻¹
strongsharp
Intense asymmetric stretching of the cumulated N=C=O system. The very strong, sharp band in a relatively clear spectral region makes isocyanates easy to identify.
Isothiocyanates
| Functional Group | Range (cm⁻¹) | Intensity | Shape | Notes |
|---|---|---|---|---|
| N=C=S stretch (isothiocyanate) | 2050–2110 | strong | sharp | Intense asymmetric stretching of the cumulated N=C=S system. Appears at lower frequency than isocyanates due to the heavier sulfur atom. |
N=C=S stretch (isothiocyanate)
2050–2110 cm⁻¹
strongsharp
Intense asymmetric stretching of the cumulated N=C=S system. Appears at lower frequency than isocyanates due to the heavier sulfur atom.
Ketones
| Functional Group | Range (cm⁻¹) | Intensity | Shape | Notes |
|---|---|---|---|---|
| C=O stretch (ketone) | 1705–1725 | strong | sharp | Strong carbonyl stretch of simple aliphatic ketones near 1715 cm⁻¹. The C=O stretch is typically the strongest band in the spectrum and is highly sensitive to electronic and structural effects. |
| C=O stretch (conjugated ketone) | 1665–1700 | strong | sharp | Carbonyl stretch lowered by conjugation with a C=C double bond or aromatic ring. The extended delocalization reduces the C=O bond order and thus the stretching frequency. |
C=O stretch (ketone)
1705–1725 cm⁻¹
strongsharp
Strong carbonyl stretch of simple aliphatic ketones near 1715 cm⁻¹. The C=O stretch is typically the strongest band in the spectrum and is highly sensitive to electronic and structural effects.
C=O stretch (conjugated ketone)
1665–1700 cm⁻¹
strongsharp
Carbonyl stretch lowered by conjugation with a C=C double bond or aromatic ring. The extended delocalization reduces the C=O bond order and thus the stretching frequency.
Esters
| Functional Group | Range (cm⁻¹) | Intensity | Shape | Notes |
|---|---|---|---|---|
| C=O stretch (ester) | 1735–1750 | strong | sharp | Carbonyl stretch of simple esters, appearing at higher frequency than ketones due to mesomeric electron donation from the ester oxygen raising the C=O force constant. |
| C=O stretch (α,β-unsaturated ester) | 1715–1735 | strong | sharp | Carbonyl stretch of esters conjugated with a double bond. Conjugation lowers the frequency compared to saturated esters by delocalizing electron density from the carbonyl. |
| C-O stretch (ester C-O-C) | 1150–1300 | strong | sharp | C-O stretching of the ester linkage. This strong band, combined with the C=O stretch near 1740 cm⁻¹, forms the two-band signature pattern diagnostic for esters. |
C=O stretch (ester)
1735–1750 cm⁻¹
strongsharp
Carbonyl stretch of simple esters, appearing at higher frequency than ketones due to mesomeric electron donation from the ester oxygen raising the C=O force constant.
C=O stretch (α,β-unsaturated ester)
1715–1735 cm⁻¹
strongsharp
Carbonyl stretch of esters conjugated with a double bond. Conjugation lowers the frequency compared to saturated esters by delocalizing electron density from the carbonyl.
C-O stretch (ester C-O-C)
1150–1300 cm⁻¹
strongsharp
C-O stretching of the ester linkage. This strong band, combined with the C=O stretch near 1740 cm⁻¹, forms the two-band signature pattern diagnostic for esters.
Anhydrides
| Functional Group | Range (cm⁻¹) | Intensity | Shape | Notes |
|---|---|---|---|---|
| C=O stretch (anhydride, symmetric) | 1800–1850 | strong | sharp | The higher-frequency symmetric C=O stretch of anhydrides. The characteristic two-band carbonyl pattern with approximately 60 cm⁻¹ separation is diagnostic for anhydrides. |
| C=O stretch (anhydride, asymmetric) | 1720–1775 | strong | sharp | The lower-frequency asymmetric C=O stretch of anhydrides. Cyclic anhydrides show a stronger high-frequency band, while acyclic anhydrides show a stronger low-frequency band. |
C=O stretch (anhydride, symmetric)
1800–1850 cm⁻¹
strongsharp
The higher-frequency symmetric C=O stretch of anhydrides. The characteristic two-band carbonyl pattern with approximately 60 cm⁻¹ separation is diagnostic for anhydrides.
C=O stretch (anhydride, asymmetric)
1720–1775 cm⁻¹
strongsharp
The lower-frequency asymmetric C=O stretch of anhydrides. Cyclic anhydrides show a stronger high-frequency band, while acyclic anhydrides show a stronger low-frequency band.
Acyl Halides
| Functional Group | Range (cm⁻¹) | Intensity | Shape | Notes |
|---|---|---|---|---|
| C=O stretch (acyl halide) | 1770–1815 | strong | sharp | Carbonyl stretch of acyl halides, at high frequency due to the strong electron-withdrawing inductive effect of the halogen, which increases the C=O force constant. |
C=O stretch (acyl halide)
1770–1815 cm⁻¹
strongsharp
Carbonyl stretch of acyl halides, at high frequency due to the strong electron-withdrawing inductive effect of the halogen, which increases the C=O force constant.
Lactones
| Functional Group | Range (cm⁻¹) | Intensity | Shape | Notes |
|---|---|---|---|---|
| C=O stretch (lactone) | 1735–1770 | strong | sharp | Carbonyl stretch of cyclic esters (lactones). Ring strain in smaller lactones shifts the frequency upward; five-membered lactones absorb near 1770 cm⁻¹, six-membered near 1740 cm⁻¹. |
C=O stretch (lactone)
1735–1770 cm⁻¹
strongsharp
Carbonyl stretch of cyclic esters (lactones). Ring strain in smaller lactones shifts the frequency upward; five-membered lactones absorb near 1770 cm⁻¹, six-membered near 1740 cm⁻¹.
Carbonates
| Functional Group | Range (cm⁻¹) | Intensity | Shape | Notes |
|---|---|---|---|---|
| C=O stretch (carbonate) | 1720–1780 | strong | sharp | Carbonyl stretch of organic carbonates. The frequency depends on whether the carbonate is cyclic or acyclic; cyclic carbonates absorb at higher frequency due to ring strain. |
C=O stretch (carbonate)
1720–1780 cm⁻¹
strongsharp
Carbonyl stretch of organic carbonates. The frequency depends on whether the carbonate is cyclic or acyclic; cyclic carbonates absorb at higher frequency due to ring strain.
Ureas
| Functional Group | Range (cm⁻¹) | Intensity | Shape | Notes |
|---|---|---|---|---|
| C=O stretch (urea) | 1640–1690 | strong | sharp | Carbonyl stretch of urea derivatives, lowered significantly by resonance donation from two flanking nitrogen atoms. The band overlaps with the amide I region. |
C=O stretch (urea)
1640–1690 cm⁻¹
strongsharp
Carbonyl stretch of urea derivatives, lowered significantly by resonance donation from two flanking nitrogen atoms. The band overlaps with the amide I region.
Nitro Compounds
| Functional Group | Range (cm⁻¹) | Intensity | Shape | Notes |
|---|---|---|---|---|
| NO₂ asymmetric stretch | 1500–1570 | strong | sharp | Strong asymmetric stretching of the nitro group. This band, paired with the symmetric stretch near 1340 cm⁻¹, forms a diagnostic pair for nitro compounds. |
| NO₂ symmetric stretch | 1310–1370 | strong | sharp | Symmetric stretching of the nitro group near 1340 cm⁻¹. Always look for the corresponding asymmetric stretch near 1530 cm⁻¹ to confirm a nitro group. |
NO₂ asymmetric stretch
1500–1570 cm⁻¹
strongsharp
Strong asymmetric stretching of the nitro group. This band, paired with the symmetric stretch near 1340 cm⁻¹, forms a diagnostic pair for nitro compounds.
NO₂ symmetric stretch
1310–1370 cm⁻¹
strongsharp
Symmetric stretching of the nitro group near 1340 cm⁻¹. Always look for the corresponding asymmetric stretch near 1530 cm⁻¹ to confirm a nitro group.
Imines
| Functional Group | Range (cm⁻¹) | Intensity | Shape | Notes |
|---|---|---|---|---|
| C=N stretch (imine/oxime) | 1620–1690 | variable | sharp | Stretching of the C=N double bond in imines and oximes. The frequency overlaps with C=C stretches and amide I, but can be distinguished by context and correlation with N-H or O-H bands. |
C=N stretch (imine/oxime)
1620–1690 cm⁻¹
variablesharp
Stretching of the C=N double bond in imines and oximes. The frequency overlaps with C=C stretches and amide I, but can be distinguished by context and correlation with N-H or O-H bands.
Alcohols
| Functional Group | Range (cm⁻¹) | Intensity | Shape | Notes |
|---|---|---|---|---|
| C-O stretch (1° alcohol) | 1040–1085 | strong | sharp | C-O stretching vibration of primary alcohols. The position near 1050 cm⁻¹ helps distinguish primary from secondary and tertiary alcohols, which absorb at progressively higher frequencies. |
| C-O stretch (2° alcohol) | 1085–1125 | strong | sharp | C-O stretching vibration of secondary alcohols near 1100 cm⁻¹. The shift from the primary alcohol position reflects increased branching at the carbon bearing the hydroxyl. |
| C-O stretch (3° alcohol) | 1125–1175 | strong | sharp | C-O stretching vibration of tertiary alcohols near 1150 cm⁻¹. The highest frequency among alcohols due to the electron-donating effects of three alkyl groups. |
| O-H bend (in-plane, alcohol) | 1230–1320 | medium | sharp | In-plane bending (deformation) of the O-H bond in alcohols. This band couples strongly with adjacent C-O and C-C stretching vibrations in the fingerprint region. |
C-O stretch (1° alcohol)
1040–1085 cm⁻¹
strongsharp
C-O stretching vibration of primary alcohols. The position near 1050 cm⁻¹ helps distinguish primary from secondary and tertiary alcohols, which absorb at progressively higher frequencies.
C-O stretch (2° alcohol)
1085–1125 cm⁻¹
strongsharp
C-O stretching vibration of secondary alcohols near 1100 cm⁻¹. The shift from the primary alcohol position reflects increased branching at the carbon bearing the hydroxyl.
C-O stretch (3° alcohol)
1125–1175 cm⁻¹
strongsharp
C-O stretching vibration of tertiary alcohols near 1150 cm⁻¹. The highest frequency among alcohols due to the electron-donating effects of three alkyl groups.
O-H bend (in-plane, alcohol)
1230–1320 cm⁻¹
mediumsharp
In-plane bending (deformation) of the O-H bond in alcohols. This band couples strongly with adjacent C-O and C-C stretching vibrations in the fingerprint region.
Ethers
| Functional Group | Range (cm⁻¹) | Intensity | Shape | Notes |
|---|---|---|---|---|
| C-O-C stretch (ether) | 1060–1150 | strong | sharp | Asymmetric stretching of the C-O-C linkage in ethers. Aliphatic ethers absorb near 1120 cm⁻¹, while aryl ethers show a second band near 1250 cm⁻¹ from the aryl-O stretch. |
C-O-C stretch (ether)
1060–1150 cm⁻¹
strongsharp
Asymmetric stretching of the C-O-C linkage in ethers. Aliphatic ethers absorb near 1120 cm⁻¹, while aryl ethers show a second band near 1250 cm⁻¹ from the aryl-O stretch.
Sulfoxides
| Functional Group | Range (cm⁻¹) | Intensity | Shape | Notes |
|---|---|---|---|---|
| S=O stretch (sulfoxide) | 1030–1070 | strong | sharp | Strong stretching of the S=O bond in sulfoxides near 1050 cm⁻¹. The single S=O band distinguishes sulfoxides from sulfones, which show two S=O bands. |
S=O stretch (sulfoxide)
1030–1070 cm⁻¹
strongsharp
Strong stretching of the S=O bond in sulfoxides near 1050 cm⁻¹. The single S=O band distinguishes sulfoxides from sulfones, which show two S=O bands.
Sulfones
| Functional Group | Range (cm⁻¹) | Intensity | Shape | Notes |
|---|---|---|---|---|
| S=O stretch (sulfone, asymmetric) | 1290–1350 | strong | sharp | Asymmetric stretching of the two S=O bonds in sulfones. This band, paired with the symmetric stretch near 1150 cm⁻¹, is diagnostic for the SO₂ group. |
| S=O stretch (sulfone, symmetric) | 1120–1170 | strong | sharp | Symmetric stretching of the two S=O bonds in sulfones near 1150 cm⁻¹. Always confirm by checking for the asymmetric stretch near 1310 cm⁻¹. |
S=O stretch (sulfone, asymmetric)
1290–1350 cm⁻¹
strongsharp
Asymmetric stretching of the two S=O bonds in sulfones. This band, paired with the symmetric stretch near 1150 cm⁻¹, is diagnostic for the SO₂ group.
S=O stretch (sulfone, symmetric)
1120–1170 cm⁻¹
strongsharp
Symmetric stretching of the two S=O bonds in sulfones near 1150 cm⁻¹. Always confirm by checking for the asymmetric stretch near 1310 cm⁻¹.
Haloalkanes
| Functional Group | Range (cm⁻¹) | Intensity | Shape | Notes |
|---|---|---|---|---|
| C-F stretch | 1000–1400 | strong | sharp | Stretching of the C-F bond. The high electronegativity of fluorine produces a very strong absorption. Multiple fluorines (as in CF₂, CF₃) give broad, intense bands in this region. |
| C-Cl stretch | 550–800 | strong | sharp | Stretching of the C-Cl bond. Multiple C-Cl bonds (as in CHCl₃) give multiple bands in this region. The heavy chlorine atom results in a lower frequency compared to C-F. |
| C-Br stretch | 500–680 | strong | sharp | Stretching of the C-Br bond at lower frequency than C-Cl due to the heavier bromine atom. The band often falls near the low-frequency limit of standard FTIR instruments. |
C-F stretch
1000–1400 cm⁻¹
strongsharp
Stretching of the C-F bond. The high electronegativity of fluorine produces a very strong absorption. Multiple fluorines (as in CF₂, CF₃) give broad, intense bands in this region.
C-Cl stretch
550–800 cm⁻¹
strongsharp
Stretching of the C-Cl bond. Multiple C-Cl bonds (as in CHCl₃) give multiple bands in this region. The heavy chlorine atom results in a lower frequency compared to C-F.
C-Br stretch
500–680 cm⁻¹
strongsharp
Stretching of the C-Br bond at lower frequency than C-Cl due to the heavier bromine atom. The band often falls near the low-frequency limit of standard FTIR instruments.
Phosphorus Compounds
| Functional Group | Range (cm⁻¹) | Intensity | Shape | Notes |
|---|---|---|---|---|
| P=O stretch | 1150–1300 | strong | sharp | Strong stretching of the P=O bond in phosphates, phosphonates, and phosphine oxides. The position is sensitive to the groups attached to phosphorus. |
P=O stretch
1150–1300 cm⁻¹
strongsharp
Strong stretching of the P=O bond in phosphates, phosphonates, and phosphine oxides. The position is sensitive to the groups attached to phosphorus.
Organosilicon
| Functional Group | Range (cm⁻¹) | Intensity | Shape | Notes |
|---|---|---|---|---|
| Si-O stretch | 1000–1100 | strong | broad | Strong, broad Si-O stretching absorption. Commonly seen in silicones, silicates, and glass. In polymeric siloxanes, the band can be very broad due to the range of Si-O-Si environments. |
| Si-CH₃ deformation | 1240–1280 | strong | sharp | Symmetric deformation of Si-CH₃ groups near 1260 cm⁻¹. This sharp, strong band is highly characteristic of silicones and trimethylsilyl groups used as protecting groups in organic synthesis. |
Si-O stretch
1000–1100 cm⁻¹
strongbroad
Strong, broad Si-O stretching absorption. Commonly seen in silicones, silicates, and glass. In polymeric siloxanes, the band can be very broad due to the range of Si-O-Si environments.
Si-CH₃ deformation
1240–1280 cm⁻¹
strongsharp
Symmetric deformation of Si-CH₃ groups near 1260 cm⁻¹. This sharp, strong band is highly characteristic of silicones and trimethylsilyl groups used as protecting groups in organic synthesis.
Epoxides
| Functional Group | Range (cm⁻¹) | Intensity | Shape | Notes |
|---|---|---|---|---|
| C-O stretch (epoxide ring) | 1230–1280 | strong | sharp | Asymmetric C-O-C stretching of the strained three-membered epoxide ring. The ring strain raises the frequency compared to acyclic ethers. A second band near 810-950 cm⁻¹ from symmetric ring stretching confirms the epoxide. |
C-O stretch (epoxide ring)
1230–1280 cm⁻¹
strongsharp
Asymmetric C-O-C stretching of the strained three-membered epoxide ring. The ring strain raises the frequency compared to acyclic ethers. A second band near 810-950 cm⁻¹ from symmetric ring stretching confirms the epoxide.
Phenols
| Functional Group | Range (cm⁻¹) | Intensity | Shape | Notes |
|---|---|---|---|---|
| C-O stretch (phenol) | 1170–1250 | strong | sharp | C-O stretching of the phenolic C-O bond, with partial double-bond character from conjugation with the aromatic ring. Confirm with the broad O-H stretch and aromatic C-H bands. |
C-O stretch (phenol)
1170–1250 cm⁻¹
strongsharp
C-O stretching of the phenolic C-O bond, with partial double-bond character from conjugation with the aromatic ring. Confirm with the broad O-H stretch and aromatic C-H bands.
Thioethers
| Functional Group | Range (cm⁻¹) | Intensity | Shape | Notes |
|---|---|---|---|---|
| C-S stretch | 570–710 | weak | sharp | Stretching of the C-S bond. The band is weak because the small dipole moment change during vibration results in low infrared absorption intensity. |
C-S stretch
570–710 cm⁻¹
weaksharp
Stretching of the C-S bond. The band is weak because the small dipole moment change during vibration results in low infrared absorption intensity.
Boronic Acids
| Functional Group | Range (cm⁻¹) | Intensity | Shape | Notes |
|---|---|---|---|---|
| B-O stretch | 1310–1380 | strong | sharp | Strong stretching of the B-O bond in boronic acids and boronate esters. The band position is sensitive to the coordination state of boron (trigonal vs. tetrahedral). |
B-O stretch
1310–1380 cm⁻¹
strongsharp
Strong stretching of the B-O bond in boronic acids and boronate esters. The band position is sensitive to the coordination state of boron (trigonal vs. tetrahedral).
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Notes on Using Correlation Tables
- Wavenumber ranges are approximate — exact peak positions depend on the molecular environment, substituent effects, and the physical state of the sample.
- Hydrogen bonding shifts O-H and N-H stretches to lower wavenumbers and broadens the bands significantly. A “free” O-H at 3650 cm⁻¹ may shift below 3000 cm⁻¹ in strongly hydrogen-bonded systems like carboxylic acid dimers.
- Conjugation with double bonds or aromatic rings lowers the C=O stretching frequency by 20–40 cm⁻¹. An α,β-unsaturated ketone absorbs near 1680 cm⁻¹ rather than 1715 cm⁻¹.
- Solid-state (KBr pellet, ATR) and solution spectra may show different peak positions and intensities. ATR correction can partially compensate, but always note which technique was used when comparing spectra.
- Never assign a functional group based on a single peak. Cross-reference multiple diagnostic bands — for example, confirm an ester by finding both a C=O stretch near 1740 cm⁻¹ and a strong C-O stretch near 1240 cm⁻¹.
Related Resources
- FTIR Spectrum Interpretation Guide — step-by-step workflow for reading FTIR spectra
- FTIR Peak Identifier — automated peak detection and functional group assignment
- Spectroscopy Unit Converter — convert between wavenumber, wavelength, frequency, and energy