FTIR Peak Identifier
Enter one or more wavenumber values from your FTIR spectrum and see every functional group whose typical range contains each peak, ranked by fit.
Separate values with commas, spaces or new lines. Accepted range: 400–4000 cm⁻¹. Example: 3400, 1720, 1250
Showing assignments for 3 wavenumbers.
Every group whose typical range contains your value is listed. The order reflects how close the value is to the middle of each range; very broad ranges, rarely encountered groups and weak bands are placed lower, and groups supported by your other entered peaks are placed higher. The fit label is not a probability.
Ketones · 1705–1725 cm⁻¹ · 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.
Check: Supported if there is no C-H pair near 2820/2720 cm⁻¹ and no broad O-H band.
Carboxylic Acids · 1700–1725 cm⁻¹ · 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.
Check: Confirm with a very broad O-H band spanning about 3300–2500 cm⁻¹.
Carbamates · 1690–1740 cm⁻¹ · strong · sharp
Carbonyl stretch of carbamates (urethanes, R-O-CO-NR₂), 1690–1740 cm⁻¹. In polyurethanes, free (non-hydrogen-bonded) urethane C=O appears near 1730 cm⁻¹ and hydrogen-bonded C=O near 1700–1710 cm⁻¹; the ratio is used to follow hydrogen bonding.
Esters · 1715–1735 cm⁻¹ · 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.
Check: Confirm with a strong C-O stretch between 1300 and 1000 cm⁻¹.
Lactams · 1695–1715 cm⁻¹ · strong · sharp
Carbonyl stretch of γ-lactams (5-membered ring, pyrrolidinones). Moderate ring strain raises frequency above acyclic amides. Found in pyroglutamate and many pharmaceuticals.
Aromatics · 1650–2000 cm⁻¹ · weak · multiple
Weak overtone and combination bands of aromatic C-H out-of-plane bending modes. The specific pattern of bands in this region is diagnostic of the ring substitution pattern (mono, ortho, meta, para). Best used by comparing to reference patterns.
Nearby: just outside the typical range (within 20 cm⁻¹)
- C=O stretch (aldehyde) (1720–1740 cm⁻¹)
- C=O stretch (anhydride, asymmetric) (1720–1790 cm⁻¹)
- C=O stretch (carbonate) (1720–1820 cm⁻¹)
- C=O stretch (conjugated/aromatic aldehyde) (1685–1710 cm⁻¹)
- C=O stretch (conjugated ketone) (1665–1700 cm⁻¹)
- C=O stretch (β-lactam, 4-membered) (1730–1790 cm⁻¹)
- C=O stretch (ester) (1735–1750 cm⁻¹)
- C=O stretch (lactone) (1735–1840 cm⁻¹)
Alkanes · 2950–2970 cm⁻¹ · 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.
Carboxylic Acids · 2500–3300 cm⁻¹ · 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.
Enols · 2500–3200 cm⁻¹ · variable · broad
Extremely broad O-H stretch in chelated enols (e.g., acetylacetone enol form, salicylaldehyde). The strong intramolecular hydrogen bond shifts the O-H to much lower frequency than free alcohols and broadens it dramatically.
Amine Salts · 2600–3100 cm⁻¹ · strong · broad
Broad N-H stretching of protonated amines (R-NH₃⁺) and amino acid zwitterions. Much broader and shifted to lower frequency than neutral amines due to strong hydrogen bonding. Multiple overlapping bands create a distinctive broad absorption.
Nearby: just outside the typical range (within 20 cm⁻¹)
- C-H stretch (CH₂ asymmetric) (2915–2940 cm⁻¹)
Alcohols, Phenols · 3200–3550 cm⁻¹ · strong · broad
Broad absorption due to hydrogen-bonded O-H groups. The breadth results from the distribution of H-bond strengths in the sample.
Check: Confirm with a C-O stretch between 1260 and 1000 cm⁻¹.
Amines · 3350–3500 cm⁻¹ · 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.
Amines · 3310–3500 cm⁻¹ · 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.
Amines · 3250–3400 cm⁻¹ · 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.
How to Use This Tool
Note peak positions
Look at your FTIR spectrum and record the wavenumber (cm⁻¹) of each significant peak. Focus on the strongest and most distinctive peaks first.
Enter wavenumbers
Type the wavenumber values into the input above. Separate multiple values with commas, spaces or new lines (for example 3400, 1720, 1250).
Review assignments
Examine every functional group whose typical range contains your peak. The list is ordered by how close your value is to the middle of each range, with very broad ranges, rarely encountered groups and weak bands placed lower and groups supported by your other entered peaks placed higher; the fit label is not a probability. Use peak intensity, shape and corroborating peaks to choose between them. Bands that may come from water vapour or CO₂ in the instrument air are flagged separately.
Confirm with context
Cross-reference assignments with peak intensity, shape, and the presence of corroborating peaks. A single peak position rarely provides a definitive identification.
Remember
Peak position alone does not confirm a functional group assignment. Always consider intensity, shape, and the presence of corroborating peaks — see our systematic interpretation guide for a structured approach. If a peak may be an artifact (water vapour, CO₂, a sloping baseline), the troubleshooting guide shows how to recognise and remove it. This tool checks against 130+ known functional group absorptions in the FTIR database.
References
- Silverstein, R. M.; Webster, F. X.; Kiemle, D. J.; Bryce, D. L. Spectrometric Identification of Organic Compounds, 8th ed. Wiley, 2014.
- Socrates, G. Infrared and Raman Characteristic Group Frequencies: Tables and Charts, 3rd ed. Wiley, 2001.