By SpectralBench Editorial Team2 min read
FTIR Spectrum Table — Infrared Functional Groups Chart
This interactive FTIR spectrum table and infrared spectrum chart covers over 80 characteristic absorption frequencies organized by functional group category. Use it as a comprehensive table for IR spectroscopy — search by group name, category, or wavenumber to instantly filter results. Click any bar in the chart or row in the table to see detailed notes, band shape, and example spectra.
The same data powers SpectralBench's FTIR Peak Identifier — the automated tool that detects peaks in your uploaded spectrum and matches them against this infrared spectrum table to assign functional groups with confidence scores. For a guided walkthrough of how to interpret FTIR spectra region by region, see our FTIR interpretation guide.
Upload a spectrum to identify peaksHow to use this chart
- Find your peak position (wavenumber in cm⁻¹) from your spectrum.
- Type the wavenumber into the search box to highlight matching groups in both the chart and the table, or browse the chart visually.
- 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.
| Functional Group | Wavenumber Range (cm⁻¹) | Intensity | Band Shape |
|---|---|---|---|
| O-H stretch (hydrogen bonded) | 3200–3550 | Strong | Broad |
| O-H stretch (carboxylic acid) | 2500–3300 | Strong | Broad |
| N-H stretch (1° amine) | 3250–3500 | Medium | Doublet |
| N-H stretch (amide) | 3180–3350 | Medium | Broad |
| C-H stretch (CH₃, CH₂) | 2845–2970 | Strong | Sharp |
| C-H stretch (aromatic) | 3000–3100 | Medium | Sharp |
| ≡C-H stretch (terminal alkyne) | 3260–3330 | Strong | Sharp |
| C≡N stretch (nitrile) | 2210–2260 | Strong | Sharp |
| C=O stretch (ketone) | 1705–1725 | Strong | Sharp |
| C=O stretch (aldehyde) | 1720–1740 | Strong | Sharp |
| C=O stretch (ester) | 1735–1750 | Strong | Sharp |
| C=O stretch (carboxylic acid) | 1700–1725 | Strong | Sharp |
| C=O stretch (amide I) | 1630–1690 | Strong | Sharp |
| C=C stretch (aromatic) | 1450–1615 | Variable | Sharp |
| C=C stretch (alkene) | 1620–1680 | Variable | Sharp |
| NO₂ stretch (nitro) | 1310–1570 | Strong | Sharp |
| C-O stretch (ester C-O-C) | 1150–1300 | Strong | Sharp |
| C-O stretch (alcohol) | 1000–1210 | Strong | Sharp |
| S=O stretch (sulfone) | 1120–1350 | Strong | Sharp |
| Si-O stretch | 1000–1100 | Strong | Broad |
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Search groups, categories, or wavenumber (e.g. "C=O", "ketone", "1715")FTIR Correlation Chart
Tap or click a row for details. Bars are color-coded by functional group category; opacity reflects band intensity.
Alcohols, Phenols
Carboxylic Acids
Amines
Amides
Alkanes
Alkenes
Aromatics
Alkynes
Aldehydes
Nitriles
Azides
Isocyanates
Isothiocyanates
Ketones
Esters
Anhydrides
Acyl Halides
Lactones
Carbonates
Ureas
Nitro Compounds
Imines
Alcohols
Ethers
Sulfoxides
Sulfones
Sulfonates
Haloalkanes
Phosphorus Compounds
Organosilicon
Epoxides
Phenols
Thioethers
Boronic Acids
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⁻¹strongsharp
Sharp absorption of non-hydrogen-bonded O-H. Typically seen in dilute solutions or gas phase.
O-H stretch (hydrogen bonded)
3200–3550 cm⁻¹strongbroad
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⁻¹strongbroad
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⁻¹strongsharp
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⁻¹mediumsharp
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)
1020–1100
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⁻¹mediumdoublet
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⁻¹mediumdoublet
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⁻¹weaksharp
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⁻¹mediumsharp
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)
1020–1100 cm⁻¹mediumsharp
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⁻¹strongsharp
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⁻¹variablebroad
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⁻¹weakbroad
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⁻¹mediumbroad
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⁻¹strongsharp
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⁻¹strongsharp
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.
CH₂ rocking (long chain)
715–725
medium
sharp
Rocking vibration of methylene chains with four or more CH₂ groups in sequence. This band near 720 cm⁻¹ is diagnostic for long-chain alkyl groups.
C-H stretch (CH₃ asymmetric)
2950–2970 cm⁻¹strongsharp
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⁻¹strongsharp
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⁻¹strongsharp
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⁻¹mediumsharp
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⁻¹mediumsharp
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⁻¹mediumsharp
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⁻¹mediumsharp
Scissoring deformation of methylene groups near 1465 cm⁻¹. In crystalline long-chain alkanes, this band may split due to crystal field effects.
CH₂ rocking (long chain)
715–725 cm⁻¹mediumsharp
Rocking vibration of methylene chains with four or more CH₂ groups in sequence. This band near 720 cm⁻¹ is diagnostic for long-chain alkyl groups.
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⁻¹mediumsharp
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⁻¹variablesharp
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⁻¹strongsharp
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⁻¹mediumsharp
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⁻¹strongsharp
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⁻¹strongsharp
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 690 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.
C-H in-plane bend (aromatic)
1000–1100
medium
sharp
In-plane bending of aromatic C-H bonds. Multiple bands in this region reflect different vibrational modes of the ring hydrogens.
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⁻¹mediumsharp
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⁻¹variablesharp
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⁻¹variablesharp
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⁻¹strongsharp
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⁻¹strongsharp
Out-of-plane ring bending of monosubstituted benzene near 690 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⁻¹strongsharp
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⁻¹strongsharp
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⁻¹strongsharp
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⁻¹strongsharp
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.
C-H in-plane bend (aromatic)
1000–1100 cm⁻¹mediumsharp
In-plane bending of aromatic C-H bonds. Multiple bands in this region reflect different vibrational modes of the ring hydrogens.
Ring breathing (aromatic)
990–1010 cm⁻¹variablesharp
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⁻¹strongsharp
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⁻¹weaksharp
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⁻¹weaksharp
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 2820 and 2720 cm⁻¹.
C-H stretch (aldehyde, Fermi doublet low)
2700–2760 cm⁻¹mediumdoublet
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⁻¹mediumdoublet
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⁻¹strongsharp
Carbonyl stretch of aldehydes, slightly higher than ketones due to less inductive donation. Confirm by looking for the characteristic Fermi doublet near 2820 and 2720 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⁻¹strongsharp
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⁻¹strongsharp
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⁻¹strongsharp
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⁻¹strongsharp
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⁻¹strongsharp
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⁻¹strongsharp
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. It appears above ketones because the electronegative ester oxygen withdraws electron density by induction, which outweighs its resonance donation and strengthens the C=O bond.
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⁻¹strongsharp
Carbonyl stretch of simple esters. It appears above ketones because the electronegative ester oxygen withdraws electron density by induction, which outweighs its resonance donation and strengthens the C=O bond.
C=O stretch (α,β-unsaturated ester)
1715–1735 cm⁻¹strongsharp
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⁻¹strongsharp
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. Acyclic anhydrides show a stronger high-frequency band, while cyclic anhydrides show a stronger low-frequency band.
C=O stretch (anhydride, symmetric)
1800–1850 cm⁻¹strongsharp
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⁻¹strongsharp
The lower-frequency asymmetric C=O stretch of anhydrides. Acyclic anhydrides show a stronger high-frequency band, while cyclic 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⁻¹strongsharp
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⁻¹strongsharp
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⁻¹strongsharp
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⁻¹strongsharp
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⁻¹strongsharp
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⁻¹strongsharp
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⁻¹variablesharp
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)
1000–1075
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)
1075–1150
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. Ring alcohols such as cyclohexanol (about 1065 cm⁻¹) and menthol (about 1045 cm⁻¹) absorb in the primary-alcohol range: the carbon bearing the hydroxyl being in a ring, or a double bond next to it, lowers the band by about one class.
C-O stretch (3° alcohol)
1100–1210
strong
sharp
C-O stretching vibration of tertiary alcohols, 1100–1210 cm⁻¹ (tert-butanol near 1200 cm⁻¹). The highest of the three alcohol types, but the range overlaps secondary alcohols between 1100 and 1150 cm⁻¹, so a band there cannot tell them apart on its own.
O-H bend (in-plane, alcohol)
1300–1420
medium
broad
In-plane bending (deformation) of the O-H bond in alcohols. The band is broad, broader than the peaks around it; it couples mainly with C-H bending vibrations and overlaps the CH₃ symmetric bend near 1375 cm⁻¹.
C-C-O skeletal deformation (alcohol)
800–900
medium
sharp
Skeletal C-C-O deformation in alcohols. This mode involves coupled bending of the C-C-O framework and is commonly observed in primary and secondary alcohols.
C-O stretch (1° alcohol)
1000–1075 cm⁻¹strongsharp
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)
1075–1150 cm⁻¹strongsharp
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. Ring alcohols such as cyclohexanol (about 1065 cm⁻¹) and menthol (about 1045 cm⁻¹) absorb in the primary-alcohol range: the carbon bearing the hydroxyl being in a ring, or a double bond next to it, lowers the band by about one class.
C-O stretch (3° alcohol)
1100–1210 cm⁻¹strongsharp
C-O stretching vibration of tertiary alcohols, 1100–1210 cm⁻¹ (tert-butanol near 1200 cm⁻¹). The highest of the three alcohol types, but the range overlaps secondary alcohols between 1100 and 1150 cm⁻¹, so a band there cannot tell them apart on its own.
O-H bend (in-plane, alcohol)
1300–1420 cm⁻¹mediumbroad
In-plane bending (deformation) of the O-H bond in alcohols. The band is broad, broader than the peaks around it; it couples mainly with C-H bending vibrations and overlaps the CH₃ symmetric bend near 1375 cm⁻¹.
C-C-O skeletal deformation (alcohol)
800–900 cm⁻¹mediumsharp
Skeletal C-C-O deformation in alcohols. This mode involves coupled bending of the C-C-O framework and is commonly observed in primary and secondary alcohols.
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⁻¹strongsharp
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⁻¹strongsharp
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⁻¹strongsharp
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⁻¹strongsharp
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.
Sulfonates
Functional Group
Range (cm⁻¹)
Intensity
Shape
Notes
SO₃ asymmetric stretch (sulfonate)
1140–1200
strong
sharp
Asymmetric stretching of the SO₃ group in sulfonates and sulfonic acids. Paired with the symmetric stretch near 1040 cm⁻¹ to confirm the functional group.
SO₃ symmetric stretch (sulfonate)
1030–1060
strong
sharp
Symmetric stretching of the SO₃ group in sulfonates and sulfonic acids.
SO₃ asymmetric stretch (sulfonate)
1140–1200 cm⁻¹strongsharp
Asymmetric stretching of the SO₃ group in sulfonates and sulfonic acids. Paired with the symmetric stretch near 1040 cm⁻¹ to confirm the functional group.
SO₃ symmetric stretch (sulfonate)
1030–1060 cm⁻¹strongsharp
Symmetric stretching of the SO₃ group in sulfonates and sulfonic acids.
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⁻¹strongsharp
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⁻¹strongsharp
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⁻¹strongsharp
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⁻¹strongsharp
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⁻¹strongbroad
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⁻¹strongsharp
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
Ring breathing (epoxide)
1230–1280
strong
sharp
Symmetric ring breathing of the three-membered epoxide ring. Bands near 950–810 cm⁻¹ (asymmetric C-O-C stretch) and 880–750 cm⁻¹ (symmetric C-O-C stretch) confirm the epoxide.
Ring breathing (epoxide)
1230–1280 cm⁻¹strongsharp
Symmetric ring breathing of the three-membered epoxide ring. Bands near 950–810 cm⁻¹ (asymmetric C-O-C stretch) and 880–750 cm⁻¹ (symmetric C-O-C stretch) confirm the epoxide.
Phenols
Functional Group
Range (cm⁻¹)
Intensity
Shape
Notes
C-O stretch (phenol)
1200–1260
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)
1200–1260 cm⁻¹strongsharp
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⁻¹weaksharp
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). Rare outside boron chemistry — confirm with other boron indicators.
B-O stretch
1310–1380 cm⁻¹strongsharp
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). Rare outside boron chemistry — confirm with other boron indicators.
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Infrared Spectrum Chart — How to Read FTIR Absorption Bands
An infrared spectrum chart plots wavenumber (cm⁻¹) against transmittance or absorbance, with each dip or peak corresponding to a specific bond vibration. The IR spectrum functional groups listed in the table above are organized by the spectral region where they absorb — from O-H and N-H stretches at high wavenumbers down through the fingerprint region below 1500 cm⁻¹. Understanding this layout makes it easier to use any FTIR data table for systematic peak assignment.
Notes on Using IR Spectroscopy 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
- ATR-FTIR Spectroscopy Guide — how ATR works and when to use it
- Spectroscopy Unit Converter — convert between wavenumber, wavelength, frequency, and energy
Frequently Asked Questions
What is an FTIR spectrum table?
An FTIR spectrum table (also called an IR correlation table) is a reference chart listing the characteristic infrared absorption frequencies of common functional groups. Each entry shows the wavenumber range (in cm⁻¹), expected intensity (strong, medium, or weak), band shape (broad, sharp, doublet), and the functional group responsible. Spectroscopists use these tables to identify unknown compounds by matching observed peaks to known absorption ranges.
How do you read an infrared spectrum chart?
Start at the high-wavenumber end (4000 cm⁻¹) and work toward the fingerprint region (below 1500 cm⁻¹). Look for broad O-H or N-H stretches around 3200–3600 cm⁻¹, sharp C-H stretches near 2800–3100 cm⁻¹, strong carbonyl (C=O) bands between 1630–1850 cm⁻¹, and the complex fingerprint region below 1500 cm⁻¹. Match each observed band's position, intensity, and shape against a reference table to assign functional groups.
How do I identify functional groups in an IR spectrum?
Identify functional groups by matching observed absorption bands to known wavenumber ranges: O-H stretches produce broad bands near 3200–3550 cm⁻¹, C=O stretches give strong sharp peaks between 1630–1850 cm⁻¹, and C-H stretches appear as sharp bands near 2800–3100 cm⁻¹. Always consider intensity and band shape alongside position — a broad band at 3300 cm⁻¹ suggests O-H (alcohol), while a medium doublet at the same position suggests N-H (primary amine). Confirm assignments by looking for corroborating peaks from the same functional group.
What wavenumber range indicates an O-H group in FTIR?
Free (non-hydrogen-bonded) O-H stretches appear as a sharp peak near 3610–3670 cm⁻¹. Hydrogen-bonded O-H in alcohols and phenols produces a broad band at 3200–3550 cm⁻¹. Carboxylic acid O-H is even broader, spanning 2500–3300 cm⁻¹ and often overlapping with C-H stretches. The breadth of the O-H band directly reflects the strength and variety of hydrogen bonding in the sample.
What is the difference between FTIR and IR spectroscopy?
IR (infrared) spectroscopy is the general technique of measuring how molecules absorb infrared light. FTIR (Fourier-Transform Infrared) is the modern implementation that uses an interferometer and Fourier transform mathematics to collect the entire spectrum simultaneously, rather than scanning one wavelength at a time. FTIR is faster, more sensitive, and has better wavelength accuracy than older dispersive IR instruments. Today, virtually all IR spectrometers use FTIR technology.