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〰️IR Spectroscopy Table - Functional Group Absorption Chart & Peak Finder

Infrared (IR) spectroscopy measures how molecular vibrations absorb mid-infrared light between 4000 and 400 cm⁻¹. Each bond type absorbs at a characteristic wavenumber, so an IR spectrum reveals which functional groups a molecule contains. Spectra are plotted with wavenumber decreasing from 4000 on the left to 400 on the right and transmittance on the y-axis, so absorptions point down. The region above 1500 cm⁻¹ holds the diagnostic bands: a broad, strong alcohol O–H at 3200–3550, a very broad carboxylic acid O–H from 2500 to 3300, two N–H bands for a primary amine at 3300–3500, unsaturated C–H above 3000 and alkyl C–H below it, the aldehyde C–H doublet at 2720/2820, C≡N near 2250, and the strong C=O stretch at 1650–1850 — acid chloride ~1800, ester ~1740, aldehyde ~1725, ketone ~1715, acid ~1710, amide ~1660, each lowered 20–30 cm⁻¹ by conjugation. For example, a strong band at 1715 with C–H at 2950 and no broad O–H points to a saturated ketone such as acetone, while a very broad 2500–3300 band plus 1710 means a carboxylic acid. This tool collects 60+ correlation bands from standard texts (Pavia, Silverstein) with range, typical position, intensity and shape on an interactive chart you can filter and search; a peak finder that ranks candidate functional groups from your observed peaks and explains the reasoning; a simulated spectrum for chosen groups or a SMILES string (ethanol, acetone, ethyl acetate, benzaldehyde…); and a wavenumber, wavelength, frequency and energy converter.

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How to use IR Spectroscopy Table

  1. 1Browse the correlation chart, filter by group type or search for 1715, carbonyl or O–H, and hover over a bar for range, intensity and notes
  2. 2In the peak finder enter the peaks from your spectrum, such as 3300 broad, 1050 s, and read the ranked functional groups and the possible assignment of each peak
  3. 3Use the simulated spectrum to see what a molecule's spectrum typically looks like from a SMILES string or chosen groups, and the converter for wavenumber, wavelength, frequency and energy

FAQ

How do you read an IR spectrum?
Start in the functional group region (4000–1500 cm⁻¹). Look for a broad O–H at 3200–3650, N–H at 3300–3500, a strong C=O at 1650–1850 and triple bonds at 2100–2260, and use 3000 cm⁻¹ to tell unsaturated from saturated C–H. Then confirm with supporting bands — the aldehyde 2720/2820 doublet, the very broad acid O–H at 2500–3300, or the two strong ester C–O bands at 1000–1300. The fingerprint region below 1500 is best used for matching against a reference spectrum.
Why do carbonyl groups absorb at different wavenumbers?
The C=O frequency follows bond strength. Electron-withdrawing groups raise it (acid chlorides ~1800, anhydrides 1820 and 1760, esters ~1740), while resonance from the nitrogen lone pair lowers amides to ~1660. Conjugation with C=C or an aromatic ring lowers the frequency by 20–30 cm⁻¹, and ring strain raises it — cyclopentanone absorbs near 1745.
How can I tell an alcohol O–H from a carboxylic acid O–H?
A hydrogen-bonded alcohol or phenol O–H is a strong, broad, rounded band at 3200–3550 cm⁻¹. A carboxylic acid O–H is far broader, running from about 2500 to 3300 and swallowing the C–H stretches, and it always comes with a strong C=O near 1710. Only in dilute solution or the gas phase does an alcohol show a sharp free O–H near 3600.
How reliable are the peak finder and simulated spectra?
Both are built on textbook correlation tables. The peak finder scores each functional group by whether its characteristic bands are present, whether intensity and shape agree, and how many of your peaks it explains — a guide to your reasoning, not a definitive identification. Simulated spectra place Lorentzian bands at typical positions to show the general look of a spectrum. Real band positions shift with hydrogen bonding, conjugation and sampling, so combine IR with the molecular formula, NMR and MS, or compare with reference spectra from SDBS or the NIST WebBook.